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<h1>Python Enhancement Proposals</h1>
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<li><a href="https://www.python.org/" title="The Python Programming Language">Python</a> » </li>
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<li><a href="../pep-0000/">PEP Index</a> » </li>
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<h1 class="page-title">PEP 419 – Protecting cleanup statements from interruptions</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">Paul Colomiets <paul at colomiets.name></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="Inactive draft that may be taken up again at a later time">Deferred</abbr></dd>
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||
<dt class="field-odd">Type<span class="colon">:</span></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>
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||
<dt class="field-even">Created<span class="colon">:</span></dt>
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||
<dd class="field-even">06-Apr-2012</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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</dl>
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<hr class="docutils" />
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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="#pep-deferral">PEP Deferral</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a><ul>
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<li><a class="reference internal" href="#coroutine-use-case">Coroutine Use Case</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="#frame-flag-f-in-cleanup">Frame Flag ‘f_in_cleanup’</a></li>
|
||
<li><a class="reference internal" href="#function-sys-setcleanuphook">Function ‘sys.setcleanuphook’</a></li>
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||
<li><a class="reference internal" href="#inspect-module-enhancements">Inspect Module Enhancements</a></li>
|
||
</ul>
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</li>
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<li><a class="reference internal" href="#example">Example</a></li>
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<li><a class="reference internal" href="#unresolved-issues">Unresolved Issues</a><ul>
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||
<li><a class="reference internal" href="#interruption-inside-with-statement-expression">Interruption Inside With Statement Expression</a></li>
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<li><a class="reference internal" href="#exception-propagation">Exception Propagation</a></li>
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||
<li><a class="reference internal" href="#interruption-between-acquiring-resource-and-try-block">Interruption Between Acquiring Resource and Try Block</a></li>
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||
<li><a class="reference internal" href="#handling-eintr-inside-a-finally">Handling EINTR Inside a Finally</a></li>
|
||
<li><a class="reference internal" href="#setting-interruption-context-inside-finally-itself">Setting Interruption Context Inside Finally Itself</a></li>
|
||
<li><a class="reference internal" href="#modifying-keyboardinterrupt">Modifying KeyboardInterrupt</a></li>
|
||
</ul>
|
||
</li>
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||
<li><a class="reference internal" href="#alternative-python-implementations-support">Alternative Python Implementations Support</a></li>
|
||
<li><a class="reference internal" href="#alternative-names">Alternative Names</a></li>
|
||
<li><a class="reference internal" href="#alternative-proposals">Alternative Proposals</a><ul>
|
||
<li><a class="reference internal" href="#propagating-f-in-cleanup-flag-automatically">Propagating ‘f_in_cleanup’ Flag Automatically</a></li>
|
||
<li><a class="reference internal" href="#add-bytecodes-incr-cleanup-decr-cleanup">Add Bytecodes ‘INCR_CLEANUP’, ‘DECR_CLEANUP’</a></li>
|
||
<li><a class="reference internal" href="#expose-f-in-cleanup-as-a-counter">Expose ‘f_in_cleanup’ as a Counter</a></li>
|
||
<li><a class="reference internal" href="#add-code-object-flag-co-cleanup">Add code object flag ‘CO_CLEANUP’</a></li>
|
||
<li><a class="reference internal" href="#have-cleanup-callback-on-frame-object-itself">Have Cleanup Callback on Frame Object Itself</a></li>
|
||
<li><a class="reference internal" href="#no-cleanup-hook">No Cleanup Hook</a></li>
|
||
</ul>
|
||
</li>
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||
<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="abstract">
|
||
<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
|
||
<p>This PEP proposes a way to protect Python code from being interrupted
|
||
inside a finally clause or during context manager cleanup.</p>
|
||
</section>
|
||
<section id="pep-deferral">
|
||
<h2><a class="toc-backref" href="#pep-deferral" role="doc-backlink">PEP Deferral</a></h2>
|
||
<p>Further exploration of the concepts covered in this PEP has been deferred
|
||
for lack of a current champion interested in promoting the goals of the PEP
|
||
and collecting and incorporating feedback, and with sufficient available
|
||
time to do so effectively.</p>
|
||
</section>
|
||
<section id="rationale">
|
||
<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
|
||
<p>Python has two nice ways to do cleanup. One is a <code class="docutils literal notranslate"><span class="pre">finally</span></code>
|
||
statement and the other is a context manager (usually called using a
