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<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta name="color-scheme" content="light dark"> <title>PEP 3136 – Labeled break and continue | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-3136/"> <link rel="stylesheet" href="../_static/style.css" type="text/css"> <link rel="stylesheet" href="../_static/mq.css" type="text/css"> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" media="(prefers-color-scheme: light)" id="pyg-light"> <link rel="stylesheet" href="../_static/pygments_dark.css" type="text/css" media="(prefers-color-scheme: dark)" id="pyg-dark"> <link rel="alternate" type="application/rss+xml" title="Latest PEPs" href="https://peps.python.org/peps.rss"> <meta property="og:title" content='PEP 3136 – Labeled break and continue | peps.python.org'> <meta property="og:description" content="This PEP proposes support for labels in Python’s break and continue statements. 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Explicit labels</a></li> <li><a class="reference internal" href="#proposal-b-numeric-break-continue">Proposal B - Numeric break & continue</a></li> <li><a class="reference internal" href="#proposal-c-the-reduplicative-method">Proposal C - The reduplicative method</a></li> <li><a class="reference internal" href="#proposal-d-explicit-iterators">Proposal D - Explicit iterators</a></li> <li><a class="reference internal" href="#proposal-e-explicit-iterators-and-iterator-methods">Proposal E - Explicit iterators and iterator methods</a></li> </ul> </li> <li><a class="reference internal" href="#implementation">Implementation</a></li> <li><a class="reference internal" href="#footnotes">Footnotes</a></li> <li><a class="reference internal" href="#resources">Resources</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="rejection-notice"> <h2><a class="toc-backref" href="#rejection-notice" role="doc-backlink">Rejection Notice</a></h2> This PEP is rejected. See <a class="reference external" href="https://mail.python.org/pipermail/python-3000/2007-July/008663.html">https://mail.python.org/pipermail/python-3000/2007-July/008663.html</a>. </section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> This PEP proposes support for labels in Python’s <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> statements. It is inspired by labeled <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> in other languages, and the author’s own infrequent but persistent need for such a feature. </section> <section id="introduction"> <h2><a class="toc-backref" href="#introduction" role="doc-backlink">Introduction</a></h2> The <code class="docutils literal notranslate">break</code> statement allows the programmer to terminate a loop early, and the <code class="docutils literal notranslate">continue</code> statement allows the programmer to move to the next iteration of a loop early. In Python currently, <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> can apply only to the innermost enclosing loop. Adding support for labels to the <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> statements is a logical extension to the existing behavior of the <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> statements. Labeled <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> can improve the readability and flexibility of complex code which uses nested loops. For brevity’s sake, the examples and discussion in this PEP usually refers to the <code class="docutils literal notranslate">break</code> statement. However, all of the examples and motivations apply equally to labeled <code class="docutils literal notranslate">continue</code>. </section> <section id="motivation"> <h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2> If the programmer wishes to move to the next iteration of an outer enclosing loop, or terminate multiple loops at once, he or she has a few less-than elegant options. Here’s one common way of imitating labeled <code class="docutils literal notranslate">break</code> in Python (For this and future examples, <code class="docutils literal notranslate">...