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Simple statements</a><ul> <li><a class="reference internal" href="#expression-statements">7.1. Expression statements</a></li> <li><a class="reference internal" href="#assignment-statements">7.2. Assignment statements</a><ul> <li><a class="reference internal" href="#augmented-assignment-statements">7.2.1. Augmented assignment statements</a></li> <li><a class="reference internal" href="#annotated-assignment-statements">7.2.2. Annotated assignment statements</a></li> </ul> </li> <li><a class="reference internal" href="#the-assert-statement">7.3. The <code class="xref std std-keyword docutils literal notranslate">assert</code> statement</a></li> <li><a class="reference internal" href="#the-pass-statement">7.4. The <code class="xref std std-keyword docutils literal notranslate">pass</code> statement</a></li> <li><a class="reference internal" href="#the-del-statement">7.5. The <code class="xref std std-keyword docutils literal notranslate">del</code> statement</a></li> <li><a class="reference internal" href="#the-return-statement">7.6. The <code class="xref std std-keyword docutils literal notranslate">return</code> statement</a></li> <li><a class="reference internal" href="#the-yield-statement">7.7. The <code class="xref std std-keyword docutils literal notranslate">yield</code> statement</a></li> <li><a class="reference internal" href="#the-raise-statement">7.8. The <code class="xref std std-keyword docutils literal notranslate">raise</code> statement</a></li> <li><a class="reference internal" href="#the-break-statement">7.9. The <code class="xref std std-keyword docutils literal notranslate">break</code> statement</a></li> <li><a class="reference internal" href="#the-continue-statement">7.10. The <code class="xref std std-keyword docutils literal notranslate">continue</code> statement</a></li> <li><a class="reference internal" href="#the-import-statement">7.11. The <code class="xref std std-keyword docutils literal notranslate">import</code> statement</a><ul> <li><a class="reference internal" href="#future-statements">7.11.1. Future statements</a></li> </ul> </li> <li><a class="reference internal" href="#the-global-statement">7.12. The <code class="xref std std-keyword docutils literal notranslate">global</code> statement</a></li> <li><a class="reference internal" href="#the-nonlocal-statement">7.13. The <code class="xref std std-keyword docutils literal notranslate">nonlocal</code> statement</a></li> <li><a class="reference internal" href="#the-type-statement">7.14. The <code class="xref std std-keyword docutils literal notranslate">type</code> statement</a></li> </ul> </li> </ul> </div> <div> <h4>Previous topic</h4> <a href="expressions.html" title="previous chapter">6. Expressions</a> </div> <div> <h4>Next topic</h4> <a href="compound_stmts.html" title="next chapter">8. Compound statements</a> </div> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/main/Doc/reference/simple_stmts.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </nav> </div> </div> <div class="related" role="navigation" aria-label="Related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="compound_stmts.html" title="8. Compound statements" accesskey="N">next</a> |</li> <li class="right" > <a href="expressions.html" title="6. 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Simple statements</a></li> <li class="right"> <div class="inline-search" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" aria-label="Quick search" type="search" name="q" id="search-box" /> <input type="submit" value="Go" /> </form> </div> | </li> <li class="right"> <label class="theme-selector-label"> Theme <select class="theme-selector" oninput="activateTheme(this.value)"> <option value="auto" selected>Auto</option> <option value="light">Light</option> <option value="dark">Dark</option> </select> </label> |</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <section id="simple-statements"> <h1>7. Simple statements<a class="headerlink" href="#simple-statements" title="Link to this heading">¶</a></h1> A simple statement is comprised within a single logical line. Several simple statements may occur on a single line separated by semicolons. The syntax for simple statements is: <pre> simple_stmt ::= <a class="reference internal" href="#grammar-token-python-grammar-expression_stmt"><code class="xref docutils literal notranslate">expression_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-assert_stmt"><code class="xref docutils literal notranslate">assert_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-assignment_stmt"><code class="xref docutils literal notranslate">assignment_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-augmented_assignment_stmt"><code class="xref docutils literal notranslate">augmented_assignment_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-annotated_assignment_stmt"><code class="xref docutils literal notranslate">annotated_assignment_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-pass_stmt"><code class="xref docutils literal notranslate">pass_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-del_stmt"><code class="xref docutils literal notranslate">del_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-return_stmt"><code class="xref docutils literal notranslate">return_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-yield_stmt"><code class="xref docutils literal notranslate">yield_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-raise_stmt"><code class="xref docutils literal notranslate">raise_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-break_stmt"><code class="xref docutils literal notranslate">break_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-continue_stmt"><code class="xref docutils literal notranslate">continue_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-import_stmt"><code class="xref docutils literal notranslate">import_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-future_stmt"><code class="xref docutils literal notranslate">future_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-global_stmt"><code class="xref docutils literal notranslate">global_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-nonlocal_stmt"><code class="xref docutils literal notranslate">nonlocal_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-type_stmt"><code class="xref docutils literal notranslate">type_stmt</code></a> </pre> <section id="expression-statements"> <h2>7.1. Expression statements<a class="headerlink" href="#expression-statements" title="Link to this heading">¶</a></h2> Expression statements are used (mostly interactively) to compute and write a value, or (usually) to call a procedure (a function that returns no meaningful result; in Python, procedures return the value <code class="docutils literal notranslate">None</code>). Other uses of expression statements are allowed and occasionally useful. The syntax for an expression statement is: <pre> expression_stmt ::= <a class="reference internal" href="expressions.html#grammar-token-python-grammar-starred_expression"><code class="xref docutils literal notranslate">starred_expression</code></a> </pre> An expression statement evaluates the expression list (which may be a single expression). In interactive mode, if the value is not <code class="docutils literal notranslate">None</code>, it is converted to a string using the built-in <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate">repr()</code></a> function and the resulting string is written to standard output on a line by itself (except if the result is <code class="docutils literal notranslate">None</code>, so that procedure calls do not cause any output.) </section> <section id="assignment-statements"> <h2>7.2. Assignment statements<a class="headerlink" href="#assignment-statements" title="Link to this heading">¶</a></h2> Assignment statements are used to (re)bind names to values and to modify attributes or items of mutable objects: <pre> assignment_stmt ::= (<a class="reference internal" href="#grammar-token-python-grammar-target_list"><code class="xref docutils literal notranslate">target_list</code></a> "=")+ (<a class="reference internal" href="expressions.html#grammar-token-python-grammar-starred_expression"><code class="xref docutils literal