|
||
<code class="docutils literal notranslate"><span class="pre">with</span></code> statement). However, neither is protected from interruption
|
||
by <code class="docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code> or <code class="docutils literal notranslate"><span class="pre">GeneratorExit</span></code> caused by
|
||
<code class="docutils literal notranslate"><span class="pre">generator.throw()</span></code>. For example:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s1">'starting'</span><span class="p">)</span>
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||
<span class="n">do_something</span><span class="p">()</span>
|
||
<span class="k">finally</span><span class="p">:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s1">'finished'</span><span class="p">)</span>
|
||
<span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>If <code class="docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code> occurs just after the second <code class="docutils literal notranslate"><span class="pre">print()</span></code>
|
||
call, the lock will not be released. Similarly, the following code
|
||
using the <code class="docutils literal notranslate"><span class="pre">with</span></code> statement is affected:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">threading</span> <span class="kn">import</span> <span class="n">Lock</span>
|
||
|
||
<span class="k">class</span> <span class="nc">MyLock</span><span class="p">:</span>
|
||
|
||
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_lock_impl</span> <span class="o">=</span> <span class="n">Lock</span><span class="p">()</span>
|
||
|
||
<span class="k">def</span> <span class="fm">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_lock_impl</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s2">"LOCKED"</span><span class="p">)</span>
|
||
|
||
<span class="k">def</span> <span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="s2">"UNLOCKING"</span><span class="p">)</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_lock_impl</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
|
||
|
||
<span class="n">lock</span> <span class="o">=</span> <span class="n">MyLock</span><span class="p">()</span>
|
||
<span class="k">with</span> <span class="n">lock</span><span class="p">:</span>
|
||
<span class="n">do_something</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>If <code class="docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code> occurs near any of the <code class="docutils literal notranslate"><span class="pre">print()</span></code> calls, the
|
||
lock will never be released.</p>
|
||
<section id="coroutine-use-case">
|
||
<h3><a class="toc-backref" href="#coroutine-use-case" role="doc-backlink">Coroutine Use Case</a></h3>
|
||
<p>A similar case occurs with coroutines. Usually coroutine libraries
|
||
want to interrupt the coroutine with a timeout. The
|
||
<code class="docutils literal notranslate"><span class="pre">generator.throw()</span></code> method works for this use case, but there is no
|
||
way of knowing if the coroutine is currently suspended from inside a
|
||
<code class="docutils literal notranslate"><span class="pre">finally</span></code> clause.</p>
|
||
<p>An example that uses yield-based coroutines follows. The code looks
|
||
similar using any of the popular coroutine libraries Monocle <a class="footnote-reference brackets" href="#id4" id="id1">[1]</a>,
|
||
Bluelet <a class="footnote-reference brackets" href="#id5" id="id2">[2]</a>, or Twisted <a class="footnote-reference brackets" href="#id6" id="id3">[3]</a>.</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">run_locked</span><span class="p">():</span>
|
||
<span class="k">yield</span> <span class="n">connection</span><span class="o">.</span><span class="n">sendall</span><span class="p">(</span><span class="s1">'LOCK'</span><span class="p">)</span>
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="k">yield</span> <span class="n">do_something</span><span class="p">()</span>
|
||
<span class="k">yield</span> <span class="n">do_something_else</span><span class="p">()</span>
|
||
<span class="k">finally</span><span class="p">:</span>
|
||
<span class="k">yield</span> <span class="n">connection</span><span class="o">.</span><span class="n">sendall</span><span class="p">(</span><span class="s1">'UNLOCK'</span><span class="p">)</span>
|
||
|
||
<span class="k">with</span> <span class="n">timeout</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span>
|
||
<span class="k">yield</span> <span class="n">run_locked</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>In the example above, <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">something</span></code> means to pause executing the
|
||
current coroutine and to execute coroutine <code class="docutils literal notranslate"><span class="pre">something</span></code> until it
|
||
finishes execution. Therefore, the coroutine library itself needs to
|
||
maintain a stack of generators. The <code class="docutils literal notranslate"><span class="pre">connection.sendall()</span></code> call waits
|
||
until the socket is writable and does a similar thing to what
|
||
<code class="docutils literal notranslate"><span class="pre">socket.sendall()</span></code> does.</p>
|
||
<p>The <code class="docutils literal notranslate"><span class="pre">with</span></code> statement ensures that all code is executed within 5
|
||
seconds timeout. It does so by registering a callback in the main
|
||