</code> denotes an arbitrary number of intervening lines of code): <div class="highlight-default notranslate"><div class="highlight"><pre>for a in a_list: time_to_break_out_of_a = False ... for b in b_list: ... if condition_one(a, b): break ... if condition_two(a, b): time_to_break_out_of_a = True break ... if time_to_break_out_of_a: break ... </pre></div> </div> This requires five lines and an extra variable, <code class="docutils literal notranslate">time_to_break_out_of_a</code>, to keep track of when to break out of the outer (a) loop. And those five lines are spread across many lines of code, making the control flow difficult to understand. This technique is also error-prone. A programmer modifying this code might inadvertently put new code after the end of the inner (b) loop but before the test for <code class="docutils literal notranslate">time_to_break_out_of_a</code>, instead of after the test. This means that code which should have been skipped by breaking out of the outer loop gets executed incorrectly. This could also be written with an exception. The programmer would declare a special exception, wrap the inner loop in a try, and catch the exception and break when you see it: <div class="highlight-default notranslate"><div class="highlight"><pre>class BreakOutOfALoop(Exception): pass for a in a_list: ... try: for b in b_list: ... if condition_one(a, b): break ... if condition_two(a, b): raise BreakOutOfALoop ... except BreakOutOfALoop: break ... </pre></div> </div> Again, though; this requires five lines and a new, single-purpose exception class (instead of a new variable), and spreads basic control flow out over many lines. And it breaks out of the inner loop with <code class="docutils literal notranslate">break</code> and out of the other loop with an exception, which is inelegant. <a class="footnote-reference brackets" href="#toowtdi" id="id1">[1]</a> This next strategy might be the most elegant solution, assuming condition_two() is inexpensive to compute: <div class="highlight-default notranslate"><div class="highlight"><pre>for a in a_list: ... for b in b_list: ... if condition_one(a, b): break ... if condition_two(a, b): break ... if condition_two(a, b) break ... </pre></div> </div> Breaking twice is still inelegant. This implementation also relies on the fact that the inner (b) loop bleeds b into the outer for loop, which (although explicitly supported) is both surprising to novices, and in my opinion counter-intuitive and poor practice. The programmer must also still remember to put in both breaks on condition two and not insert code before the second break. A single conceptual action, breaking out of both loops on condition_two(), requires four lines of code at two indentation levels, possibly separated by many intervening lines at the end of the inner (b) loop. <section id="other-languages"> <h3><a class="toc-backref" href="#other-languages" role="doc-backlink">Other languages</a></h3> Now, put aside whatever dislike you may have for other programming languages, and consider the syntax of labeled <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code>. In Perl: <div class="highlight-default notranslate"><div class="highlight"><pre>ALOOP: foreach $a (@a_array){ ... BLOOP: foreach $b (@b_array){ ... if (condition_one($a,$b)){ last BLOOP; # same as plain old last; } ... if (condition_two($a,$b)){ last ALOOP; } ... } ... } </pre></div> </div> (Notes: Perl uses <code class="docutils literal notranslate">last</code> instead of <code class="docutils literal notranslate">break</code>. The BLOOP labels could be omitted; <code class="docutils literal notranslate">last</code> and <code class="docutils literal notranslate">continue</code> apply to the innermost loop by default.) PHP uses a number denoting the number of loops to break out of, rather than a label: <div class="highlight-default notranslate"><div class="highlight"><pre>foreach ($a_array as $a){ .... foreach ($b_array as $b){ .... if (condition_one($a, $b)){ break 1; # same as plain old break } .... if (condition_two($a, $b)){ break 2; } .... } ... } </pre></div> </div> C/C++, Java, and Ruby all have similar constructions. The control flow regarding when to break out of the outer (a) loop is fully encapsulated in the <code class="docutils literal notranslate">break</code> statement which gets executed when the break condition is satisfied. The depth of the break statement does not matter. Control flow is not spread out. No extra variables, exceptions, or re-checking or storing of control conditions is required. There is no danger that code will get inadvertently inserted after the end of the inner (b) loop and before the break condition is re-checked inside the outer (a) loop. These are the benefits that labeled <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> would bring to Python. </section> </section> <section id="what-this-pep-is-not"> <h2><a class="toc-backref" href="#what-this-pep-is-not" role="doc-backlink">What this PEP is not</a></h2> This PEP is not a proposal to add GOTO to Python. GOTO allows a programmer to jump to an arbitrary block or line of code, and generally makes control flow more difficult to follow. Although <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> (with or without support for labels) can be considered a type of GOTO, it is much more