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<h1 class="page-title">PEP 679 – Allow parentheses in assert statements</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Pablo Galindo Salgado <pablogsal at python.org></dd>
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<dt class="field-even">Discussions-To<span class="colon">:</span></dt>
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<dd class="field-even"><a class="reference external" href="https://discuss.python.org/t/pep-679-allow-parentheses-in-assert-statements/13003">Discourse thread</a></dd>
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<dt class="field-odd">Status<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Proposal under active discussion and revision">Draft</abbr></dd>
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<dt class="field-even">Type<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd>
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<dt class="field-odd">Created<span class="colon">:</span></dt>
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<dd class="field-odd">07-Jan-2022</dd>
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<dt class="field-even">Python-Version<span class="colon">:</span></dt>
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<dd class="field-even">3.12</dd>
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</dl>
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<section id="contents">
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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
|
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<li><a class="reference internal" href="#motivation">Motivation</a></li>
|
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
|
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<li><a class="reference internal" href="#specification">Specification</a></li>
|
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<li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li>
|
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<li><a class="reference internal" href="#security-implications">Security Implications</a></li>
|
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<li><a class="reference internal" href="#how-to-teach-this">How to Teach This</a></li>
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<li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li>
|
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<li><a class="reference internal" href="#references">References</a></li>
|
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
||
</ul>
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</details></section>
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<section id="abstract">
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<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
|
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<p>This PEP proposes to allow parentheses surrounding the two-argument form of
|
||
assert statements. This will cause the interpreter to reinterpret what before
|
||
would have been an assert with a two-element tuple that will always be True
|
||
(<code class="docutils literal notranslate"><span class="pre">assert</span> <span class="pre">(expression,</span> <span class="pre">message)</span></code>) to an assert statement with a subject and a
|
||
failure message, equivalent to the statement with the parentheses removed
|
||
(<code class="docutils literal notranslate"><span class="pre">assert</span> <span class="pre">expression,</span> <span class="pre">message</span></code>).</p>
|
||
</section>
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<section id="motivation">
|
||
<h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2>
|
||
<p>It is a common user mistake when using the form of the assert statement that includes
|
||
the error message to surround it with parentheses. Unfortunately, this mistake
|
||
passes undetected as the assert will always pass, because it is
|
||
interpreted as an assert statement where the expression is a two-tuple, which
|
||
always has truth-y value.</p>
|
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<p>The mistake most often happens when extending the test or description beyond a
|
||
single line, as parentheses are the natural way to do that.</p>
|
||
<p>This is so common that a <code class="docutils literal notranslate"><span class="pre">SyntaxWarning</span></code> is <a class="reference external" href="https://bugs.python.org/issue35029">now emitted by the compiler</a>.</p>
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<p>Additionally, some other statements in the language allow parenthesized forms
|
||
in one way or another like <code class="docutils literal notranslate"><span class="pre">import</span></code> statements (<code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">x</span> <span class="pre">import</span> <span class="pre">(a,b,c)</span></code>) and
|
||
<code class="docutils literal notranslate"><span class="pre">del</span></code> statements (<code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">(a,b,c)</span></code>).</p>
|
||
<p>Allowing parentheses not only will remove the common mistake but also will allow
|
||
users and auto-formatters to format long assert statements over multiple lines
|
||
in what the authors of this document believe will be a more natural way.
