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<h1 class="page-title">PEP 3152 – Cofunctions</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">Gregory Ewing <greg.ewing at canterbury.ac.nz></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Formally declined and will not be accepted">Rejected</abbr></dd>
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<dt class="field-odd">Type<span class="colon">:</span></dt>
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<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>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">13-Feb-2009</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">3.3</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even"><p></p></dd>
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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><ul>
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<li><a class="reference internal" href="#rejection">Rejection</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#specification">Specification</a><ul>
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<li><a class="reference internal" href="#cofunction-definitions">Cofunction definitions</a></li>
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<li><a class="reference internal" href="#cocalls">Cocalls</a></li>
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<li><a class="reference internal" href="#new-builtins-attributes-and-c-api-functions">New builtins, attributes and C API functions</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#motivation-and-rationale">Motivation and Rationale</a></li>
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<li><a class="reference internal" href="#prototype-implementation">Prototype Implementation</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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</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>A syntax is proposed for defining and calling a special type of
|
||
generator called a ‘cofunction’. It is designed to provide a
|
||
streamlined way of writing generator-based coroutines, and allow the
|
||
early detection of certain kinds of error that are easily made when
|
||
writing such code, which otherwise tend to cause hard-to-diagnose
|
||
symptoms.</p>
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<p>This proposal builds on the ‘yield from’ mechanism described in PEP
|
||
380, and describes some of the semantics of cofunctions in terms of
|
||
it. However, it would be possible to define and implement cofunctions
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||
independently of <a class="pep reference internal" href="../pep-0380/" title="PEP 380 – Syntax for Delegating to a Subgenerator">PEP 380</a> if so desired.</p>
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<section id="rejection">
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<h3><a class="toc-backref" href="#rejection" role="doc-backlink">Rejection</a></h3>
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<p>See <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2015-April/139503.html">https://mail.python.org/pipermail/python-dev/2015-April/139503.html</a></p>
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||
</section>
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</section>
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<section id="specification">
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<h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2>
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<section id="cofunction-definitions">
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<h3><a class="toc-backref" href="#cofunction-definitions" role="doc-backlink">Cofunction definitions</a></h3>
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<p>A new keyword <code class="docutils literal notranslate"><span class="pre">codef</span></code> is introduced which is used in place of
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||
<code class="docutils literal notranslate"><span class="pre">def</span></code> to define a cofunction. A cofunction is a special kind of
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generator having the following characteristics:</p>
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<ol class="arabic simple">
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||
<li>A cofunction is always a generator, even if it does not contain any
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||
<code class="docutils literal notranslate"><span class="pre">yield</span></code> or <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code> expressions.</li>
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<li>A cofunction cannot be called the same way as an ordinary function.
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An exception is raised if an ordinary call to a cofunction is
|
||
attempted.</li>
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||
</ol>
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||
</section>
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||
<section id="cocalls">
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<h3><a class="toc-backref" href="#cocalls" role="doc-backlink">Cocalls</a></h3>
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||
<p>Calls from one cofunction to another are made by marking the call with
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||
a new keyword <code class="docutils literal notranslate"><span class="pre">cocall</span></code>. The expression</p>
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||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">cocall</span> <span class="n">f</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">)</span>
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||
</pre></div>
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||
</div>
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||
<p>is semantically equivalent to</p>
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||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">yield from</span> <span class="n">f</span><span class="o">.</span><span class="n">__cocall__</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">)</span>
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||
</pre></div>
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||
</div>
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||
<p>except that the object returned by __cocall__ is expected to be an
|
||
iterator, so the step of calling iter() on it is skipped.</p>
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||
<p>The full syntax of a cocall expression is described by the following
|
||
grammar lines:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">atom</span><span class="p">:</span> <span class="n">cocall</span> <span class="o">|</span> <span class="o"><</span><span class="n">existing</span> <span class="n">alternatives</span> <span class="k">for</span> <span class="n">atom</span><span class="o">></span>
|
||
<span class="n">cocall</span><span class="p">:</span> <span class="s1">'cocall'</span> <span class="n">atom</span> <span class="n">cotrailer</span><span class="o">*</span> <span class="s1">'('</span> <span class="p">[</span><span class="n">arglist</span><span class="p">]</span> <span class="s1">')'</span>