loop, which calls <code class="docutils literal notranslate"><span class="pre">generator.throw()</span></code> on the top-most frame in the
|
||
coroutine stack when a timeout happens.</p>
|
||
<p>The <code class="docutils literal notranslate"><span class="pre">greenlets</span></code> extension works in a similar way, except that it
|
||
doesn’t need <code class="docutils literal notranslate"><span class="pre">yield</span></code> to enter a new stack frame. Otherwise
|
||
considerations are similar.</p>
|
||
</section>
|
||
</section>
|
||
<section id="specification">
|
||
<h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2>
|
||
<section id="frame-flag-f-in-cleanup">
|
||
<h3><a class="toc-backref" href="#frame-flag-f-in-cleanup" role="doc-backlink">Frame Flag ‘f_in_cleanup’</a></h3>
|
||
<p>A new flag on the frame object is proposed. It is set to <code class="docutils literal notranslate"><span class="pre">True</span></code> if
|
||
this frame is currently executing a <code class="docutils literal notranslate"><span class="pre">finally</span></code> clause. Internally,
|
||
the flag must be implemented as a counter of nested finally statements
|
||
currently being executed.</p>
|
||
<p>The internal counter also needs to be incremented during execution of
|
||
the <code class="docutils literal notranslate"><span class="pre">SETUP_WITH</span></code> and <code class="docutils literal notranslate"><span class="pre">WITH_CLEANUP</span></code> bytecodes, and decremented
|
||
when execution for these bytecodes is finished. This allows to also
|
||
protect <code class="docutils literal notranslate"><span class="pre">__enter__()</span></code> and <code class="docutils literal notranslate"><span class="pre">__exit__()</span></code> methods.</p>
|
||
</section>
|
||
<section id="function-sys-setcleanuphook">
|
||
<h3><a class="toc-backref" href="#function-sys-setcleanuphook" role="doc-backlink">Function ‘sys.setcleanuphook’</a></h3>
|
||
<p>A new function for the <code class="docutils literal notranslate"><span class="pre">sys</span></code> module is proposed. This function sets
|
||
a callback which is executed every time <code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code> becomes
|
||
false. Callbacks get a frame object as their sole argument, so that
|
||
they can figure out where they are called from.</p>
|
||
<p>The setting is thread local and must be stored in the
|
||
<code class="docutils literal notranslate"><span class="pre">PyThreadState</span></code> structure.</p>
|
||
</section>
|
||
<section id="inspect-module-enhancements">
|
||
<h3><a class="toc-backref" href="#inspect-module-enhancements" role="doc-backlink">Inspect Module Enhancements</a></h3>
|
||
<p>Two new functions are proposed for the <code class="docutils literal notranslate"><span class="pre">inspect</span></code> module:
|
||
<code class="docutils literal notranslate"><span class="pre">isframeincleanup()</span></code> and <code class="docutils literal notranslate"><span class="pre">getcleanupframe()</span></code>.</p>
|
||
<p><code class="docutils literal notranslate"><span class="pre">isframeincleanup()</span></code>, given a frame or generator object as its sole
|
||
argument, returns the value of the <code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code> attribute of a
|
||
frame itself or of the <code class="docutils literal notranslate"><span class="pre">gi_frame</span></code> attribute of a generator.</p>
|
||
<p><code class="docutils literal notranslate"><span class="pre">getcleanupframe()</span></code>, given a frame object as its sole argument,
|
||
returns the innermost frame which has a true value of
|
||
<code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code>, or <code class="docutils literal notranslate"><span class="pre">None</span></code> if no frames in the stack have a nonzero
|
||
value for that attribute. It starts to inspect from the specified
|
||
frame and walks to outer frames using <code class="docutils literal notranslate"><span class="pre">f_back</span></code> pointers, just like
|
||
<code class="docutils literal notranslate"><span class="pre">getouterframes()</span></code> does.</p>
|
||
</section>
|
||
</section>
|
||
<section id="example">
|
||
<h2><a class="toc-backref" href="#example" role="doc-backlink">Example</a></h2>
|
||
<p>An example implementation of a SIGINT handler that interrupts safely
|
||
might look like:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">inspect</span><span class="o">,</span> <span class="nn">sys</span><span class="o">,</span> <span class="nn">functools</span>
|
||
|
||
<span class="k">def</span> <span class="nf">sigint_handler</span><span class="p">(</span><span class="n">sig</span><span class="p">,</span> <span class="n">frame</span><span class="p">):</span>
|
||
<span class="k">if</span> <span class="n">inspect</span><span class="o">.</span><span class="n">getcleanupframe</span><span class="p">(</span><span class="n">frame</span><span class="p">)</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
|
||
<span class="k">raise</span> <span class="ne">KeyboardInterrupt</span><span class="p">()</span>
|
||
<span class="n">sys</span><span class="o">.</span><span class="n">setcleanuphook</span><span class="p">(</span><span class="n">functools</span><span class="o">.</span><span class="n">partial</span><span class="p">(</span><span class="n">sigint_handler</span><span class="p">,</span> <span class="mi">0</span><span class="p">))</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>A coroutine example is out of scope of this document, because its