notranslate">starred_expression</code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-yield_expression"><code class="xref docutils literal notranslate">yield_expression</code></a>) target_list ::= <a class="reference internal" href="#grammar-token-python-grammar-target"><code class="xref docutils literal notranslate">target</code></a> ("," <a class="reference internal" href="#grammar-token-python-grammar-target"><code class="xref docutils literal notranslate">target</code></a>)* [","] target ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> | "(" [<a class="reference internal" href="#grammar-token-python-grammar-target_list"><code class="xref docutils literal notranslate">target_list</code></a>] ")" | "[" [<a class="reference internal" href="#grammar-token-python-grammar-target_list"><code class="xref docutils literal notranslate">target_list</code></a>] "]" | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-attributeref"><code class="xref docutils literal notranslate">attributeref</code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-subscription"><code class="xref docutils literal notranslate">subscription</code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-slicing"><code class="xref docutils literal notranslate">slicing</code></a> | "*" <a class="reference internal" href="#grammar-token-python-grammar-target"><code class="xref docutils literal notranslate">target</code></a> </pre> (See section <a class="reference internal" href="expressions.html#primaries">Primaries</a> for the syntax definitions for attributeref, subscription, and slicing.) An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right. Assignment is defined recursively depending on the form of the target (list). When a target is part of a mutable object (an attribute reference, subscription or slicing), the mutable object must ultimately perform the assignment and decide about its validity, and may raise an exception if the assignment is unacceptable. The rules observed by various types and the exceptions raised are given with the definition of the object types (see section <a class="reference internal" href="datamodel.html#types">The standard type hierarchy</a>). Assignment of an object to a target list, optionally enclosed in parentheses or square brackets, is recursively defined as follows. <ul class="simple"> <li>If the target list is a single target with no trailing comma, optionally in parentheses, the object is assigned to that target.</li> <li>Else: <ul> <li>If the target list contains one target prefixed with an asterisk, called a “starred” target: The object must be an iterable with at least as many items as there are targets in the target list, minus one. The first items of the iterable are assigned, from left to right, to the targets before the starred target. The final items of the iterable are assigned to the targets after the starred target. A list of the remaining items in the iterable is then assigned to the starred target (the list can be empty).</li> <li>Else: The object must be an iterable with the same number of items as there are targets in the target list, and the items are assigned, from left to right, to the corresponding targets.</li> </ul> </li> </ul> Assignment of an object to a single target is recursively defined as follows. <ul> <li>If the target is an identifier (name): <ul class="simple"> <li>If the name does not occur in a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate">global</code></a> or <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate">nonlocal</code></a> statement in the current code block: the name is bound to the object in the current local namespace.</li> <li>Otherwise: the name is bound to the object in the global namespace or the outer namespace determined by <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate">nonlocal</code></a>, respectively.</li> </ul> The name is rebound if it was already bound. This may cause the reference count for the object previously bound to the name to reach zero, causing the object to be deallocated and its destructor (if it has one) to be called. </li> <li id="index-8">If the target is an attribute reference: The primary expression in the reference is evaluated. It should yield an object with assignable attributes; if this is not the case, <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> is raised. That object is then asked to assign the assigned object to the given attribute; if it cannot perform the assignment, it raises an exception (usually but not necessarily <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a>). Note: If the object is a class instance and the attribute reference occurs on both sides of the assignment operator, the right-hand side expression, <code class="docutils literal notranslate">a.x</code> can access either an instance attribute or (if no instance attribute exists) a class attribute. The left-hand side target <code class="docutils literal notranslate">a.x</code> is always set as an instance attribute, creating it if necessary. Thus, the two occurrences of <code class="docutils literal notranslate">a.x</code> do not necessarily refer to the same attribute: if the right-hand side expression refers to a class attribute, the left-hand side creates a new instance attribute as the target of the assignment: <div class="highlight-python3 notranslate"><div class="highlight"><pre>class Cls: x = 3 # class variable inst = Cls() inst.x = inst.x + 1 # writes inst.x as 4 leaving Cls.x as 3 </pre></div> </div> This description does not necessarily apply to descriptor attributes, such as properties created with <a class="reference internal" href="../library/functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate">property()</code></a>. </li> <li id="index-9">If the target is a subscription: The primary expression in the reference is evaluated. It should yield either a mutable sequence object (such as a list) or a mapping object (such as a dictionary). Next, the subscript expression is evaluated. If the primary is a mutable sequence object (such as a list), the subscript must yield an integer. If it is negative, the sequence’s length is added to it. The resulting value must be a nonnegative integer less than the sequence’s length, and the sequence is asked to assign the assigned object to its item with that index. If the index is out of range, <a class="reference internal" href="../library/exceptions.html#IndexError" title="IndexError"><code class="xref py py-exc docutils literal notranslate">IndexError</code></a> is raised (assignment to a subscripted sequence cannot add new items to a list). If the primary is a mapping object (such as a dictionary), the subscript must have a type compatible with the mapping’s key type, and the mapping is then asked to create a key/value pair which maps the subscript to the assigned object. This can either replace an existing key/value pair with the same key value, or insert a new key/value pair (if no key with the same value existed). For user-defined objects, the <a class="reference internal" href="datamodel.html#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate">__setitem__()</code></a> method is called with appropriate arguments. </li> <li id="index-12">If the target is a slicing: The primary expression in the reference is evaluated. It should yield a mutable sequence object (such as a list). The assigned object should be a sequence object of the same type. Next, the lower and upper bound expressions are evaluated, insofar they are present; defaults are zero and the sequence’s length. The bounds should evaluate to integers. If either bound is negative, the sequence’s length is added to it. The resulting bounds are clipped to lie between zero and the sequence’s length, inclusive. Finally, the sequence object is asked to replace the slice with the items of the assigned sequence. The length of the slice may be different from the length of the assigned sequence, thus changing the length of the target sequence, if the target sequence allows it.