restricted. Another Python construct, <code class="docutils literal notranslate">yield</code>, could also be considered a form of GOTO – an even less restrictive one. The goal of this PEP is to propose an extension to the existing control flow tools <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code>, to make control flow easier to understand, not more difficult. Labeled <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> cannot transfer control to another function or method. They cannot even transfer control to an arbitrary line of code in the current scope. Currently, they can only affect the behavior of a loop, and are quite different and much more restricted than GOTO. This extension allows them to affect any enclosing loop in the current name-space, but it does not change their behavior to that of GOTO. </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> Under all of these proposals, <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> by themselves will continue to behave as they currently do, applying to the innermost loop by default. <section id="proposal-a-explicit-labels"> <h3><a class="toc-backref" href="#proposal-a-explicit-labels" role="doc-backlink">Proposal A - Explicit labels</a></h3> The for and while loop syntax will be followed by an optional <code class="docutils literal notranslate">as</code> or <code class="docutils literal notranslate">label</code> (contextual) keyword <a class="footnote-reference brackets" href="#keyword" id="id2">[2]</a> and then an identifier, which may be used to identify the loop out of which to break (or which should be continued). The <code class="docutils literal notranslate">break</code> (and <code class="docutils literal notranslate">continue</code>) statements will be followed by an optional identifier that refers to the loop out of which to break (or which should be continued). Here is an example using the <code class="docutils literal notranslate">as</code> keyword: <div class="highlight-default notranslate"><div class="highlight"><pre>for a in a_list as a_loop: ... for b in b_list as b_loop: ... if condition_one(a, b): break b_loop # same as plain old break ... if condition_two(a, b): break a_loop ... ... </pre></div> </div> Or, with <code class="docutils literal notranslate">label</code> instead of <code class="docutils literal notranslate">as</code>: <div class="highlight-default notranslate"><div class="highlight"><pre>for a in a_list label a_loop: ... for b in b_list label b_loop: ... if condition_one(a, b): break b_loop # same as plain old break ... if condition_two(a, b): break a_loop ... ... </pre></div> </div> This has all the benefits outlined above. It requires modifications to the language syntax: the syntax of <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> syntax statements and for and while statements. It requires either a new conditional keyword <code class="docutils literal notranslate">label</code> or an extension to the conditional keyword <code class="docutils literal notranslate">as</code>. <a class="footnote-reference brackets" href="#as" id="id3">[3]</a> It is unlikely to require any changes to existing Python programs. Passing an identifier not defined in the local scope to <code class="docutils literal notranslate">break</code> or <code class="docutils literal notranslate">continue</code> would raise a NameError. </section> <section id="proposal-b-numeric-break-continue"> <h3><a class="toc-backref" href="#proposal-b-numeric-break-continue" role="doc-backlink">Proposal B - Numeric break & continue</a></h3> Rather than altering the syntax of <code class="docutils literal notranslate">for</code> and <code class="docutils literal notranslate">while</code> loops, <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> would take a numeric argument denoting the enclosing loop which is being controlled, similar to PHP. It seems more Pythonic to me for <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> to refer to loops indexing from zero, as opposed to indexing from one as PHP does. <div class="highlight-default notranslate"><div class="highlight"><pre>for a in a_list: ... for b in b_list: ... if condition_one(a,b): break 0 # same as plain old break ... if condition_two(a,b): break 1 ... ... </pre></div> </div> Passing a number that was too large, or less than zero, or non-integer to <code class="docutils literal notranslate">break</code> or <code class="docutils literal notranslate">continue</code> would (probably) raise an IndexError. This proposal would not require any changes to existing Python programs. </section> <section id="proposal-c-the-reduplicative-method"> <h3><a class="toc-backref" href="#proposal-c-the-reduplicative-method" role="doc-backlink">Proposal