|
||
implementation depends very much on a trampoline (or main loop) used
|
||
by coroutine library.</p>
|
||
</section>
|
||
<section id="unresolved-issues">
|
||
<h2><a class="toc-backref" href="#unresolved-issues" role="doc-backlink">Unresolved Issues</a></h2>
|
||
<section id="interruption-inside-with-statement-expression">
|
||
<h3><a class="toc-backref" href="#interruption-inside-with-statement-expression" role="doc-backlink">Interruption Inside With Statement Expression</a></h3>
|
||
<p>Given the statement</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">):</span>
|
||
<span class="n">do_something</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Python can be interrupted after <code class="docutils literal notranslate"><span class="pre">open()</span></code> is called, but before the
|
||
<code class="docutils literal notranslate"><span class="pre">SETUP_WITH</span></code> bytecode is executed. There are two possible
|
||
decisions:</p>
|
||
<ul>
|
||
<li>Protect <code class="docutils literal notranslate"><span class="pre">with</span></code> expressions. This would require another bytecode,
|
||
since currently there is no way of recognizing the start of the
|
||
<code class="docutils literal notranslate"><span class="pre">with</span></code> expression.</li>
|
||
<li>Let the user write a wrapper if he considers it important for the
|
||
use-case. A safe wrapper might look like this:<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">FileWrapper</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
|
||
|
||
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">filename</span> <span class="o">=</span> <span class="n">filename</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">mode</span> <span class="o">=</span> <span class="n">mode</span>
|
||
|
||
<span class="k">def</span> <span class="fm">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">file</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">filename</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">mode</span><span class="p">)</span>
|
||
|
||
<span class="k">def</span> <span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">file</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Alternatively it can be written using the <code class="docutils literal notranslate"><span class="pre">contextmanager()</span></code>
|
||
decorator:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nd">@contextmanager</span>
|
||
<span class="k">def</span> <span class="nf">open_wrapper</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="p">):</span>
|
||
<span class="n">file</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">mode</span><span class="p">)</span>
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="k">yield</span> <span class="n">file</span>
|
||
<span class="k">finally</span><span class="p">:</span>
|
||
<span class="n">file</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>This code is safe, as the first part of the generator (before yield)
|
||
is executed inside the <code class="docutils literal notranslate"><span class="pre">SETUP_WITH</span></code> bytecode of the caller.</p>
|
||
</li>
|
||
</ul>
|
||
</section>
|
||
<section id="exception-propagation">
|
||
<h3><a class="toc-backref" href="#exception-propagation" role="doc-backlink">Exception Propagation</a></h3>
|
||
<p>Sometimes a <code class="docutils literal notranslate"><span class="pre">finally</span></code> clause or an <code class="docutils literal notranslate"><span class="pre">__enter__()</span></code>/<code class="docutils literal notranslate"><span class="pre">__exit__()</span></code>
|
||
method can raise an exception. Usually this is not a problem, since
|
||
more important exceptions like <code class="docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code> or <code class="docutils literal notranslate"><span class="pre">SystemExit</span></code>
|
||
should be raised instead. But it may be nice to be able to keep the
|
||
original exception inside a <code class="docutils literal notranslate"><span class="pre">__context__</span></code> attribute. So the cleanup
|
||
hook signature may grow an exception argument:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">sigint_handler</span><span class="p">(</span><span class="n">sig</span><span class="p">,</span> <span class="n">frame</span><span class="p">)</span>
|
||
<span class="k">if</span> <span class="n">inspect</span><span class="o">.</span><span class="n">getcleanupframe</span><span class="p">(</span><span class="n">frame</span><span class="p">)</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
|
||
<span class="k">raise</span> <span class="ne">KeyboardInterrupt</span><span class="p">()</span>
|
||
<span class="n">sys</span><span class="o">.</span><span class="n">setcleanuphook</span><span class="p">(</span><span class="n">retry_sigint</span><span class="p">)</span>
|
||
|
||
<span class="k">def</span> <span class="nf">retry_sigint</span><span class="p">(</span><span class="n">frame</span><span class="p">,</span> <span class="n">exception</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
|
||
<span class="k">if</span> <span class="n">inspect</span><span class="o">.</span><span class="n">getcleanupframe</span><span class="p">(</span><span class="n">frame</span><span class="p">)</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
|
||
<span class="k">raise</span> <span class="ne">KeyboardInterrupt</span><span class="p">()</span> <span class="kn">from</span> <span class="nn">exception</span>