</li> </ul> <div class="impl-detail compound"> CPython implementation detail: In the current implementation, the syntax for targets is taken to be the same as for expressions, and invalid syntax is rejected during the code generation phase, causing less detailed error messages. </div> Although the definition of assignment implies that overlaps between the left-hand side and the right-hand side are ‘simultaneous’ (for example <code class="docutils literal notranslate">a, b = b, a</code> swaps two variables), overlaps within the collection of assigned-to variables occur left-to-right, sometimes resulting in confusion. For instance, the following program prints <code class="docutils literal notranslate">[0, 2]</code>: <div class="highlight-python3 notranslate"><div class="highlight"><pre>x = [0, 1] i = 0 i, x[i] = 1, 2 # i is updated, then x[i] is updated print(x) </pre></div> </div> <div class="admonition seealso"> See also <dl class="simple"> <dt><a class="pep reference external" href="https://peps.python.org/pep-3132/">PEP 3132</a> - Extended Iterable Unpacking</dt><dd>The specification for the <code class="docutils literal notranslate">*target</code> feature. </dd> </dl> </div> <section id="augmented-assignment-statements"> <h3>7.2.1. Augmented assignment statements<a class="headerlink" href="#augmented-assignment-statements" title="Link to this heading">¶</a></h3> Augmented assignment is the combination, in a single statement, of a binary operation and an assignment statement: <pre> augmented_assignment_stmt ::= <a class="reference internal" href="#grammar-token-python-grammar-augtarget"><code class="xref docutils literal notranslate">augtarget</code></a> <a class="reference internal" href="#grammar-token-python-grammar-augop"><code class="xref docutils literal notranslate">augop</code></a> (<a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression_list"><code class="xref docutils literal notranslate">expression_list</code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-yield_expression"><code class="xref docutils literal notranslate">yield_expression</code></a>) augtarget ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-attributeref"><code class="xref docutils literal notranslate">attributeref</code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-subscription"><code class="xref docutils literal notranslate">subscription</code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-slicing"><code class="xref docutils literal notranslate">slicing</code></a> augop ::= "+=" | "-=" | "*=" | "@=" | "/=" | "//=" | "%=" | "**=" | ">>=" | "<<=" | "&=" | "^=" | "|=" </pre> (See section <a class="reference internal" href="expressions.html#primaries">Primaries</a> for the syntax definitions of the last three symbols.) An augmented assignment evaluates the target (which, unlike normal assignment statements, cannot be an unpacking) and the expression list, performs the binary operation specific to the type of assignment on the two operands, and assigns the result to the original target. The target is only evaluated once. An augmented assignment statement like <code class="docutils literal notranslate">x += 1</code> can be rewritten as <code class="docutils literal notranslate">x = x + 1</code> to achieve a similar, but not exactly equal effect. In the augmented version, <code class="docutils literal notranslate">x</code> is only evaluated once. Also, when possible, the actual operation is performed in-place, meaning that rather than creating a new object and assigning that to the target, the old object is modified instead. Unlike normal assignments, augmented assignments evaluate the left-hand side before evaluating the right-hand side. For example, <code class="docutils literal notranslate">a[i] += f(x)</code> first looks-up <code class="docutils literal notranslate">a[i]</code>, then it evaluates <code class="docutils literal notranslate">f(x)</code> and performs the addition, and lastly, it writes the result back to <code class="docutils literal notranslate">a[i]</code>. With the exception of assigning to tuples and multiple targets in a single statement, the assignment done by augmented assignment statements is handled the same way as normal assignments. Similarly, with the exception of the possible in-place behavior, the binary operation performed by augmented assignment is the same as the normal binary operations. For targets which are attribute references, the same <a class="reference internal" href="#attr-target-note">caveat about class and instance attributes</a> applies as for regular assignments. </section> <section id="annotated-assignment-statements"> <h3>7.2.2. Annotated assignment statements<a class="headerlink" href="#annotated-assignment-statements" title="Link to this heading">¶</a></h3> <a class="reference internal" href="../glossary.html#term-variable-annotation">Annotation</a> assignment is the combination, in a single statement, of a variable or attribute annotation and an optional assignment statement: <pre> annotated_assignment_stmt ::= <a class="reference internal" href="#grammar-token-python-grammar-augtarget"><code class="xref docutils literal notranslate">augtarget</code></a> ":" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a> ["=" (<a class="reference internal" href="expressions.html#grammar-token-python-grammar-starred_expression"><code class="xref docutils literal notranslate">starred_expression</code></a> | <a class="reference internal" href="expressions.html#grammar-token-python-grammar-yield_expression"><code class="xref docutils literal notranslate">yield_expression</code></a>)] </pre> The difference from normal <a class="reference internal" href="#assignment">Assignment statements</a> is that only a single target is allowed. The assignment target is considered “simple” if it consists of a single name that is not enclosed in parentheses. For simple assignment targets, if in class or module scope, the annotations are evaluated and stored in a special class or module attribute <code class="xref py py-attr docutils literal notranslate">__annotations__</code> that is a dictionary mapping from variable names (mangled if private) to evaluated annotations. This attribute is writable and is automatically created at the start of class or module body execution, if annotations are found statically. If the assignment target is not simple (an attribute, subscript node, or parenthesized name), the annotation is evaluated if in class or module scope, but not stored. If a name is annotated in a function scope, then this name is local for that scope. Annotations are never evaluated and stored in function scopes. If the right hand side is present, an annotated assignment performs the actual assignment before evaluating annotations (where applicable). If the right hand side is not present for an expression target, then the interpreter evaluates the target except for the last <a class="reference internal" href="datamodel.html#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate">__setitem__()</code></a> or <a class="reference internal" href="datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate">__setattr__()</code></a> call. <div class="admonition seealso"> See also <dl class="simple"> <dt><a class="pep reference external" href="https://peps.python.org/pep-0526/">PEP 526</a> - Syntax for Variable Annotations</dt><dd>The proposal that added syntax for annotating the types of variables (including class variables and instance variables), instead of expressing them through comments. </dd> <dt><a class="pep reference external" href="https://peps.python.org/pep-0484/">PEP 484</a> - Type hints</dt><dd>The proposal that added the <a class="reference internal" href="../library/typing.html#module-typing" title="typing: Support for type hints (see :pep:`484`)."