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Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 535</li> </ul> <button id="colour-scheme-cycler" onClick="setColourScheme(nextColourScheme())"> <svg aria-hidden="true" class="colour-scheme-icon-when-auto"><use href="#svg-sun-half"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-dark"><use href="#svg-moon"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-light"><use href="#svg-sun"></use></svg> Toggle light / dark / auto colour theme </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 535 – Rich comparison chaining</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author:</dt> <dd class="field-odd">Alyssa Coghlan <ncoghlan at gmail.com></dd> <dt class="field-even">Status:</dt> <dd class="field-even"><abbr title="Inactive draft that may be taken up again at a later time">Deferred</abbr></dd> <dt class="field-odd">Type:</dt> <dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd> <dt class="field-even">Requires:</dt> <dd class="field-even"><a class="reference external" href="../pep-0532/">532</a></dd> <dt class="field-odd">Created:</dt> <dd class="field-odd">12-Nov-2016</dd> <dt class="field-even">Python-Version:</dt> <dd class="field-even">3.8</dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#pep-deferral">PEP Deferral</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#relationship-with-other-peps">Relationship with other PEPs</a></li> <li><a class="reference internal" href="#specification">Specification</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#implementation">Implementation</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="pep-deferral"> <h2><a class="toc-backref" href="#pep-deferral" role="doc-backlink">PEP Deferral</a></h2> Further consideration of this PEP has been deferred until Python 3.8 at the earliest. </section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> Inspired by <a class="pep reference internal" href="../pep-0335/" title="PEP 335 – Overloadable Boolean Operators">PEP 335</a>, and building on the circuit breaking protocol described in <a class="pep reference internal" href="../pep-0532/" title="PEP 532 – A circuit breaking protocol and binary operators">PEP 532</a>, this PEP proposes a change to the definition of chained comparisons, where the comparison chaining will be updated to use the left-associative circuit breaking operator (<code class="docutils literal notranslate">else</code>) rather than the logical disjunction operator (<code class="docutils literal notranslate">and</code>) if the left hand comparison returns a circuit breaker as its result. While there are some practical complexities arising from the current handling of single-valued arrays in NumPy, this change should be sufficient to allow elementwise chained comparison operations for matrices, where the result is a matrix of boolean values, rather than raising <code class="docutils literal notranslate">ValueError</code> or tautologically returning <code class="docutils literal notranslate">True</code> (indicating a non-empty matrix). </section> <section id="relationship-with-other-peps"> <h2><a class="toc-backref" href="#relationship-with-other-peps" role="doc-backlink">Relationship with other PEPs</a></h2> This PEP has been extracted from earlier iterations of <a class="pep reference internal" href="../pep-0532/" title="PEP 532 – A circuit breaking protocol and binary operators">PEP 532</a>, as a follow-on use case for the circuit breaking protocol, rather than an essential part of its introduction. The specific proposal in this PEP to handle the element-wise comparison use case by changing the semantic definition of comparison chaining is drawn directly from Guido’s rejection of <a class="pep reference internal" href="../pep-0335/" title="PEP 335 – Overloadable Boolean Operators">PEP 335</a>. </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> A chained comparison like <code class="docutils literal notranslate">0 < x < 10</code> written as: <div class="highlight-default notranslate"><div class="highlight"><pre>LEFT_BOUND LEFT_OP EXPR RIGHT_OP RIGHT_BOUND </pre></div> </div> is currently roughly semantically equivalent to: <div class="highlight-default notranslate"><div class="highlight"><pre>_expr = EXPR _lhs_result = LEFT_BOUND LEFT_OP _expr _expr_result = _lhs_result and (_expr RIGHT_OP RIGHT_BOUND) </pre></div> </div> Using the circuit breaking concepts introduced in <a class="pep reference internal" href="../pep-0532/" title="PEP 532 – A circuit breaking protocol and binary operators">PEP 532</a>, this PEP proposes that comparison chaining be changed to explicitly check if the left comparison returns a circuit breaker, and if so, use <code class="docutils literal notranslate">else</code> rather than <code class="docutils literal notranslate">and</code> to implement the comparison chaining: <div class="highlight-default notranslate"><div class="highlight"><pre>_expr = EXPR _lhs_result = LEFT_BOUND LEFT_OP _expr if hasattr(type(_lhs_result), "__else__"): _expr_result = _lhs_result else (_expr RIGHT_OP RIGHT_BOUND) else: _expr_result = _lhs_result and (_expr RIGHT_OP RIGHT_BOUND) </pre></div> </div> This allows types like NumPy arrays to control the behaviour of chained comparisons by returning suitably defined circuit breakers from comparison operations. The expansion of this logic to an arbitrary number of chained comparison operations would be the same as the existing expansion for <code class="docutils literal