|
||
</pre></div>
|
||
</div>
|
||
<div class="admonition note">
|
||
<p class="admonition-title">Note</p>
|
||
<p>There is no need to have three arguments like in the <code class="docutils literal notranslate"><span class="pre">__exit__</span></code>
|
||
method since there is a <code class="docutils literal notranslate"><span class="pre">__traceback__</span></code> attribute in exception in
|
||
Python 3.</p>
|
||
</div>
|
||
<p>However, this will set the <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> for the exception, which is
|
||
not exactly what’s intended. So some hidden interpreter logic may be
|
||
used to put a <code class="docutils literal notranslate"><span class="pre">__context__</span></code> attribute on every exception raised in a
|
||
cleanup hook.</p>
|
||
</section>
|
||
<section id="interruption-between-acquiring-resource-and-try-block">
|
||
<h3><a class="toc-backref" href="#interruption-between-acquiring-resource-and-try-block" role="doc-backlink">Interruption Between Acquiring Resource and Try Block</a></h3>
|
||
<p>The example from the first section is not totally safe. Let’s take a
|
||
closer look:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="n">do_something</span><span class="p">()</span>
|
||
<span class="k">finally</span><span class="p">:</span>
|
||
<span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>The problem might occur if the code is interrupted just after
|
||
<code class="docutils literal notranslate"><span class="pre">lock.acquire()</span></code> is executed but before the <code class="docutils literal notranslate"><span class="pre">try</span></code> block is
|
||
entered.</p>
|
||
<p>There is no way the code can be fixed unmodified. The actual fix
|
||
depends very much on the use case. Usually code can be fixed using a
|
||
<code class="docutils literal notranslate"><span class="pre">with</span></code> statement:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">lock</span><span class="p">:</span>
|
||
<span class="n">do_something</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>However, for coroutines one usually can’t use the <code class="docutils literal notranslate"><span class="pre">with</span></code> statement
|
||
because you need to <code class="docutils literal notranslate"><span class="pre">yield</span></code> for both the acquire and release
|
||
operations. So the code might be rewritten like this:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
|
||
<span class="k">yield</span> <span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
|
||
<span class="n">do_something</span><span class="p">()</span>
|
||
<span class="k">finally</span><span class="p">:</span>
|
||
<span class="k">yield</span> <span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>The actual locking code might need more code to support this use case,
|
||
but the implementation is usually trivial, like this: check if the
|
||
lock has been acquired and unlock if it is.</p>
|
||
</section>
|
||
<section id="handling-eintr-inside-a-finally">
|
||
<h3><a class="toc-backref" href="#handling-eintr-inside-a-finally" role="doc-backlink">Handling EINTR Inside a Finally</a></h3>
|
||
<p>Even if a signal handler is prepared to check the <code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code>
|
||
flag, <code class="docutils literal notranslate"><span class="pre">InterruptedError</span></code> might be raised in the cleanup handler,
|
||
because the respective system call returned an <code class="docutils literal notranslate"><span class="pre">EINTR</span></code> error. The
|
||
primary use cases are prepared to handle this:</p>
|
||
<ul class="simple">
|
||
<li>Posix mutexes never return <code class="docutils literal notranslate"><span class="pre">EINTR</span></code></li>
|
||
<li>Networking libraries are always prepared to handle <code class="docutils literal notranslate"><span class="pre">EINTR</span></code></li>
|
||
<li>Coroutine libraries are usually interrupted with the <code class="docutils literal notranslate"><span class="pre">throw()</span></code>
|
||
method, not with a signal</li>
|
||
</ul>
|
||
<p>The platform-specific function <code class="docutils literal notranslate"><span class="pre">siginterrupt()</span></code> might be used to
|
||
remove the need to handle <code class="docutils literal notranslate"><span class="pre">EINTR</span></code>. However, it may have hardly
|
||
predictable consequences, for example <code class="docutils literal notranslate"><span class="pre">SIGINT</span></code> a handler is never
|
||
called if the main thread is stuck inside an IO routine.</p>
|
||
<p>A better approach would be to have the code, which is usually used in
|
||
cleanup handlers, be prepared to handle <code class="docutils literal notranslate"><span class="pre">InterruptedError</span></code>
|
||
explicitly. An example of such code might be a file-based lock
|
||
implementation.</p>
|
||
<p><code class="docutils literal notranslate"><span class="pre">signal.pthread_sigmask</span></code> can be used to block signals inside
|
||
cleanup handlers which can be interrupted with <code class="docutils literal notranslate"><span class="pre">EINTR</span></code>.</p>
|
||
</section>
|
||
<section id="setting-interruption-context-inside-finally-itself">
|
||
<h3><a class="toc-backref" href="#setting-interruption-context-inside-finally-itself" role="doc-backlink">Setting Interruption Context Inside Finally Itself</a></h3>
|
||
<p>Some coroutine libraries may need to set a timeout for the finally