C - The reduplicative method</a></h3> The syntax of <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> would be altered to allow multiple <code class="docutils literal notranslate">break</code> and continue statements on the same line. Thus, <code class="docutils literal notranslate">break break</code> would break out of the first and second enclosing loops. <div class="highlight-default notranslate"><div class="highlight"><pre>for a in a_list: ... for b in b_list: ... if condition_one(a,b): break # plain old break ... if condition_two(a,b): break break ... ... </pre></div> </div> This would also allow the programmer to break out of the inner loop and continue the next outermost simply by writing <code class="docutils literal notranslate">break continue</code>, <a class="footnote-reference brackets" href="#breakcontinue" id="id4">[4]</a> and so on. I’m not sure what exception would be raised if the programmer used more <code class="docutils literal notranslate">break</code> or <code class="docutils literal notranslate">continue</code> statements than existing loops (perhaps a SyntaxError?). I expect this proposal to get rejected because it will be judged too difficult to understand. This proposal would not require any changes to existing Python programs. </section> <section id="proposal-d-explicit-iterators"> <h3><a class="toc-backref" href="#proposal-d-explicit-iterators" role="doc-backlink">Proposal D - Explicit iterators</a></h3> Rather than embellishing for and while loop syntax with labels, the programmer wishing to use labeled breaks would be required to create the iterator explicitly and assign it to an identifier if he or she wanted to <code class="docutils literal notranslate">break</code> out of or <code class="docutils literal notranslate">continue</code> that loop from within a deeper loop. <div class="highlight-default notranslate"><div class="highlight"><pre>a_iter = iter(a_list) for a in a_iter: ... b_iter = iter(b_list) for b in b_iter: ... if condition_one(a,b): break b_iter # same as plain old break ... if condition_two(a,b): break a_iter ... ... </pre></div> </div> Passing a non-iterator object to <code class="docutils literal notranslate">break</code> or <code class="docutils literal notranslate">continue</code> would raise a TypeError; and a nonexistent identifier would raise a NameError. This proposal requires only one extra line to create a labeled loop, and no extra lines to break out of a containing loop, and no changes to existing Python programs. </section> <section id="proposal-e-explicit-iterators-and-iterator-methods"> <h3><a class="toc-backref" href="#proposal-e-explicit-iterators-and-iterator-methods" role="doc-backlink">Proposal E - Explicit iterators and iterator methods</a></h3> This is a variant of Proposal D. Iterators would need be created explicitly if anything other that the most basic use of <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code> was required. Instead of modifying the syntax of <code class="docutils literal notranslate">break</code> and <code class="docutils literal notranslate">continue</code>, <code class="docutils literal notranslate">.break()</code> and <code class="docutils literal notranslate">.continue()</code> methods could be added to the Iterator type. <div class="highlight-default notranslate"><div class="highlight"><pre>a_iter = iter(a_list) for a in a_iter: ... b_iter = iter(b_list) for b in b_iter: ... if condition_one(a,b): b_iter.break() # same as plain old break ... if condition_two(a,b): a_iter.break() ... ... </pre></div> </div> I expect that this proposal will get rejected on the grounds of sheer ugliness. However, it requires no changes to the language syntax whatsoever, nor does it require any changes to existing Python programs. </section> </section> <section id="implementation"> <h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2> I have never looked at the Python language implementation itself, so I have no idea how difficult this would be to implement. If this PEP is accepted, but no one is available to write the feature, I will try to implement it myself. </section> <section id="footnotes"> <h2><a class="toc-backref" href="#footnotes" role="doc-backlink">Footnotes</a></h2> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="toowtdi" role="doc-footnote"> <dt class="label" id="toowtdi">[<a href="#id1">1</a>]</dt> <dd>Breaking some loops with exceptions is inelegant because it’s a violation of There’s Only One Way To Do It.