|
||
Although is possible to currently format long <code class="docutils literal notranslate"><span class="pre">assert</span></code> statements over
|
||
multiple lines as:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">assert</span> <span class="p">(</span>
|
||
<span class="n">very</span> <span class="n">very</span> <span class="n">long</span>
|
||
<span class="n">expression</span>
|
||
<span class="p">),</span> <span class="p">(</span>
|
||
<span class="s2">"very very long "</span>
|
||
<span class="s2">"message"</span>
|
||
<span class="p">)</span>
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||
</pre></div>
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||
</div>
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<p>the authors of this document believe the parenthesized form is more clear and more consistent with
|
||
the formatting of other grammar constructs:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">assert</span> <span class="p">(</span>
|
||
<span class="n">very</span> <span class="n">very</span> <span class="n">long</span>
|
||
<span class="n">expression</span><span class="p">,</span>
|
||
|
||
<span class="s2">"very very long "</span>
|
||
<span class="s2">"message"</span><span class="p">,</span>
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||
<span class="p">)</span>
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||
</pre></div>
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</div>
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<p>This change has been originally discussed and proposed in <a class="reference internal" href="#bpo-46167" id="id1"><span>[bpo-46167]</span></a>.</p>
|
||
</section>
|
||
<section id="rationale">
|
||
<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
|
||
<p>This change can be implemented in the parser or in the compiler. We have
|
||
selected implementing this change in the parser because doing it in the compiler
|
||
will require re-interpreting the AST of an assert statement with a two-tuple:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">Module</span><span class="p">(</span>
|
||
<span class="n">body</span><span class="o">=</span><span class="p">[</span>
|
||
<span class="n">Assert</span><span class="p">(</span>
|
||
<span class="n">test</span><span class="o">=</span><span class="n">Tuple</span><span class="p">(</span>
|
||
<span class="n">elts</span><span class="o">=</span><span class="p">[</span>
|
||
<span class="n">Name</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="s1">'x'</span><span class="p">,</span> <span class="n">ctx</span><span class="o">=</span><span class="n">Load</span><span class="p">()),</span>
|
||
<span class="n">Name</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="s1">'y'</span><span class="p">,</span> <span class="n">ctx</span><span class="o">=</span><span class="n">Load</span><span class="p">())],</span>
|
||
<span class="n">ctx</span><span class="o">=</span><span class="n">Load</span><span class="p">()))],</span>
|
||
<span class="n">type_ignores</span><span class="o">=</span><span class="p">[])</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>as the AST of an assert statement with an expression and a message:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">Module</span><span class="p">(</span>
|
||
<span class="n">body</span><span class="o">=</span><span class="p">[</span>
|
||
<span class="n">Assert</span><span class="p">(</span>
|
||
<span class="n">test</span><span class="o">=</span><span class="n">Name</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="s1">'x'</span><span class="p">,</span> <span class="n">ctx</span><span class="o">=</span><span class="n">Load</span><span class="p">()),</span>
|
||
<span class="n">msg</span><span class="o">=</span><span class="n">Name</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="s1">'y'</span><span class="p">,</span> <span class="n">ctx</span><span class="o">=</span><span class="n">Load</span><span class="p">()))],</span>
|
||
<span class="n">type_ignores</span><span class="o">=</span><span class="p">[])</span>
|
||
</pre></div>
|
||
</div>
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||
<p>The problem with this approach is that the AST of the first form will
|
||
technically be “incorrect” as we already have a specialized form for the AST of
|
||
an assert statement with a test and a message (the second one). This
|
||
means that many tools that deal with ASTs will need to be aware of this change
|
||
in semantics, which will be confusing as there is already a correct form that
|
||
better expresses the new meaning.</p>
|
||
</section>
|
||
<section id="specification">
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||
<h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2>
|
||
<p>This PEP proposes changing the grammar of the <code class="docutils literal notranslate"><span class="pre">assert</span></code> statement to:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">|</span> <span class="s1">'assert'</span> <span class="s1">'('</span> <span class="n">expression</span> <span class="s1">','</span> <span class="n">expression</span> <span class="p">[</span><span class="s1">','</span><span class="p">]</span> <span class="s1">')'</span> <span class="o">&</span><span class="p">(</span><span class="n">NEWLINE</span> <span class="o">|</span> <span class="s1">';'</span><span class="p">)</span>