|
||
clause itself. For example:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
|
||
<span class="n">do_something</span><span class="p">()</span>
|
||
<span class="k">finally</span><span class="p">:</span>
|
||
<span class="k">with</span> <span class="n">timeout</span><span class="p">(</span><span class="mf">0.5</span><span class="p">):</span>
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="k">yield</span> <span class="n">do_slow_cleanup</span><span class="p">()</span>
|
||
<span class="k">finally</span><span class="p">:</span>
|
||
<span class="k">yield</span> <span class="n">do_fast_cleanup</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>With current semantics, timeout will either protect the whole <code class="docutils literal notranslate"><span class="pre">with</span></code>
|
||
block or nothing at all, depending on the implementation of each
|
||
library. What the author intended is to treat <code class="docutils literal notranslate"><span class="pre">do_slow_cleanup</span></code> as
|
||
ordinary code, and <code class="docutils literal notranslate"><span class="pre">do_fast_cleanup</span></code> as a cleanup (a
|
||
non-interruptible one).</p>
|
||
<p>A similar case might occur when using greenlets or tasklets.</p>
|
||
<p>This case can be fixed by exposing <code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code> as a counter, and
|
||
by calling a cleanup hook on each decrement. A coroutine library may
|
||
then remember the value at timeout start, and compare it on each hook
|
||
execution.</p>
|
||
<p>But in practice, the example is considered to be too obscure to take
|
||
into account.</p>
|
||
</section>
|
||
<section id="modifying-keyboardinterrupt">
|
||
<h3><a class="toc-backref" href="#modifying-keyboardinterrupt" role="doc-backlink">Modifying KeyboardInterrupt</a></h3>
|
||
<p>It should be decided if the default <code class="docutils literal notranslate"><span class="pre">SIGINT</span></code> handler should be
|
||
modified to use the described mechanism. The initial proposition is
|
||
to keep old behavior, for two reasons:</p>
|
||
<ul class="simple">
|
||
<li>Most application do not care about cleanup on exit (either they do
|
||
not have external state, or they modify it in crash-safe way).</li>
|
||
<li>Cleanup may take too much time, not giving user a chance to
|
||
interrupt an application.</li>
|
||
</ul>
|
||
<p>The latter case can be fixed by allowing an unsafe break if a
|
||
<code class="docutils literal notranslate"><span class="pre">SIGINT</span></code> handler is called twice, but it seems not worth the
|
||
complexity.</p>
|
||
</section>
|
||
</section>
|
||
<section id="alternative-python-implementations-support">
|
||
<h2><a class="toc-backref" href="#alternative-python-implementations-support" role="doc-backlink">Alternative Python Implementations Support</a></h2>
|
||
<p>We consider <code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code> an implementation detail. The actual
|
||
implementation may have some fake frame-like object passed to signal
|
||
handler, cleanup hook and returned from <code class="docutils literal notranslate"><span class="pre">getcleanupframe()</span></code>. The
|
||
only requirement is that the <code class="docutils literal notranslate"><span class="pre">inspect</span></code> module functions work as
|
||
expected on these objects. For this reason, we also allow to pass a
|
||
generator object to the <code class="docutils literal notranslate"><span class="pre">isframeincleanup()</span></code> function, which removes
|
||
the need to use the <code class="docutils literal notranslate"><span class="pre">gi_frame</span></code> attribute.</p>
|
||
<p>It might be necessary to specify that <code class="docutils literal notranslate"><span class="pre">getcleanupframe()</span></code> must
|
||
return the same object that will be passed to cleanup hook at the next
|
||
invocation.</p>
|
||
</section>
|
||
<section id="alternative-names">
|
||
<h2><a class="toc-backref" href="#alternative-names" role="doc-backlink">Alternative Names</a></h2>
|
||
<p>The original proposal had a <code class="docutils literal notranslate"><span class="pre">f_in_finally</span></code> frame attribute, as the
|
||
original intention was to protect <code class="docutils literal notranslate"><span class="pre">finally</span></code> clauses. But as it grew
|
||
up to protecting <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> and <code class="docutils literal notranslate"><span class="pre">__exit__</span></code> methods too, the
|
||
<code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code> name seems better. Although the <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> method
|
||
is not a cleanup routine, it at least relates to cleanup done by
|
||
context managers.</p>
|
||
<p><code class="docutils literal notranslate"><span class="pre">setcleanuphook</span></code>, <code class="docutils literal notranslate"><span class="pre">isframeincleanup</span></code> and <code class="docutils literal notranslate"><span class="pre">getcleanupframe</span></code> can
|
||
be unobscured to <code class="docutils literal notranslate"><span class="pre">set_cleanup_hook</span></code>, <code class="docutils literal notranslate"><span class="pre">is_frame_in_cleanup</span></code> and
|
||
<code class="docutils literal notranslate"><span class="pre">get_cleanup_frame</span></code>, although they follow the naming convention of
|
||
their respective modules.</p>
|
||
</section>
|
||