</aside> <aside class="footnote brackets" id="keyword" role="doc-footnote"> <dt class="label" id="keyword">[<a href="#id2">2</a>]</dt> <dd>Or really any new contextual keyword that the community likes: <code class="docutils literal notranslate">as</code>, <code class="docutils literal notranslate">label</code>, <code class="docutils literal notranslate">labeled</code>, <code class="docutils literal notranslate">loop</code>, <code class="docutils literal notranslate">name</code>, <code class="docutils literal notranslate">named</code>, <code class="docutils literal notranslate">walrus</code>, whatever.</aside> <aside class="footnote brackets" id="as" role="doc-footnote"> <dt class="label" id="as">[<a href="#id3">3</a>]</dt> <dd>The use of <code class="docutils literal notranslate">as</code> in a similar context has been proposed here, <a class="reference external" href="http://sourceforge.net/tracker/index.php?func=detail&aid=1714448&group_id=5470&atid=355470">http://sourceforge.net/tracker/index.php?func=detail&aid=1714448&group_id=5470&atid=355470</a> but to my knowledge this idea has not been written up as a PEP.</aside> <aside class="footnote brackets" id="breakcontinue" role="doc-footnote"> <dt class="label" id="breakcontinue">[<a href="#id4">4</a>]</dt> <dd>To continue the Nth outer loop, you would write break N-1 times and then continue. Only one <code class="docutils literal notranslate">continue</code> would be allowed, and only at the end of a sequence of breaks. <code class="docutils literal notranslate">continue break</code> or <code class="docutils literal notranslate">continue continue</code> makes no sense.</aside> </aside> </section> <section id="resources"> <h2><a class="toc-backref" href="#resources" role="doc-backlink">Resources</a></h2> This issue has come up before, although it has never been resolved, to my knowledge. <ul class="simple"> <li><a class="reference external" href="http://groups.google.com/group/comp.lang.python/browse_thread/thread/6da848f762c9cf58/979ca3cd42633b52?lnk=gst&q=labeled+break&rnum=3#979ca3cd42633b52">labeled breaks</a>, on comp.lang.python, in the context of <code class="docutils literal notranslate">do...while</code> loops</li> <li><a class="reference external" href="https://mail.python.org/pipermail/python-list/1999-September/#11080">break LABEL vs. exceptions + PROPOSAL</a>, on python-list, as compared to using Exceptions for flow control</li> <li><a class="reference external" href="https://mail.python.org/pipermail/python-list/2001-April/#78439">Named code blocks</a> on python-list, a suggestion motivated by the desire for labeled break / continue</li> <li><a class="reference external" href="http://mail-archives.apache.org/mod_mbox/httpd-python-cvs/200511.mbox/%3C20051112204322.4010.qmail@minotaur.apache.org%3E">mod_python bug fix</a> An example of someone setting a flag inside an inner loop that triggers a continue in the containing loop, to work around the absence of labeled break and continue</li> </ul> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> This document has been placed in the public domain. </section> </section> <hr class="docutils" /> Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-3136.rst">https://github.com/python/peps/blob/main/peps/pep-3136.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-3136.rst">2023-09-09 17:39:29 GMT</a> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#rejection-notice">Rejection Notice</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#introduction">Introduction</a></li> <li><a class="reference internal" href="#motivation">Motivation</a><ul> <li><a class="reference internal" href="#other-languages">Other languages</a></li> </ul> </li> <li><a class="reference internal" href="#what-this-pep-is-not">What this PEP is not</a></li> <li><a class="reference internal" href="#specification">Specification</a><ul> <li><a class="reference internal" href="#proposal-a-explicit-labels">Proposal A - Explicit labels</a></li> <li><a class="reference internal" href="#proposal-b-numeric-break-continue">Proposal B - Numeric break & continue</a></li> <li><a class="reference internal" href="#proposal-c-the-reduplicative-method">Proposal C - The reduplicative method</a></li> <li><a class="reference internal" href="#proposal-d-explicit-iterators">Proposal D - Explicit iterators</a></li> <li><a class="reference internal" href="#proposal-e-explicit-iterators-and-iterator-methods">Proposal E - Explicit iterators and iterator methods</a></li> </ul> </li> <li><a class="reference internal" href="#implementation">Implementation</a></li> <li><a class="reference internal" href="#footnotes">Footnotes</a></li> <li><a class="reference internal" href="#resources">Resources</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-3136.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script src="../_static/wrap_tables.js"></script> <script src="../_static/sticky_banner.js"></script> </body> </html>