|
||
<span class="o">|</span> <span class="s1">'assert'</span> <span class="n">a</span><span class="o">=</span><span class="n">expression</span> <span class="p">[</span><span class="s1">','</span> <span class="n">expression</span> <span class="p">]</span>
|
||
</pre></div>
|
||
</div>
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||
<p>Where the first line is the new form of the assert statement that allows
|
||
parentheses. The lookahead is needed so statements like <code class="docutils literal notranslate"><span class="pre">assert</span> <span class="pre">(a,</span> <span class="pre">b)</span> <span class="pre"><=</span> <span class="pre">c,</span>
|
||
<span class="pre">"something"</span></code> are still parsed correctly and to prevent the parser to eagerly
|
||
capture the tuple as the full statement.</p>
|
||
<p>Optionally, new “invalid” rule can be added to produce custom syntax errors to
|
||
cover tuples with 0, 1, 3 or more elements.</p>
|
||
</section>
|
||
<section id="backwards-compatibility">
|
||
<h2><a class="toc-backref" href="#backwards-compatibility" role="doc-backlink">Backwards Compatibility</a></h2>
|
||
<p>The change is not technically backwards compatible, as parsing <code class="docutils literal notranslate"><span class="pre">assert</span> <span class="pre">(x,y)</span></code>
|
||
is currently interpreted as an assert statement with a 2-tuple as the subject,
|
||
while after this change it will be interpreted as <code class="docutils literal notranslate"><span class="pre">assert</span> <span class="pre">x,y</span></code>.</p>
|
||
<p>On the other hand, assert statements of this kind always pass, so they are
|
||
effectively not doing anything in user code. The authors of this document think
|
||
that this backwards incompatibility nature is beneficial, as it will highlight
|
||
these cases in user code while before they will have passed unnoticed (assuming that
|
||
these cases still exist because users are ignoring syntax warnings).</p>
|
||
</section>
|
||
<section id="security-implications">
|
||
<h2><a class="toc-backref" href="#security-implications" role="doc-backlink">Security Implications</a></h2>
|
||
<p>There are no security implications for this change.</p>
|
||
</section>
|
||
<section id="how-to-teach-this">
|
||
<h2><a class="toc-backref" href="#how-to-teach-this" role="doc-backlink">How to Teach This</a></h2>
|
||
<p>The new form of the <code class="docutils literal notranslate"><span class="pre">assert</span></code> statement will be documented as part of the language
|
||
standard.</p>
|
||
<p>When teaching the form with error message of the <code class="docutils literal notranslate"><span class="pre">assert</span></code> statement to users,
|
||
now it can be noted that adding parentheses also work as expected, which allows to break
|
||
the statement over multiple lines.</p>
|
||
</section>
|
||
<section id="reference-implementation">
|
||
<h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2>
|
||
<p>A proposed draft PR with the change exist in <a class="reference internal" href="#gh-30247" id="id2"><span>[GH-30247]</span></a>.</p>
|
||
</section>
|
||
<section id="references">
|
||
<h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2>
|
||
<div role="list" class="citation-list">
|
||
<div class="citation" id="bpo-46167" role="doc-biblioentry">
|
||
<dt class="label" id="bpo-46167">[<a href="#id1">bpo-46167</a>]</dt>
|
||
<dd><a class="reference external" href="https://bugs.python.org/issue46167">https://bugs.python.org/issue46167</a></div>
|
||
<div class="citation" id="gh-30247" role="doc-biblioentry">
|
||
<dt class="label" id="gh-30247">[<a href="#id2">GH-30247</a>]</dt>
|
||
<dd><a class="reference external" href="https://github.com/python/cpython/pull/30247">https://github.com/python/cpython/pull/30247</a></div>
|
||
</div>
|
||
</section>
|
||
<section id="copyright">
|
||
<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
|
||
<p>This document is placed in the public domain or under the
|
||
CC0-1.0-Universal license, whichever is more permissive.</p>
|
||
</section>
|
||
</section>
|
||
<hr class="docutils" />
|
||
<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0679.rst">https://github.com/python/peps/blob/main/peps/pep-0679.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0679.rst">2023-09-09 17:39:29 GMT</a></p>
|
||
|
||
</article>
|
||
<nav id="pep-sidebar">
|
||
<h2>Contents</h2>
|
||
<ul>
|
||
<li><a class="reference internal" href="#abstract">Abstract</a></li>
|
||
<li><a class="reference internal" href="#motivation">Motivation</a></li>
|
||
<li><a class="reference internal" href="#rationale">Rationale</a></li>
|
||
<li><a class="reference internal" href="#specification">Specification</a></li>
|
||
<li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li>
|
||
<li><a class="reference internal" href="#security-implications">Security Implications</a></li>
|
||
<li><a class="reference internal" href="#how-to-teach-this">How to Teach This</a></li>
|
||
<li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li>
|
||
<li><a class="reference internal" href="#references">References</a></li>
|
||
<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
||
</ul>
|
||
|
||
<br>
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