<section id="alternative-proposals">
|
||
<h2><a class="toc-backref" href="#alternative-proposals" role="doc-backlink">Alternative Proposals</a></h2>
|
||
<section id="propagating-f-in-cleanup-flag-automatically">
|
||
<h3><a class="toc-backref" href="#propagating-f-in-cleanup-flag-automatically" role="doc-backlink">Propagating ‘f_in_cleanup’ Flag Automatically</a></h3>
|
||
<p>This can make <code class="docutils literal notranslate"><span class="pre">getcleanupframe()</span></code> unnecessary. But for yield-based
|
||
coroutines you need to propagate it yourself. Making it writable
|
||
leads to somewhat unpredictable behavior of <code class="docutils literal notranslate"><span class="pre">setcleanuphook()</span></code>.</p>
|
||
</section>
|
||
<section id="add-bytecodes-incr-cleanup-decr-cleanup">
|
||
<h3><a class="toc-backref" href="#add-bytecodes-incr-cleanup-decr-cleanup" role="doc-backlink">Add Bytecodes ‘INCR_CLEANUP’, ‘DECR_CLEANUP’</a></h3>
|
||
<p>These bytecodes can be used to protect the expression inside the
|
||
<code class="docutils literal notranslate"><span class="pre">with</span></code> statement, as well as making counter increments more explicit
|
||
and easy to debug (visible inside a disassembly). Some middle ground
|
||
might be chosen, like <code class="docutils literal notranslate"><span class="pre">END_FINALLY</span></code> and <code class="docutils literal notranslate"><span class="pre">SETUP_WITH</span></code> implicitly
|
||
decrementing the counter (<code class="docutils literal notranslate"><span class="pre">END_FINALLY</span></code> is present at end of every
|
||
<code class="docutils literal notranslate"><span class="pre">with</span></code> suite).</p>
|
||
<p>However, adding new bytecodes must be considered very carefully.</p>
|
||
</section>
|
||
<section id="expose-f-in-cleanup-as-a-counter">
|
||
<h3><a class="toc-backref" href="#expose-f-in-cleanup-as-a-counter" role="doc-backlink">Expose ‘f_in_cleanup’ as a Counter</a></h3>
|
||
<p>The original intention was to expose a minimum of needed
|
||
functionality. However, as we consider the frame flag
|
||
<code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code> an implementation detail, we may expose it as a
|
||
counter.</p>
|
||
<p>Similarly, if we have a counter we may need to have the cleanup hook
|
||
called on every counter decrement. It’s unlikely to have much
|
||
performance impact as nested finally clauses are an uncommon case.</p>
|
||
</section>
|
||
<section id="add-code-object-flag-co-cleanup">
|
||
<h3><a class="toc-backref" href="#add-code-object-flag-co-cleanup" role="doc-backlink">Add code object flag ‘CO_CLEANUP’</a></h3>
|
||
<p>As an alternative to set the flag inside the <code class="docutils literal notranslate"><span class="pre">SETUP_WITH</span></code> and
|
||
<code class="docutils literal notranslate"><span class="pre">WITH_CLEANUP</span></code> bytecodes, we can introduce a flag <code class="docutils literal notranslate"><span class="pre">CO_CLEANUP</span></code>.
|
||
When the interpreter starts to execute code with <code class="docutils literal notranslate"><span class="pre">CO_CLEANUP</span></code> set,
|
||
it sets <code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code> for the whole function body. This flag is
|
||
set for code objects of <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> and <code class="docutils literal notranslate"><span class="pre">__exit__</span></code> special
|
||
methods. Technically it might be set on functions called
|
||
<code class="docutils literal notranslate"><span class="pre">__enter__</span></code> and <code class="docutils literal notranslate"><span class="pre">__exit__</span></code>.</p>
|
||
<p>This seems to be less clear solution. It also covers the case where
|
||
<code class="docutils literal notranslate"><span class="pre">__enter__</span></code> and <code class="docutils literal notranslate"><span class="pre">__exit__</span></code> are called manually. This may be
|
||
accepted either as a feature or as an unnecessary side-effect (or,
|
||
though unlikely, as a bug).</p>
|
||
<p>It may also impose a problem when <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> or <code class="docutils literal notranslate"><span class="pre">__exit__</span></code>
|
||
functions are implemented in C, as there is no code object to check
|
||
for the <code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code> flag.</p>
|
||
</section>
|
||
<section id="have-cleanup-callback-on-frame-object-itself">
|
||
<h3><a class="toc-backref" href="#have-cleanup-callback-on-frame-object-itself" role="doc-backlink">Have Cleanup Callback on Frame Object Itself</a></h3>
|
||
<p>The frame object may be extended to have a <code class="docutils literal notranslate"><span class="pre">f_cleanup_callback</span></code>
|
||
member which is called when <code class="docutils literal notranslate"><span class="pre">f_in_cleanup</span></code> is reset to 0. This
|
||
would help to register different callbacks to different coroutines.</p>
|
||
<p>Despite its apparent beauty, this solution doesn’t add anything, as
|
||
the two primary use cases are:</p>
|
||
<ul class="simple">
|
||
<li>Setting the callback in a signal handler. The callback is
|
||
inherently a single one for this case.</li>
|
||
<li>Use a single callback per loop for the coroutine use case. Here, in
|
||
almost all cases, there is only one loop per thread.</li>
|
||
</ul>
|
||
</section>
|
||
<section id="no-cleanup-hook">
|
||
