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Errors and Exceptions</a><ul> <li><a class="reference internal" href="#syntax-errors">8.1. Syntax Errors</a></li> <li><a class="reference internal" href="#exceptions">8.2. Exceptions</a></li> <li><a class="reference internal" href="#handling-exceptions">8.3. Handling Exceptions</a></li> <li><a class="reference internal" href="#raising-exceptions">8.4. Raising Exceptions</a></li> <li><a class="reference internal" href="#exception-chaining">8.5. Exception Chaining</a></li> <li><a class="reference internal" href="#user-defined-exceptions">8.6. User-defined Exceptions</a></li> <li><a class="reference internal" href="#defining-clean-up-actions">8.7. Defining Clean-up Actions</a></li> <li><a class="reference internal" href="#predefined-clean-up-actions">8.8. Predefined Clean-up Actions</a></li> <li><a class="reference internal" href="#raising-and-handling-multiple-unrelated-exceptions">8.9. Raising and Handling Multiple Unrelated Exceptions</a></li> <li><a class="reference internal" href="#enriching-exceptions-with-notes">8.10. Enriching Exceptions with Notes</a></li> </ul> </li> </ul> </div> <div> <h4>Previous topic</h4> <a href="inputoutput.html" title="previous chapter">7. Input and Output</a> </div> <div> <h4>Next topic</h4> <a href="classes.html" title="next chapter">9. Classes</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/tutorial/errors.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="classes.html" title="9. Classes" accesskey="N">next</a> |</li> <li class="right" > <a href="inputoutput.html" title="7. Input and Output" accesskey="P">previous</a> |</li> <li><img src="../_static/py.svg" alt="Python logo" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li class="switchers"> <div class="language_switcher_placeholder"></div> <div class="version_switcher_placeholder"></div> </li> <li> </li> <li id="cpython-language-and-version"> <a href="../index.html">3.13.0 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">The Python Tutorial</a> »</li> <li class="nav-item nav-item-this"><a href="">8. Errors and Exceptions</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="errors-and-exceptions"> <h1>8. Errors and Exceptions<a class="headerlink" href="#errors-and-exceptions" title="Link to this heading">¶</a></h1> Until now error messages haven’t been more than mentioned, but if you have tried out the examples you have probably seen some. There are (at least) two distinguishable kinds of errors: syntax errors and exceptions. <section id="syntax-errors"> <h2>8.1. Syntax Errors<a class="headerlink" href="#syntax-errors" title="Link to this heading">¶</a></h2> Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> while True print('Hello world') File "<stdin>", line 1 while True print('Hello world') ^^^^^ SyntaxError: invalid syntax </pre></div> </div> The parser repeats the offending line and displays little ‘arrow’s pointing at the token in the line where the error was detected. The error may be caused by the absence of a token before the indicated token. In the example, the error is detected at the function <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate">print()</code></a>, since a colon (<code class="docutils literal notranslate">':'</code>) is missing before it. File name and line number are printed so you know where to look in case the input came from a script. </section> <section id="exceptions"> <h2>8.2. Exceptions<a class="headerlink" href="#exceptions" title="Link to this heading">¶</a></h2> Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not unconditionally fatal: you will soon learn how to handle them in Python programs. Most exceptions are not handled by programs, however, and result in error messages as shown here: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> 10 * (1/0) Traceback (most recent call last): File "<stdin>", line 1, in <module> 10 * (1/0) ~^~ ZeroDivisionError: division by zero >>> 4 + spam*3 Traceback (most recent call last): File "<stdin>", line 1, in <module> 4 + spam*3 ^^^^ NameError: name 'spam' is not defined >>> '2' + 2 Traceback (most recent call last): File "<stdin>", line 1, in <module> '2' + 2 ~~~~^~~ TypeError: can only concatenate str (not "int") to str </pre></div> </div> The last line of the error message indicates what happened. Exceptions come in different types, and the type is printed as part of the message: the types in the example are <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate">ZeroDivisionError</code></a>, <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate">NameError</code></a> and <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>. The string printed as the exception type is the name of the built-in exception that occurred. This is true for all built-in exceptions, but need not be true for user-defined exceptions (although it is a useful convention). Standard exception names are built-in identifiers (not reserved keywords). The rest of the line provides detail based on the type of exception and what caused it. The preceding part of the error message shows the context where the exception occurred, in the form of a stack traceback. In general it contains a stack traceback listing source lines; however, it will not display lines read from standard input. <a class="reference internal" href="../library/exceptions.html#bltin-exceptions">Built-in Exceptions</a> lists the