><code class="xref py py-mod docutils literal notranslate">typing</code></a> module to provide a standard syntax for type annotations that can be used in static analysis tools and IDEs. </dd> </dl> </div> <div class="versionchanged"> Changed in version 3.8: Now annotated assignments allow the same expressions in the right hand side as regular assignments. Previously, some expressions (like un-parenthesized tuple expressions) caused a syntax error. </div> </section> </section> <section id="the-assert-statement"> <h2>7.3. The <code class="xref std std-keyword docutils literal notranslate">assert</code> statement<a class="headerlink" href="#the-assert-statement" title="Link to this heading">¶</a></h2> Assert statements are a convenient way to insert debugging assertions into a program: <pre> assert_stmt ::= "assert" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a> ["," <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>] </pre> The simple form, <code class="docutils literal notranslate">assert expression</code>, is equivalent to <div class="highlight-python3 notranslate"><div class="highlight"><pre>if __debug__: if not expression: raise AssertionError </pre></div> </div> The extended form, <code class="docutils literal notranslate">assert expression1, expression2</code>, is equivalent to <div class="highlight-python3 notranslate"><div class="highlight"><pre>if __debug__: if not expression1: raise AssertionError(expression2) </pre></div> </div> These equivalences assume that <a class="reference internal" href="../library/constants.html#debug__" title="__debug__"><code class="xref py py-const docutils literal notranslate">__debug__</code></a> and <a class="reference internal" href="../library/exceptions.html#AssertionError" title="AssertionError"><code class="xref py py-exc docutils literal notranslate">AssertionError</code></a> refer to the built-in variables with those names. In the current implementation, the built-in variable <code class="docutils literal notranslate">__debug__</code> is <code class="docutils literal notranslate">True</code> under normal circumstances, <code class="docutils literal notranslate">False</code> when optimization is requested (command line option <a class="reference internal" href="../using/cmdline.html#cmdoption-O"><code class="xref std std-option docutils literal notranslate">-O</code></a>). The current code generator emits no code for an <a class="reference internal" href="#assert"><code class="xref std std-keyword docutils literal notranslate">assert</code></a> statement when optimization is requested at compile time. Note that it is unnecessary to include the source code for the expression that failed in the error message; it will be displayed as part of the stack trace. Assignments to <a class="reference internal" href="../library/constants.html#debug__" title="__debug__"><code class="xref py py-const docutils literal notranslate">__debug__</code></a> are illegal. The value for the built-in variable is determined when the interpreter starts. </section> <section id="the-pass-statement"> <h2>7.4. The <code class="xref std std-keyword docutils literal notranslate">pass</code> statement<a class="headerlink" href="#the-pass-statement" title="Link to this heading">¶</a></h2> <pre id="index-20"> pass_stmt ::= "pass" </pre> <a class="reference internal" href="#pass"><code class="xref std std-keyword docutils literal notranslate">pass</code></a> is a null operation — when it is executed, nothing happens. It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed, for example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>def f(arg): pass # a function that does nothing (yet) class C: pass # a class with no methods (yet) </pre></div> </div> </section> <section id="the-del-statement"> <h2>7.5. The <code class="xref std std-keyword docutils literal notranslate">del</code> statement<a class="headerlink" href="#the-del-statement" title="Link to this heading">¶</a></h2> <pre id="index-21"> del_stmt ::= "del" <a class="reference internal" href="#grammar-token-python-grammar-target_list"><code class="xref docutils literal notranslate">target_list</code></a> </pre> Deletion is recursively defined very similar to the way assignment is defined. Rather than spelling it out in full details, here are some hints. Deletion of a target list recursively deletes each target, from left to right. Deletion of a name removes the binding of that name from the local or global namespace, depending on whether the name occurs in a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate">global</code></a> statement in the same code block. If the name is unbound, a <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate">NameError</code></a> exception will be raised. Deletion of attribute references, subscriptions and slicings is passed to the primary object involved; deletion of a slicing is in general equivalent to assignment of an empty slice of the right type (but even this is determined by the sliced object). <div class="versionchanged"> Changed in version 3.2: Previously it was illegal to delete a name from the local namespace if it occurs as a free variable in a nested block. </div> </section> <section id="the-return-statement"> <h2>7.6. The <code class="xref std std-keyword docutils literal notranslate">return</code> statement<a class="headerlink" href="#the-return-statement" title="Link to this heading">¶</a></h2> <pre id="index-24"> return_stmt ::= "return" [<a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression_list"><code class="xref docutils literal notranslate">expression_list</code></a>] </pre> <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> may only occur syntactically nested in a function definition, not within a nested class definition. If an expression list is present, it is evaluated, else <code class="docutils literal notranslate">None</code> is substituted. <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> leaves the current function call with the expression list (or <code class="docutils literal notranslate">None</code>) as return value. When <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> passes control out of a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement with a <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause, that <code class="xref std std-keyword docutils literal notranslate">finally</code> clause is executed before really leaving the function. In a generator function, the <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> statement indicates that the generator is done and will cause <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a> to be raised. The returned value (if any) is used as an argument to construct <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a> and becomes the <a class="reference internal" href="../library/exceptions.html#StopIteration.value" title="StopIteration.value"><code class="xref py py-attr docutils literal notranslate">StopIteration.value</code></a> attribute. In an asynchronous generator function, an empty <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> statement indicates that the asynchronous generator is done and will cause <a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate">StopAsyncIteration</code></a> to be raised. A non-empty <code class="xref std std-keyword docutils literal notranslate">return</code> statement is a syntax error in an asynchronous generator function. </section> <section id="the-yield-statement"> <h2>7.7. The <code class="xref std std-keyword docutils literal notranslate">yield</code> statement<a class="headerlink" href="#the-yield-statement" title="Link to this heading">¶</a></h2> <pre id="index-26"> yield_stmt ::= <a class="reference internal" href="expressions.html#grammar-token-python-grammar-yield_expression"><code class="xref docutils literal notranslate">yield_expression</code></a> </pre> A <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate">yield</code></a> statement is semantically equivalent to a <a class="reference internal" href="expressions.html#yieldexpr">yield expression</a>. The <code class="docutils literal notranslate">yield</code> statement can be used to omit the parentheses that would otherwise be required in the equivalent yield expression statement. For example, the yield statements <div class="highlight-python3 notranslate"><div class="highlight"><pre>yield <expr> yield from <expr> </pre></div> </div> are equivalent to the yield expression statements <div class="highlight-python3 notranslate"><div class="highlight"><pre>(yield <expr>) (yield from <expr>) </pre></div> </div> Yield expressions and statements are only used when defining a <a class="reference internal" href="../glossary.html#term-generator">generator</a> function, and are only used in the body of the generator function. Using <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate">yield</code></a> in a function definition is sufficient to cause that definition to create a generator function instead of a normal function. For full details of <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate">yield</code></a> semantics, refer