<h3><a class="toc-backref" href="#no-cleanup-hook" role="doc-backlink">No Cleanup Hook</a></h3>
|
||
<p>The original proposal included no cleanup hook specification, as there
|
||
are a few ways to achieve the same using current tools:</p>
|
||
<ul class="simple">
|
||
<li>Using <code class="docutils literal notranslate"><span class="pre">sys.settrace()</span></code> and the <code class="docutils literal notranslate"><span class="pre">f_trace</span></code> callback. This may
|
||
impose some problem to debugging, and has a big performance impact
|
||
(although interrupting doesn’t happen very often).</li>
|
||
<li>Sleeping a bit more and trying again. For a coroutine library this
|
||
is easy. For signals it may be achieved using <code class="docutils literal notranslate"><span class="pre">signal.alert</span></code>.</li>
|
||
</ul>
|
||
<p>Both methods are considered too impractical and a way to catch exit
|
||
from <code class="docutils literal notranslate"><span class="pre">finally</span></code> clauses is proposed.</p>
|
||
</section>
|
||
</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="id4" role="doc-footnote">
|
||
<dt class="label" id="id4">[<a href="#id1">1</a>]</dt>
|
||
<dd>Monocle
|
||
<a class="reference external" href="https://github.com/saucelabs/monocle">https://github.com/saucelabs/monocle</a></aside>
|
||
<aside class="footnote brackets" id="id5" role="doc-footnote">
|
||
<dt class="label" id="id5">[<a href="#id2">2</a>]</dt>
|
||
<dd>Bluelet
|
||
<a class="reference external" href="https://github.com/sampsyo/bluelet">https://github.com/sampsyo/bluelet</a></aside>
|
||
<aside class="footnote brackets" id="id6" role="doc-footnote">
|
||
<dt class="label" id="id6">[<a href="#id3">3</a>]</dt>
|
||
<dd>Twisted: inlineCallbacks
|
||
<a class="reference external" href="https://twisted.org/documents/8.1.0/api/twisted.internet.defer.html">https://twisted.org/documents/8.1.0/api/twisted.internet.defer.html</a></aside>
|
||
</aside>
|
||
<p>[4] Original discussion
|
||
<a class="reference external" href="https://mail.python.org/pipermail/python-ideas/2012-April/014705.html">https://mail.python.org/pipermail/python-ideas/2012-April/014705.html</a></p>
|
||
<p>[5] Implementation of PEP 419
|
||
<a class="reference external" href="https://github.com/python/cpython/issues/58935">https://github.com/python/cpython/issues/58935</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-0419.rst">https://github.com/python/peps/blob/main/peps/pep-0419.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0419.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="#pep-deferral">PEP Deferral</a></li>
|
||
<li><a class="reference internal" href="#rationale">Rationale</a><ul>
|
||
<li><a class="reference internal" href="#coroutine-use-case">Coroutine Use Case</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#specification">Specification</a><ul>
|
||
<li><a class="reference internal" href="#frame-flag-f-in-cleanup">Frame Flag ‘f_in_cleanup’</a></li>
|
||
<li><a class="reference internal" href="#function-sys-setcleanuphook">Function ‘sys.setcleanuphook’</a></li>
|
||
<li><a class="reference internal" href="#inspect-module-enhancements">Inspect Module Enhancements</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#example">Example</a></li>
|
||
<li><a class="reference internal" href="#unresolved-issues">Unresolved Issues</a><ul>
|
||
<li><a class="reference internal" href="#interruption-inside-with-statement-expression">Interruption Inside With Statement Expression</a></li>
|
||
<li><a class="reference internal" href="#exception-propagation">Exception Propagation</a></li>
|
||
<li><a class="reference internal" href="#interruption-between-acquiring-resource-and-try-block">Interruption Between Acquiring Resource and Try Block</a></li>
|
||
<li><a class="reference internal" href="#handling-eintr-inside-a-finally">Handling EINTR Inside a Finally</a></li>
|
||
<li><a class="reference internal" href="#setting-interruption-context-inside-finally-itself">Setting Interruption Context Inside Finally Itself</a></li>
|
||
<li><a class="reference internal" href="#modifying-keyboardinterrupt">Modifying KeyboardInterrupt</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#alternative-python-implementations-support">Alternative Python Implementations Support</a></li>
|
||
<li><a class="reference internal" href="#alternative-names">Alternative Names</a></li>
|
||
<li><a class="reference internal" href="#alternative-proposals">Alternative Proposals</a><ul>
|
||
<li><a class="reference internal" href="#propagating-f-in-cleanup-flag-automatically">Propagating ‘f_in_cleanup’ Flag Automatically</a></li>
|
||
<li><a class="reference internal" href="#add-bytecodes-incr-cleanup-decr-cleanup">Add Bytecodes ‘INCR_CLEANUP’, ‘DECR_CLEANUP’</a></li>
|
||
<li><a class="reference internal" href="#expose-f-in-cleanup-as-a-counter">Expose ‘f_in_cleanup’ as a Counter</a></li>
|
||
<li><a class="reference internal" href="#add-code-object-flag-co-cleanup">Add code object flag ‘CO_CLEANUP’</a></li>
|
||
<li><a class="reference internal" href="#have-cleanup-callback-on-frame-object-itself">Have Cleanup Callback on Frame Object Itself</a></li>
|
||
<li><a class="reference internal" href="#no-cleanup-hook">No Cleanup Hook</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#references">References</a></li>
|
||
<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
||
</ul>
|
||
|
||
<br>
|
||
<a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0419.rst">Page Source (GitHub)</a>
|
||
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