built-in exceptions and their meanings. </section> <section id="handling-exceptions"> <h2>8.3. Handling Exceptions<a class="headerlink" href="#handling-exceptions" title="Link to this heading">¶</a></h2> It is possible to write programs that handle selected exceptions. Look at the following example, which asks the user for input until a valid integer has been entered, but allows the user to interrupt the program (using <kbd class="kbd compound docutils literal notranslate"><kbd class="kbd docutils literal notranslate">Control</kbd>-<kbd class="kbd docutils literal notranslate">C</kbd></kbd> or whatever the operating system supports); note that a user-generated interruption is signalled by raising the <a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate">KeyboardInterrupt</code></a> exception. <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> while True: ... try: ... x = int(input("Please enter a number: ")) ... break ... except ValueError: ... print("Oops! That was no valid number. Try again...") ... </pre></div> </div> The <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement works as follows. <ul class="simple"> <li>First, the try clause (the statement(s) between the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> and <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> keywords) is executed.</li> <li>If no exception occurs, the except clause is skipped and execution of the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement is finished.</li> <li>If an exception occurs during execution of the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> clause, the rest of the clause is skipped. Then, if its type matches the exception named after the <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> keyword, the except clause is executed, and then execution continues after the try/except block.</li> <li>If an exception occurs which does not match the exception named in the except clause, it is passed on to outer <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statements; if no handler is found, it is an unhandled exception and execution stops with an error message.</li> </ul> A <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement may have more than one except clause, to specify handlers for different exceptions. At most one handler will be executed. Handlers only handle exceptions that occur in the corresponding try clause, not in other handlers of the same <code class="xref std std-keyword docutils literal notranslate">try</code> statement. An except clause may name multiple exceptions as a parenthesized tuple, for example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>... except (RuntimeError, TypeError, NameError): ... pass </pre></div> </div> A class in an <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> clause matches exceptions which are instances of the class itself or one of its derived classes (but not the other way around — an except clause listing a derived class does not match instances of its base classes). For example, the following code will print B, C, D in that order: <div class="highlight-python3 notranslate"><div class="highlight"><pre>class B(Exception): pass class C(B): pass class D(C): pass for cls in [B, C, D]: try: raise cls() except D: print("D") except C: print("C") except B: print("B") </pre></div> </div> Note that if the except clauses were reversed (with <code class="docutils literal notranslate">except B</code> first), it would have printed B, B, B — the first matching except clause is triggered. When an exception occurs, it may have associated values, also known as the exception’s arguments. The presence and types of the arguments depend on the exception type. The except clause may specify a variable after the exception name. The variable is bound to the exception instance which typically has an <code class="docutils literal notranslate">args</code> attribute that stores the arguments. For convenience, builtin exception types define <a class="reference internal" href="../reference/datamodel.html#object.__str__" title="object.__str__"><code class="xref py py-meth docutils literal notranslate">__str__()</code></a> to print all the arguments without explicitly accessing <code class="docutils literal notranslate">.args</code>. <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try: ... raise Exception('spam', 'eggs') ... except Exception as inst: ... print(type(inst)) # the exception type ... print(inst.args) # arguments stored in .args ... print(inst) # __str__ allows args to be printed directly, ... # but may be overridden in exception subclasses ... x, y = inst.args # unpack args ... print('x =', x) ... print('y =', y) ... <class 'Exception'> ('spam', 'eggs') ('spam', 'eggs') x = spam y = eggs </pre></div> </div> The exception’s <a class="reference internal" href="../reference/datamodel.html#object.__str__" title="object.