to the <a class="reference internal" href="expressions.html#yieldexpr">Yield expressions</a> section. </section> <section id="the-raise-statement"> <h2>7.8. The <code class="xref std std-keyword docutils literal notranslate">raise</code> statement<a class="headerlink" href="#the-raise-statement" title="Link to this heading">¶</a></h2> <pre id="index-27"> raise_stmt ::= "raise" [<a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a> ["from" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>]] </pre> If no expressions are present, <a class="reference internal" href="#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> re-raises the exception that is currently being handled, which is also known as the active exception. If there isn’t currently an active exception, a <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate">RuntimeError</code></a> exception is raised indicating that this is an error. Otherwise, <a class="reference internal" href="#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> evaluates the first expression as the exception object. It must be either a subclass or an instance of <a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-class docutils literal notranslate">BaseException</code></a>. If it is a class, the exception instance will be obtained when needed by instantiating the class with no arguments. The type of the exception is the exception instance’s class, the value is the instance itself. A traceback object is normally created automatically when an exception is raised and attached to it as the <a class="reference internal" href="../library/exceptions.html#BaseException.__traceback__" title="BaseException.__traceback__"><code class="xref py py-attr docutils literal notranslate">__traceback__</code></a> attribute. You can create an exception and set your own traceback in one step using the <a class="reference internal" href="../library/exceptions.html#BaseException.with_traceback" title="BaseException.with_traceback"><code class="xref py py-meth docutils literal notranslate">with_traceback()</code></a> exception method (which returns the same exception instance, with its traceback set to its argument), like so: <div class="highlight-python3 notranslate"><div class="highlight"><pre>raise Exception("foo occurred").with_traceback(tracebackobj) </pre></div> </div> The <code class="docutils literal notranslate">from</code> clause is used for exception chaining: if given, the second expression must be another exception class or instance. If the second expression is an exception instance, it will be attached to the raised exception as the <a class="reference internal" href="../library/exceptions.html#BaseException.__cause__" title="BaseException.__cause__"><code class="xref py py-attr docutils literal notranslate">__cause__</code></a> attribute (which is writable). If the expression is an exception class, the class will be instantiated and the resulting exception instance will be attached to the raised exception as the <code class="xref py py-attr docutils literal notranslate">__cause__</code> attribute. If the raised exception is not handled, both exceptions will be printed: <div class="highlight-pycon notranslate"><div class="highlight"><pre>>>> try: ... print(1 / 0) ... except Exception as exc: ... raise RuntimeError("Something bad happened") from exc ... Traceback (most recent call last): File "<stdin>", line 2, in <module> print(1 / 0) ~~^~~ ZeroDivisionError: division by zero The above exception was the direct cause of the following exception: Traceback (most recent call last): File "<stdin>", line 4, in <module> raise RuntimeError("Something bad happened") from exc RuntimeError: Something bad happened </pre></div> </div> A similar mechanism works implicitly if a new exception is raised when an exception is already being handled. An exception may be handled when an <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> or <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause, or a <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement, is used. The previous exception is then attached as the new exception’s <a class="reference internal" href="../library/exceptions.html#BaseException.__context__" title="BaseException.__context__"><code class="xref py py-attr docutils literal notranslate">__context__</code></a> attribute: <div class="highlight-pycon notranslate"><div class="highlight"><pre>>>> try: ... print(1 / 0) ... except: ... raise RuntimeError("Something bad happened") ... Traceback (most recent call last): File "<stdin>", line 2, in <module> print(1 / 0) ~~^~~ ZeroDivisionError: division by zero During handling of the above exception, another exception occurred: Traceback (most recent call last): File "<stdin>", line 4, in <module> raise RuntimeError("Something bad happened") RuntimeError: Something bad happened </pre></div> </div> Exception chaining can be explicitly suppressed by specifying <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> in the <code class="docutils literal notranslate">from</code> clause: <div class="highlight-pycon notranslate"><div class="highlight"><pre>>>> try: ... print(1 / 0) ... except: ... raise RuntimeError("Something bad happened") from None ... Traceback (most recent call last): File "<stdin>", line 4, in <module> RuntimeError: Something bad happened </pre></div> </div> Additional information on exceptions can be found in section <a class="reference internal" href="executionmodel.html#exceptions">Exceptions</a>, and information about handling exceptions is in section <a class="reference internal" href="compound_stmts.html#try">The try statement</a>. <div class="versionchanged"> Changed in version 3.3: <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> is now permitted as <code class="docutils literal notranslate">Y</code> in <code class="docutils literal notranslate">raise X from Y</code>. Added the <a class="reference internal" href="../library/exceptions.html#BaseException.__suppress_context__" title="BaseException.__suppress_context__"><code class="xref py py-attr docutils literal notranslate">__suppress_context__</code></a> attribute to suppress automatic display of the exception context. </div> <div class="versionchanged"> Changed in version 3.11: If the traceback of the active exception is modified in an <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> clause, a subsequent <code class="docutils literal notranslate">raise</code> statement re-raises the exception with the modified traceback. Previously, the exception was re-raised with the traceback it had when it was caught. </div> </section> <section id="the-break-statement"> <h2>7.9. The <code class="xref std std-keyword docutils literal notranslate">break</code> statement<a class="headerlink" href="#the-break-statement" title="Link to this heading">¶</a></h2> <pre id="index-30"> break_stmt ::= "break" </pre> <a class="reference internal" href="#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> may only occur syntactically nested in a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> or <a class="reference internal" href="compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate">while</code></a> loop, but not nested in a function or class definition within that loop. It terminates the nearest enclosing loop, skipping the optional <code class="xref std std-keyword docutils literal notranslate">else</code> clause if the loop has one. If a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> loop is terminated by <a class="reference internal" href="#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a>, the loop control target keeps its current value. When <a class="reference internal" href="#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> passes control out of a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement with a <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause, that <code class="xref std std-keyword docutils literal notranslate">finally</code> clause is executed before really leaving the loop. </section> <section id="the-continue-statement"> <h2>7.10. The <code class="xref std std-keyword docutils literal notranslate">continue</code> statement<a class="headerlink" href="#the-continue-statement" title="Link to this heading">¶</a></h2> <pre id="index-33"> continue_stmt ::= "continue" </pre> <a class="reference internal" href="#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> may only occur syntactically nested in a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> or <a class="reference internal" href="compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate">while</code></a> loop, but not nested in a function or class definition within that loop. It continues with the next cycle of the nearest enclosing loop. When <a class="reference internal" href="#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> passes control out of a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement with a <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause, that <code class="xref std std-keyword docutils literal notranslate">finally</code> clause is executed before really starting the next loop cycle. </section> <section id="the-import-statement"> <h2>7.11. The <code class="xref std std-keyword docutils literal notranslate">import</code> statement<a class="headerlink" href="#the-import-statement" title="Link to this heading">¶</a></h2> <pre id="index-34"> import_stmt ::= "import" <a class="reference internal" href="#grammar-token-python-grammar-module"><code class="xref docutils literal notranslate">module</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>] ("," <a class="reference internal" href="#grammar-token-python-grammar-module"><code class="xref docutils literal notranslate">module</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>])* | "from" <a class="reference internal" href="#grammar-token-python-grammar-relative_module"><code class="xref docutils literal notranslate">relative_module</code></a> "import" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>] ("," <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>])* | "from" <a class="reference internal" href="#grammar-token-python-grammar-relative_module"><code class="xref docutils literal notranslate">relative_module</code></a> "import" "(" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>] ("," <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>])* [","] ")" | "from" <a class="reference internal" href="#grammar-token-python-grammar-relative_module"><code class="xref docutils literal notranslate">relative_module</code></a> "import" "*" module ::= (<a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> ".")* <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> relative_module ::= "."* <a class="reference internal" href="#grammar-token-python-grammar-module"><code class="xref docutils literal notranslate">module</code></a> | "."+ </pre> The basic import statement (no <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate">from</code></a> clause) is executed in two steps: <ol class="arabic simple"> <li>find a module, loading and initializing it if necessary</li> <li>define a name or names in the local namespace for the scope where the <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate">import</code></a> statement occurs.</li> </ol> When the statement contains multiple clauses (separated by commas) the two steps are carried out separately for each clause, just as though the clauses had been separated out into individual import statements. The details of the first step, finding and loading modules, are described in greater detail in the section on the <a class="reference internal" href="import.html#importsystem">import system</a>, which also describes the various types of packages and modules that can be imported, as well as all the hooks that can be used to customize the import system. Note that failures in this step may indicate either that the module could not be located, or that an error occurred while initializing the module, which includes execution of the module’s code. If the requested module is retrieved successfully, it will be made available in the local namespace in one of three ways: <ul class="simple" id="index-35"> <li>If the module name is followed by <code class="xref std std-keyword docutils literal notranslate">as</code>, then the name following <code class="xref std std-keyword docutils literal notranslate">as</code> is bound directly to the imported module.</li> <li>If no other name is specified, and the module being imported is a top level module, the module’s name is bound in the local namespace as a reference to the imported module</li> <li>If the module being imported is not a top level module, then the name of the top level package that contains the module is bound in the local namespace as a reference to the top level package. The imported module must be accessed using its full qualified name rather than directly</li> </ul> The <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate">from</code></a> form uses a slightly more complex process: <ol class="arabic simple"> <li>find the module specified in the <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate">from</code></a> clause, loading and initializing it if necessary;</li> <li>for each of the identifiers specified in the <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate">import</code></a> clauses: <ol class="arabic simple"> <li>check if the imported module has an attribute by that name</li> <li>if not, attempt to import a submodule with that name and then check the imported module again for that attribute</li> <li>if the attribute is not found, <a class="reference internal" href="../library/exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal notranslate">ImportError</code></a> is raised.</li> <li>otherwise, a reference to that value is stored in the local namespace, using the name in the <code class="xref std std-keyword docutils literal notranslate">as</code> clause if it is present, otherwise using the attribute name</li> </ol> </li> </ol> Examples: <div class="highlight-python3 notranslate"><div class="highlight"><pre>import foo # foo imported and bound locally import foo.bar.baz # foo, foo.bar, and foo.bar.baz imported, foo bound locally import foo.bar.baz as fbb # foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as fbb from foo.bar import baz # foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as baz from foo import attr # foo imported and foo.attr bound as attr </pre></div> </div> If the list of identifiers is replaced by a star (<code class="docutils literal notranslate">'*'</code>), all public names defined in the module are bound in the local namespace for the scope where the <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate">import</code></a> statement occurs. The public names defined by a module are determined by checking the module’s namespace for a variable named <code class="docutils literal notranslate">__all__</code>; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in <code class="docutils literal notranslate">__all__</code> are all considered public and are required to exist. If <code class="docutils literal notranslate">__all__</code> is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character (<code class="docutils literal notranslate">'_'</code>). <code class="docutils literal notranslate">__all__</code> should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module). The wild card form of import — <code class="docutils literal notranslate">from module import *</code> — is only allowed at the module level. Attempting to use it in class or function definitions will raise a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a>. When specifying what module to import you do not have to specify the absolute name of the module. When a module or package is contained within another package it is possible to make a relative import within the same top package without having to mention the package name. By using leading dots in the specified module or package after <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate">from</code></a> you can specify how high to traverse up the current package hierarchy without specifying exact names. One leading dot means the current package where the module making the import exists. Two dots means up one package level. Three dots is up two levels, etc. So if you execute <code class="docutils literal notranslate">from . import mod</code> from a module in the <code class="docutils literal notranslate">pkg</code> package then you will end up importing <code class="docutils