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||
<span class="n">cotrailer</span><span class="p">:</span> <span class="s1">'['</span> <span class="n">subscriptlist</span> <span class="s1">']'</span> <span class="o">|</span> <span class="s1">'.'</span> <span class="n">NAME</span>
|
||
</pre></div>
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||
</div>
|
||
<p>The <code class="docutils literal notranslate"><span class="pre">cocall</span></code> keyword is syntactically valid only inside a
|
||
cofunction. A SyntaxError will result if it is used in any other
|
||
context.</p>
|
||
<p>Objects which implement __cocall__ are expected to return an object
|
||
obeying the iterator protocol. Cofunctions respond to __cocall__ the
|
||
same way as ordinary generator functions respond to __call__, i.e. by
|
||
returning a generator-iterator.</p>
|
||
<p>Certain objects that wrap other callable objects, notably bound
|
||
methods, will be given __cocall__ implementations that delegate to the
|
||
underlying object.</p>
|
||
</section>
|
||
<section id="new-builtins-attributes-and-c-api-functions">
|
||
<h3><a class="toc-backref" href="#new-builtins-attributes-and-c-api-functions" role="doc-backlink">New builtins, attributes and C API functions</a></h3>
|
||
<p>To facilitate interfacing cofunctions with non-coroutine code, there will
|
||
be a built-in function <code class="docutils literal notranslate"><span class="pre">costart</span></code> whose definition is equivalent to</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">costart</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span>
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||
<span class="k">return</span> <span class="n">obj</span><span class="o">.</span><span class="n">__cocall__</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">)</span>
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||
</pre></div>
|
||
</div>
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||
<p>There will also be a corresponding C API function</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">PyObject</span> <span class="o">*</span><span class="n">PyObject_CoCall</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">obj</span><span class="p">,</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">kwds</span><span class="p">)</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>It is left unspecified for now whether a cofunction is a distinct type
|
||
of object or, like a generator function, is simply a specially-marked
|
||
function instance. If the latter, a read-only boolean attribute
|
||
<code class="docutils literal notranslate"><span class="pre">__iscofunction__</span></code> should be provided to allow testing whether a
|
||
given function object is a cofunction.</p>
|
||
</section>
|
||
</section>
|
||
<section id="motivation-and-rationale">
|
||
<h2><a class="toc-backref" href="#motivation-and-rationale" role="doc-backlink">Motivation and Rationale</a></h2>
|
||
<p>The <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code> syntax is reasonably self-explanatory when used for
|
||
the purpose of delegating part of the work of a generator to another
|
||
function. It can also be used to good effect in the implementation of
|
||
generator-based coroutines, but it reads somewhat awkwardly when used
|
||
for that purpose, and tends to obscure the true intent of the code.</p>
|
||
<p>Furthermore, using generators as coroutines is somewhat error-prone.
|
||
If one forgets to use <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code> when it should have been used, or
|
||
uses it when it shouldn’t have, the symptoms that result can be
|
||
obscure and confusing.</p>
|
||
<p>Finally, sometimes there is a need for a function to be a coroutine
|
||
even though it does not yield anything, and in these cases it is
|
||
necessary to resort to kludges such as <code class="docutils literal notranslate"><span class="pre">if</span> <span class="pre">0:</span> <span class="pre">yield</span></code> to force it to
|
||
be a generator.</p>
|
||
<p>The <code class="docutils literal notranslate"><span class="pre">codef</span></code> and <code class="docutils literal notranslate"><span class="pre">cocall</span></code> constructs address the first issue by
|
||
making the syntax directly reflect the intent, that is, that the
|
||
function forms part of a coroutine.</p>
|
||
<p>The second issue is addressed by making it impossible to mix coroutine
|
||
and non-coroutine code in ways that don’t make sense. If the rules
|
||
are violated, an exception is raised that points out exactly what and
|
||
where the problem is.</p>
|
||
<p>Lastly, the need for dummy yields is eliminated by making the form of
|
||
definition determine whether the function is a coroutine, rather than
|
||
what it contains.</p>
|
||
</section>
|
||
<section id="prototype-implementation">
|
||
<h2><a class="toc-backref" href="#prototype-implementation" role="doc-backlink">Prototype Implementation</a></h2>
|
||
<p>An implementation in the form of patches to Python 3.1.2 can be found
|
||
here:</p>
|
||
<p><a class="reference external" href="http://www.cosc.canterbury.ac.nz/greg.ewing/python/generators/cofunctions.html">http://www.cosc.canterbury.ac.nz/greg.ewing/python/generators/cofunctions.html</a></p>
|
||
</section>
|
||
<section id="copyright">
|
||
<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
|
||
<p>This document has been placed in the public domain.</p>
|
||
</section>
|
||
</section>
|
||
<hr class="docutils" />
|
||
<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-3152.rst">https://github.com/python/peps/blob/main/peps/pep-3152.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-3152.rst">2023-09-09 17:39:29 GMT</a></p>
|
||
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</article>
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<nav id="pep-sidebar">
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<h2>Contents</h2>
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<ul>
|
||
<li><a class="reference internal" href="#abstract">Abstract</a><ul>
|
||
<li><a class="reference internal" href="#rejection">Rejection</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#specification">Specification</a><ul>
|
||
<li><a class="reference internal" href="#cofunction-definitions">Cofunction definitions</a></li>
|
||
<li><a class="reference internal" href="#cocalls">Cocalls</a></li>
|
||
<li><a class="reference internal" href="#new-builtins-attributes-and-c-api-functions">New builtins, attributes and C API functions</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#motivation-and-rationale">Motivation and Rationale</a></li>
|
||
<li><a class="reference internal" href="#prototype-implementation">Prototype Implementation</a></li>
|
||
<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
||
</ul>
|
||
|
||
<br>
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<a id="source" href="https://github.com/python/peps/blob/main/peps/pep-3152.rst">Page Source (GitHub)</a>
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