__str__"><code class="xref py py-meth docutils literal notranslate">__str__()</code></a> output is printed as the last part (‘detail’) of the message for unhandled exceptions. <a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-exc docutils literal notranslate">BaseException</code></a> is the common base class of all exceptions. One of its subclasses, <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate">Exception</code></a>, is the base class of all the non-fatal exceptions. Exceptions which are not subclasses of <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate">Exception</code></a> are not typically handled, because they are used to indicate that the program should terminate. They include <a class="reference internal" href="../library/exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate">SystemExit</code></a> which is raised by <a class="reference internal" href="../library/sys.html#sys.exit" title="sys.exit"><code class="xref py py-meth docutils literal notranslate">sys.exit()</code></a> and <a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate">KeyboardInterrupt</code></a> which is raised when a user wishes to interrupt the program. <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate">Exception</code></a> can be used as a wildcard that catches (almost) everything. However, it is good practice to be as specific as possible with the types of exceptions that we intend to handle, and to allow any unexpected exceptions to propagate on. The most common pattern for handling <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate">Exception</code></a> is to print or log the exception and then re-raise it (allowing a caller to handle the exception as well): <div class="highlight-python3 notranslate"><div class="highlight"><pre>import sys try: f = open('myfile.txt') s = f.readline() i = int(s.strip()) except OSError as err: print("OS error:", err) except ValueError: print("Could not convert data to an integer.") except Exception as err: print(f"Unexpected {err=}, {type(err)=}") raise </pre></div> </div> The <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> … <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> statement has an optional else clause, which, when present, must follow all except clauses. It is useful for code that must be executed if the try clause does not raise an exception. For example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>for arg in sys.argv[1:]: try: f = open(arg, 'r') except OSError: print('cannot open', arg) else: print(arg, 'has', len(f.readlines()), 'lines') f.close() </pre></div> </div> The use of the <code class="xref std std-keyword docutils literal notranslate">else</code> clause is better than adding additional code to the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the <code class="xref std std-keyword docutils literal notranslate">try</code> … <code class="xref std std-keyword docutils literal notranslate">except</code> statement. Exception handlers do not handle only exceptions that occur immediately in the try clause, but also those that occur inside functions that are called (even indirectly) in the try clause. For example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> def this_fails(): ... x = 1/0 ... >>> try: ... this_fails() ... except ZeroDivisionError as err: ... print('Handling run-time error:', err) ... Handling run-time error: division by zero </pre></div> </div> </section> <section id="raising-exceptions"> <h2>8.4. Raising Exceptions<a class="headerlink" href="#raising-exceptions" title="Link to this heading">¶</a></h2> The <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> statement allows the programmer to force a specified exception to occur. For example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> raise NameError('HiThere') Traceback (most recent call last): File "<stdin>", line 1, in <module> raise NameError('HiThere') NameError: HiThere </pre></div> </div> The sole argument to <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from <a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-class docutils literal notranslate">BaseException</code></a>, such as <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate">Exception</code></a> or one of its subclasses). If an exception class is passed, it will be implicitly instantiated by calling its constructor with no arguments: <div class="highlight-python3 notranslate"><div class="highlight"><pre>raise ValueError # shorthand for 'raise ValueError()' </pre></div> </div> If you need to determine whether an exception was raised but don’t intend to handle it, a simpler form of the <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> statement allows you to re-raise the exception: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try: ... raise NameError('HiThere') ... except NameError: ... print('An exception flew by!') ... raise ... An exception flew by! Traceback (most recent call last): File "<stdin>", line 2, in <module> raise NameError('HiThere') NameError: HiThere </pre></div> </div> </section> <section id="exception-chaining"> <h2>8.5. Exception Chaining<a class="headerlink" href="#exception-chaining" title="Link to this heading">¶</a></h2> If an unhandled exception occurs inside an <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> section, it will have the exception being handled attached to it and included in the error message: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try: ... open("database.sqlite") ... except OSError: ... raise RuntimeError("unable to handle error") ... Traceback (most recent call last): File "<stdin>", line 2, in <module> open("database.sqlite") ~~~~^^^^^^^^^^^^^^^^^^^ FileNotFoundError: [Errno 2] No such file or directory: 'database.sqlite' During handling of the above exception, another exception occurred: Traceback (most recent call last): File "<stdin>", line 4, in <module> raise RuntimeError("unable to handle