literal notranslate">pkg.mod</code>. If you execute <code class="docutils literal notranslate">from ..subpkg2 import mod</code> from within <code class="docutils literal notranslate">pkg.subpkg1</code> you will import <code class="docutils literal notranslate">pkg.subpkg2.mod</code>. The specification for relative imports is contained in the <a class="reference internal" href="import.html#relativeimports">Package Relative Imports</a> section. <a class="reference internal" href="../library/importlib.html#importlib.import_module" title="importlib.import_module"><code class="xref py py-func docutils literal notranslate">importlib.import_module()</code></a> is provided to support applications that determine dynamically the modules to be loaded. Raises an <a class="reference internal" href="../library/sys.html#auditing">auditing event</a> <code class="docutils literal notranslate">import</code> with arguments <code class="docutils literal notranslate">module</code>, <code class="docutils literal notranslate">filename</code>, <code class="docutils literal notranslate">sys.path</code>, <code class="docutils literal notranslate">sys.meta_path</code>, <code class="docutils literal notranslate">sys.path_hooks</code>. <section id="future-statements"> <h3>7.11.1. Future statements<a class="headerlink" href="#future-statements" title="Link to this heading">¶</a></h3> A future statement is a directive to the compiler that a particular module should be compiled using syntax or semantics that will be available in a specified future release of Python where the feature becomes standard. The future statement is intended to ease migration to future versions of Python that introduce incompatible changes to the language. It allows use of the new features on a per-module basis before the release in which the feature becomes standard. <pre> future_stmt ::= "from" "__future__" "import" <a class="reference internal" href="#grammar-token-python-grammar-feature"><code class="xref docutils literal notranslate">feature</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>] ("," <a class="reference internal" href="#grammar-token-python-grammar-feature"><code class="xref docutils literal notranslate">feature</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>])* | "from" "__future__" "import" "(" <a class="reference internal" href="#grammar-token-python-grammar-feature"><code class="xref docutils literal notranslate">feature</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>] ("," <a class="reference internal" href="#grammar-token-python-grammar-feature"><code class="xref docutils literal notranslate">feature</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>])* [","] ")" feature ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> </pre> A future statement must appear near the top of the module. The only lines that can appear before a future statement are: <ul class="simple"> <li>the module docstring (if any),</li> <li>comments,</li> <li>blank lines, and</li> <li>other future statements.</li> </ul> The only feature that requires using the future statement is <code class="docutils literal notranslate">annotations</code> (see <a class="pep reference external" href="https://peps.python.org/pep-0563/">PEP 563</a>). All historical features enabled by the future statement are still recognized by Python 3. The list includes <code class="docutils literal notranslate">absolute_import</code>, <code class="docutils literal notranslate">division</code>, <code class="docutils literal notranslate">generators</code>, <code class="docutils literal notranslate">generator_stop</code>, <code class="docutils literal notranslate">unicode_literals</code>, <code class="docutils literal notranslate">print_function</code>, <code class="docutils literal notranslate">nested_scopes</code> and <code class="docutils literal notranslate">with_statement</code>. They are all redundant because they are always enabled, and only kept for backwards compatibility. A future statement is recognized and treated specially at compile time: Changes to the semantics of core constructs are often implemented by generating different code. It may even be the case that a new feature introduces new incompatible syntax (such as a new reserved word), in which case the compiler may need to parse the module differently. Such decisions cannot be pushed off until runtime. For any given release, the compiler knows which feature names have been defined, and raises a compile-time error if a future statement contains a feature not known to it. The direct runtime semantics are the same as for any import statement: there is a standard module <a class="reference internal" href="../library/__future__.html#module-__future__" title="__future__: Future statement definitions"><code class="xref py py-mod docutils literal notranslate">__future__</code></a>, described later, and it will be imported in the usual way at the time the future statement is executed. The interesting runtime semantics depend on the specific feature enabled by the future statement. Note that there is nothing special about the statement: <div class="highlight-python3 notranslate"><div class="highlight"><pre>import __future__ [as name] </pre></div> </div> That is not a future statement; it’s an ordinary import statement with no special semantics or syntax restrictions. Code compiled by calls to the built-in functions <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate">exec()</code></a> and <a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate">compile()</code></a> that occur in a module <code class="xref py py-mod docutils literal notranslate">M</code> containing a future statement will, by default, use the new syntax or semantics associated with the future statement. This can be controlled by optional arguments to <a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate">compile()</code></a> — see the documentation of that function for details. A future statement typed at an interactive interpreter prompt will take effect for the rest of the interpreter session. If an interpreter is started with the <a class="reference internal" href="../using/cmdline.html#cmdoption-i"><code class="xref std std-option docutils literal notranslate">-i</code></a> option, is passed a script name to execute, and the script includes a future statement, it will be in effect in the interactive session started after the script is executed. <div class="admonition seealso"> See also <dl class="simple"> <dt><a class="pep reference external" href="https://peps.python.org/pep-0236/">PEP 236</a> - Back to the __future__</dt><dd>The original proposal for the __future__ mechanism. </dd> </dl> </div> </section> </section> <section id="the-global-statement"> <h2>7.12. The <code class="xref std std-keyword docutils literal notranslate">global</code> statement<a class="headerlink" href="#the-global-statement" title="Link to this heading">¶</a></h2> <pre id="index-43"> global_stmt ::= "global" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> ("," <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>)* </pre> The <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate">global</code></a> statement causes the listed identifiers to be interpreted as globals. It would be impossible to assign to a global variable without <code class="xref std std-keyword docutils literal notranslate">global</code>, although free variables may refer to globals without being declared global. The <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate">global</code></a> statement applies to the entire scope of a function or class body. A <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a> is raised if a variable is used or assigned to prior to its global declaration in the scope. Programmer’s note: <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate">global</code></a> is a directive to the parser. It applies only to code parsed at the same time as the <code class="xref std std-keyword docutils literal notranslate">global</code> statement. In particular, a <code class="xref std std-keyword docutils literal notranslate">global</code> statement contained in a string or code object supplied to the built-in <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate">exec()</code></a> function does not affect the code block