notranslate">and</code>. </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> In ultimately rejecting <a class="pep reference internal" href="../pep-0335/" title="PEP 335 – Overloadable Boolean Operators">PEP 335</a>, Guido van Rossum noted <a class="footnote-reference brackets" href="#id2" id="id1">[1]</a>: <blockquote> <div>The NumPy folks brought up a somewhat separate issue: for them, the most common use case is chained comparisons (e.g. A </blockquote> To understand this observation, we first need to look at how comparisons work with NumPy arrays: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> import numpy as np >>> increasing = np.arange(5) >>> increasing array([0, 1, 2, 3, 4]) >>> decreasing = np.arange(4, -1, -1) >>> decreasing array([4, 3, 2, 1, 0]) >>> increasing < decreasing array([ True, True, False, False, False], dtype=bool) </pre></div> </div> Here we see that NumPy array comparisons are element-wise by default, comparing each element in the left hand array to the corresponding element in the right hand array, and producing a matrix of boolean results. If either side of the comparison is a scalar value, then it is broadcast across the array and compared to each individual element: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> 0 < increasing array([False, True, True, True, True], dtype=bool) >>> increasing < 4 array([ True, True, True, True, False], dtype=bool) </pre></div> </div> However, this broadcasting idiom breaks down if we attempt to use chained comparisons: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> 0 < increasing < 4 Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all() </pre></div> </div> The problem is that internally, Python implicitly expands this chained comparison into the form: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> 0 < increasing and increasing < 4 Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all() </pre></div> </div> And NumPy only permits implicit coercion to a boolean value for single-element arrays where <code class="docutils literal notranslate">a.any()</code> and <code class="docutils literal notranslate">a.all()</code> can be assured of having the same result: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> np.array([False]) and np.array([False]) array([False], dtype=bool) >>> np.array([False]) and np.array([True]) array([False], dtype=bool) >>> np.array([True]) and np.array([False]) array([False], dtype=bool) >>> np.array([True]) and np.array([True]) array([ True], dtype=bool) </pre></div> </div> The proposal in this PEP would allow this situation to be changed by updating the definition of element-wise comparison operations in NumPy to return a dedicated subclass that implements the new circuit breaking protocol and also changes the result array’s interpretation in a boolean context to always return <code class="docutils literal notranslate">False</code> and hence never trigger the short-circuiting behaviour: <div class="highlight-default notranslate"><div class="highlight"><pre>class ComparisonResultArray(np.ndarray): def __bool__(self): # Element-wise comparison chaining never short-circuits return False def _raise_NotImplementedError(self): msg = ("Comparison array truth values are ambiguous outside " "chained comparisons. Use a.any() or a.all()") raise NotImplementedError(msg) def __not__(self): self._raise_NotImplementedError() def __then__(self, result): self._raise_NotImplementedError() def __else__(self, result): return np.logical_and(self, other.view(ComparisonResultArray)) </pre></div> </div> With this change, the chained comparison example above would be able to return: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> 0 < increasing < 4 ComparisonResultArray([ False, True, True, True, False], dtype=bool) </pre></div> </div> </section> <section id="implementation"> <h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2> Actual implementation has been deferred pending in-principle interest in the idea of making the changes proposed in <a class="pep reference internal" href="../pep-0532/" title="PEP 532 – A circuit breaking protocol and binary operators">PEP 532</a>. …TBD… </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="id2" role="doc-footnote"> <dt class="label" id="id2">[<a href="#id1">1</a>]</dt> <dd>PEP 335 rejection notification (<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2012-March/117510.html">https://mail.python.org/pipermail/python-dev/2012-March/117510.html</a>)</aside> </aside> </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 under the terms of the CC0 1.0 license: <a class="reference external" href="https://creativecommons.org/publicdomain/zero/1.0/">https://creativecommons.org/publicdomain/zero/1.0/</a> </section> </section> <hr class="docutils" /> Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0535.rst">https://github.com/python/peps/blob/main/peps/pep-0535.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0535.rst">2025-02-01 08:59:27 GMT</a> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#pep-deferral">PEP Deferral</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#relationship-with-other-peps">Relationship with other PEPs</a></li> <li><a class="reference internal" href="#specification">Specification</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#implementation">Implementation</a></li> <li><a class="reference internal" href="#references">References</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-0535.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>