error") RuntimeError: unable to handle error </pre></div> </div> To indicate that an exception is a direct consequence of another, the <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> statement allows an optional <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">from</code></a> clause: <div class="highlight-python3 notranslate"><div class="highlight"><pre># exc must be exception instance or None. raise RuntimeError from exc </pre></div> </div> This can be useful when you are transforming exceptions. For example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> def func(): ... raise ConnectionError ... >>> try: ... func() ... except ConnectionError as exc: ... raise RuntimeError('Failed to open database') from exc ... Traceback (most recent call last): File "<stdin>", line 2, in <module> func() ~~~~^^ File "<stdin>", line 2, in func ConnectionError The above exception was the direct cause of the following exception: Traceback (most recent call last): File "<stdin>", line 4, in <module> raise RuntimeError('Failed to open database') from exc RuntimeError: Failed to open database </pre></div> </div> It also allows disabling automatic exception chaining using the <code class="docutils literal notranslate">from None</code> idiom: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try: ... open('database.sqlite') ... except OSError: ... raise RuntimeError from None ... Traceback (most recent call last): File "<stdin>", line 4, in <module> raise RuntimeError from None RuntimeError </pre></div> </div> For more information about chaining mechanics, see <a class="reference internal" href="../library/exceptions.html#bltin-exceptions">Built-in Exceptions</a>. </section> <section id="user-defined-exceptions"> <h2>8.6. User-defined Exceptions<a class="headerlink" href="#user-defined-exceptions" title="Link to this heading">¶</a></h2> Programs may name their own exceptions by creating a new exception class (see <a class="reference internal" href="classes.html#tut-classes">Classes</a> for more about Python classes). Exceptions should typically be derived from the <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate">Exception</code></a> class, either directly or indirectly. Exception classes can be defined which do anything any other class can do, but are usually kept simple, often only offering a number of attributes that allow information about the error to be extracted by handlers for the exception. Most exceptions are defined with names that end in “Error”, similar to the naming of the standard exceptions. Many standard modules define their own exceptions to report errors that may occur in functions they define. </section> <section id="defining-clean-up-actions"> <h2>8.7. Defining Clean-up Actions<a class="headerlink" href="#defining-clean-up-actions" title="Link to this heading">¶</a></h2> The <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try: ... raise KeyboardInterrupt ... finally: ... print('Goodbye, world!') ... Goodbye, world! Traceback (most recent call last): File "<stdin>", line 2, in <module> raise KeyboardInterrupt KeyboardInterrupt </pre></div> </div> If a <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause is present, the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause will execute as the last task before the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement completes. The <code class="xref std std-keyword docutils literal notranslate">finally</code> clause runs whether or not the <code class="xref std std-keyword docutils literal notranslate">try</code> statement produces an exception. The following points discuss more complex cases when an exception occurs: <ul class="simple"> <li>If an exception occurs during execution of the <code class="xref std std-keyword docutils literal notranslate">try</code> clause, the exception may be handled by an <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> clause. If the exception is not handled by an <code class="xref std std-keyword docutils literal notranslate">except</code> clause, the exception is re-raised after the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause has been executed.</li> <li>An exception could occur during execution of an <code class="xref std std-keyword docutils literal notranslate">except</code> or <code class="xref std std-keyword docutils literal notranslate">else</code> clause. Again, the exception is re-raised after the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause has been executed.</li> <li>If the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause executes a <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a>, <a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> or <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> statement, exceptions are not re-raised.</li> <li>If the <code class="xref std std-keyword docutils literal notranslate">try</code> statement reaches a <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a>, <a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> or <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> statement, the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause will execute just prior to the <code class="xref std std-keyword docutils literal notranslate">break</code>, <code class="xref std std-keyword docutils literal notranslate">continue</code> or <code class="xref std std-keyword docutils literal notranslate">return</code> statement’s execution.