containing the function call, and code contained in such a string is unaffected by <code class="xref std std-keyword docutils literal notranslate">global</code> statements in the code containing the function call. The same applies to the <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate">eval()</code></a> and <a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate">compile()</code></a> functions. </section> <section id="the-nonlocal-statement"> <h2>7.13. The <code class="xref std std-keyword docutils literal notranslate">nonlocal</code> statement<a class="headerlink" href="#the-nonlocal-statement" title="Link to this heading">¶</a></h2> <pre id="index-45"> nonlocal_stmt ::= "nonlocal" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> ("," <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>)* </pre> When the definition of a function or class is nested (enclosed) within the definitions of other functions, its nonlocal scopes are the local scopes of the enclosing functions. The <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate">nonlocal</code></a> statement causes the listed identifiers to refer to names previously bound in nonlocal scopes. It allows encapsulated code to rebind such nonlocal identifiers. If a name is bound in more than one nonlocal scope, the nearest binding is used. If a name is not bound in any nonlocal scope, or if there is no nonlocal scope, a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a> is raised. The <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate">nonlocal</code></a> statement applies to the entire scope of a function or class body. A <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a> is raised if a variable is used or assigned to prior to its nonlocal declaration in the scope. <div class="admonition seealso"> See also <dl class="simple"> <dt><a class="pep reference external" href="https://peps.python.org/pep-3104/">PEP 3104</a> - Access to Names in Outer Scopes</dt><dd>The specification for the <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate">nonlocal</code></a> statement. </dd> </dl> </div> Programmer’s note: <a class="reference internal" href="#nonlocal"><code class="xref std std-keyword docutils literal notranslate">nonlocal</code></a> is a directive to the parser and applies only to code parsed along with it. See the note for the <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate">global</code></a> statement. </section> <section id="the-type-statement"> <h2>7.14. The <code class="xref std std-keyword docutils literal notranslate">type</code> statement<a class="headerlink" href="#the-type-statement" title="Link to this heading">¶</a></h2> <pre id="index-47"> type_stmt ::= 'type' <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> [<a class="reference internal" href="compound_stmts.html#grammar-token-python-grammar-type_params"><code class="xref docutils literal notranslate">type_params</code></a>] "=" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a> </pre> The <code class="xref std std-keyword docutils literal notranslate">type</code> statement declares a type alias, which is an instance of <a class="reference internal" href="../library/typing.html#typing.TypeAliasType" title="typing.TypeAliasType"><code class="xref py py-class docutils literal notranslate">typing.TypeAliasType</code></a>. For example, the following statement creates a type alias: <div class="highlight-python3 notranslate"><div class="highlight"><pre>type Point = tuple[float, float] </pre></div> </div> This code is roughly equivalent to: <div class="highlight-python3 notranslate"><div class="highlight"><pre>annotation-def VALUE_OF_Point(): return tuple[float, float] Point = typing.TypeAliasType("Point", VALUE_OF_Point()) </pre></div> </div> <code class="docutils literal notranslate">annotation-def</code> indicates an <a class="reference internal" href="executionmodel.html#annotation-scopes">annotation scope</a>, which behaves mostly like a function, but with several small differences. The value of the type alias is evaluated in the annotation scope. It is not evaluated when the type alias is created, but only when the value is accessed through the type alias’s <code class="xref py py-attr docutils literal notranslate">__value__</code> attribute (see <a class="reference internal" href="executionmodel.html#lazy-evaluation">Lazy evaluation</a>). This allows the type alias to refer to names that are not yet defined. Type aliases may be made generic by adding a <a class="reference internal" href="compound_stmts.html#type-params">type parameter list</a> after the name. See <a class="reference internal" href="compound_stmts.html#generic-type-aliases">Generic type aliases</a> for more. <code class="xref std std-keyword docutils literal notranslate">type</code> is a <a class="reference internal" href="lexical_analysis.html#soft-keywords">soft keyword</a>. <div class="versionadded"> Added in version 3.12. </div> <div class="admonition seealso"> See also <dl class="simple"> <dt><a class="pep reference external" href="https://peps.python.org/pep-0695/">PEP 695</a> - Type Parameter Syntax</dt><dd>Introduced the <code class="xref std std-keyword docutils literal notranslate">type</code> statement and syntax for generic classes and functions. </dd> </dl> </div> </section> </section> <div class="clearer"></div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="Main"> <div class="sphinxsidebarwrapper"> <div> <h3><a href="../contents.html">Table of Contents</a></h3> <ul> <li><a class="reference internal" href="#">7. Simple statements</a><ul> <li><a class="reference internal" href="#expression-statements">7.1. Expression statements</a></li> <li><a class="reference internal" href="#assignment-statements">7.2. Assignment statements</a><ul> <li><a class="reference internal" href="#augmented-assignment-statements">7.2.1. Augmented assignment statements</a></li> <li><a class="reference internal" href="#annotated-assignment-statements">7.2.2. Annotated assignment statements</a></li> </ul> </li> <li><a class="reference internal" href="#the-assert-statement">7.3. The <code class="xref std std-keyword docutils literal notranslate">assert</code> statement</a></li> <li><a class="reference internal" href="#the-pass-statement">7.4. The <code class="xref std std-keyword docutils literal notranslate">pass</code> statement</a></li> <li><a class="reference internal" href="#the-del-statement">7.5. The <code class="xref std std-keyword docutils literal notranslate">del</code> statement</a></li> <li><a class="reference internal" href="#the-return-statement">7.6. The <code class="xref std std-keyword docutils literal notranslate">return</code> statement</a></li> <li><a class="reference internal" href="#the-yield-statement">7.7. The <code class="xref std std-keyword docutils literal notranslate">yield</code> statement</a></li> <li><a class="reference internal" href="#the-raise-statement">7.8. The <code class="xref std std-keyword docutils literal notranslate">raise</code> statement</a></li> <li><a class="reference internal" href="#the-break-statement">7.9. The <code class="xref std std-keyword docutils literal notranslate">break</code> statement</a></li> <li><a class="reference internal" href="#the-continue-statement">7.10. The <code class="xref std std-keyword docutils literal notranslate">continue</code> statement</a></li> <li><a class="reference internal" href="#the-import-statement">7.11. The <code class="xref std std-keyword docutils literal notranslate">import</code> statement</a><ul> <li><a class="reference internal" href="#future-statements">7.11.1. Future statements</a></li> </ul> </li> <li><a class="reference internal" href="#the-global-statement">7.12. The <code class="xref std std-keyword docutils literal notranslate">global</code> statement</a></li> <li><a class="reference internal" href="#the-nonlocal-statement">7.13. The <code class="xref std std-keyword docutils literal notranslate">nonlocal</code> statement</a></li> <li><a class="reference internal" href="#the-type-statement">7.14. The <code class="xref std std-keyword docutils literal notranslate">type</code> statement</a></li> </ul> </li> </ul> </div> <div> <h4>Previous topic</h4> <a href="expressions.html" title="previous chapter">6. 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