</li> <li>If a <code class="xref std std-keyword docutils literal notranslate">finally</code> clause includes a <code class="xref std std-keyword docutils literal notranslate">return</code> statement, the returned value will be the one from the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause’s <code class="xref std std-keyword docutils literal notranslate">return</code> statement, not the value from the <code class="xref std std-keyword docutils literal notranslate">try</code> clause’s <code class="xref std std-keyword docutils literal notranslate">return</code> statement.</li> </ul> For example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> def bool_return(): ... try: ... return True ... finally: ... return False ... >>> bool_return() False </pre></div> </div> A more complicated example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> def divide(x, y): ... try: ... result = x / y ... except ZeroDivisionError: ... print("division by zero!") ... else: ... print("result is", result) ... finally: ... print("executing finally clause") ... >>> divide(2, 1) result is 2.0 executing finally clause >>> divide(2, 0) division by zero! executing finally clause >>> divide("2", "1") executing finally clause Traceback (most recent call last): File "<stdin>", line 1, in <module> divide("2", "1") ~~~~~~^^^^^^^^^^ File "<stdin>", line 3, in divide result = x / y ~~^~~ TypeError: unsupported operand type(s) for /: 'str' and 'str' </pre></div> </div> As you can see, the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause is executed in any event. The <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> raised by dividing two strings is not handled by the <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> clause and therefore re-raised after the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause has been executed. In real world applications, the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful. </section> <section id="predefined-clean-up-actions"> <h2>8.8. Predefined Clean-up Actions<a class="headerlink" href="#predefined-clean-up-actions" title="Link to this heading">¶</a></h2> Some objects define standard clean-up actions to be undertaken when the object is no longer needed, regardless of whether or not the operation using the object succeeded or failed. Look at the following example, which tries to open a file and print its contents to the screen. <div class="highlight-python3 notranslate"><div class="highlight"><pre>for line in open("myfile.txt"): print(line, end="") </pre></div> </div> The problem with this code is that it leaves the file open for an indeterminate amount of time after this part of the code has finished executing. This is not an issue in simple scripts, but can be a problem for larger applications. The <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly. <div class="highlight-python3 notranslate"><div class="highlight"><pre>with open("myfile.txt") as f: for line in f: print(line, end="") </pre></div> </div> After the statement is executed, the file f is always closed, even if a problem was encountered while processing the lines. Objects which, like files, provide predefined clean-up actions will indicate this in their documentation. </section> <section id="raising-and-handling-multiple-unrelated-exceptions"> <h2>8.9. Raising and Handling Multiple Unrelated Exceptions<a class="headerlink" href="#raising-and-handling-multiple-unrelated-exceptions" title="Link to this heading">¶</a></h2> There are situations where it is necessary to report several exceptions that have occurred. This is often the case in concurrency frameworks, when several tasks may have failed in parallel, but there are also other use cases where it is desirable to continue execution and collect multiple errors rather than raise the first exception. The builtin <a class="reference internal" href="../library/exceptions.html#ExceptionGroup" title="ExceptionGroup"><code class="xref py py-exc docutils literal notranslate">ExceptionGroup</code></a> wraps a list of exception instances so that they can be raised together. It is an exception itself, so it can be caught like any other exception. <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> def f(): ... excs = [OSError('error 1'), SystemError('error 2')] ... raise ExceptionGroup('there were problems', excs) ... >>> f() + Exception Group Traceback (most recent call last): | File "<stdin>", line 1, in <module> | f() | ~^^ | File "<stdin>", line 3, in f | raise ExceptionGroup('there were problems', excs) | ExceptionGroup: there were problems (2 sub-exceptions) +-+---------------- 1 ---------------- | OSError: error 1 +---------------- 2 ---------------- | SystemError: error 2 +------------------------------------ >>> try: ... f() ... except Exception as e: ... print(f'caught {type(e)}: e') ... caught <class 'ExceptionGroup'>: e >>> </pre></div> </div> By using <code class="docutils literal notranslate">except*</code> instead of <code class="docutils literal notranslate">except</code>, we can selectively handle only the exceptions in the group that match a certain type. In the following example, which shows a nested exception group, each <code class="docutils literal notranslate">except*</code> clause extracts from the group exceptions of a certain type while letting all other exceptions propagate to other clauses and eventually to be reraised. <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> def f(): ... raise ExceptionGroup( ... "group1", ... [ ... OSError(1), ... SystemError(2), ... ExceptionGroup( ... "group2", ... [ ... OSError(3), ... RecursionError(4) ... ] ... ) ... ] ... ) ... >>> try: ... f() ... except* OSError as e: ... print("There were OSErrors") ... except* SystemError as e: ... print("There were SystemErrors") ... There were OSErrors There were SystemErrors + Exception Group Traceback (most recent call last): | File "<stdin>", line 2, in <module> | f() | ~^^ | File "<stdin>", line 2, in f | raise ExceptionGroup( | ...<12 lines>... | ) | ExceptionGroup: group1 (1 sub-exception) +-+---------------- 1 ---------------- | ExceptionGroup: group2 (1 sub-exception) +-+---------------- 1 ---------------- | RecursionError: 4 +------------------------------------ >>> </pre></div> </div> Note that the exceptions nested in an exception group must be instances, not types. This is because in practice the exceptions would typically be ones that have already been raised and caught by the program, along the following pattern: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> excs = [] ... for test in tests: ... try: ... test.run() ... except Exception as e: ... excs.append(e) ... >>> if excs: ... raise ExceptionGroup("Test Failures", excs) ... </pre></div> </div> </section> <section id="enriching-exceptions-with-notes"> <h2>8.10. Enriching Exceptions with Notes<a class="headerlink" href="#enriching-exceptions-with-notes" title="Link to this heading">¶</a></h2> When an exception is created in order to be raised, it is usually initialized with information that describes the error that has occurred. There are cases where it is useful to add information after the exception was caught. For this purpose, exceptions have a method <code class="docutils literal notranslate">add_note(note)</code> that accepts a string and adds it to the exception’s notes list. The standard traceback rendering includes all notes, in the order they were added, after the exception. <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try: ... raise TypeError('bad type') ... except Exception as e: ... e.add_note('Add some information') ... e.add_note('Add some more information') ... raise ... Traceback (most recent call last): File "<stdin>", line 2, in <module> raise TypeError('bad type') TypeError: bad type Add some information Add some more information >>> </pre></div> </div> For example, when collecting exceptions into an exception group, we may want to add context information for the individual errors. In the following each exception in the group has a note indicating when this error has occurred. <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> def f(): ... raise OSError('operation failed') ... >>> excs = [] >>> for i in range(3): ... try: ... f() ... except Exception as e: ... e.add_note(f'Happened in Iteration {i+1}') ... excs.append(e) ... >>> raise ExceptionGroup('We have some problems', excs) + Exception Group Traceback (most recent call last): | File "<stdin>", line 1, in <module> | raise ExceptionGroup('We have some problems', excs) | ExceptionGroup: We have some problems (3 sub-exceptions) +-+---------------- 1 ---------------- | Traceback (most recent call last): | File "<stdin>", line 3, in <module> | f() | ~^^ | File "<stdin>", line 2, in f | raise OSError('operation failed') | OSError: operation failed | Happened in Iteration 1 +---------------- 2 ---------------- | Traceback (most recent call last): | File "<stdin>", line 3, in <module> | f() | ~^^ | File "<stdin>", line 2, in f | raise OSError('operation failed') | OSError: operation failed | Happened in Iteration 2 +---------------- 3 ---------------- | Traceback (most recent call last): | File "<stdin>", line 3, in <module> | f() | ~^^ | File "<stdin>", line 2, in f | raise OSError('operation failed') | OSError: operation failed | Happened in Iteration 3 +------------------------------------ >>> </pre></div> </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="#">8. Errors and Exceptions</a><ul> <li><a class="reference internal" href="#syntax-errors">8.1. Syntax Errors</a></li> <li><a class="reference internal" href="#exceptions">8.2. Exceptions</a></li> <li><a class="reference internal" href="#handling-exceptions">8.3. Handling Exceptions</a></li> <li><a class="reference internal" href="#raising-exceptions">8.4. Raising Exceptions</a></li> <li><a class="reference internal" href="#exception-chaining">8.5. Exception Chaining</a></li> <li><a class="reference internal" href="#user-defined-exceptions">8.6. User-defined Exceptions</a></li> <li><a class="reference internal" href="#defining-clean-up-actions">8.7. Defining Clean-up Actions</a></li> <li><a class="reference internal" href="#predefined-clean-up-actions">8.8. Predefined Clean-up Actions</a></li> <li><a class="reference internal" href="#raising-and-handling-multiple-unrelated-exceptions">8.9. Raising and Handling Multiple Unrelated Exceptions</a></li> <li><a class="reference internal" href="#enriching-exceptions-with-notes">8.10. Enriching Exceptions with Notes</a></li> </ul> </li> </ul> </div> <div> <h4>Previous topic</h4> <a href="inputoutput.html" title="previous chapter">7. Input and Output</a> </div> <div> <h4>Next topic</h4> <a href="classes.html" title="next chapter">9. 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