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<h1 class="page-title">PEP 319 – Python Synchronize/Asynchronize Block</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">Michel Pelletier <michel at users.sourceforge.net></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">24-Feb-2003</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.4</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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</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="#pronouncement">Pronouncement</a></li>
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<li><a class="reference internal" href="#synchronization-targets">Synchronization Targets</a></li>
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<li><a class="reference internal" href="#other-patterns-that-synchronize">Other Patterns that Synchronize</a></li>
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<li><a class="reference internal" href="#formal-syntax">Formal Syntax</a></li>
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<li><a class="reference internal" href="#proposed-implementation">Proposed Implementation</a></li>
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<li><a class="reference internal" href="#backward-compatibility">Backward Compatibility</a></li>
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<li><a class="reference internal" href="#pep-310-reliable-acquisition-release-pairs">PEP 310 Reliable Acquisition/Release Pairs</a></li>
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<li><a class="reference internal" href="#how-java-does-it">How Java Does It</a></li>
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<li><a class="reference internal" href="#how-jython-does-it">How Jython Does It</a></li>
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<li><a class="reference internal" href="#summary-of-proposed-changes-to-python">Summary of Proposed Changes to Python</a></li>
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<li><a class="reference internal" href="#risks">Risks</a></li>
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<li><a class="reference internal" href="#dissenting-opinion">Dissenting Opinion</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>
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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>This PEP proposes adding two new keywords to Python, ‘synchronize’
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and ‘asynchronize’.</p>
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</section>
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<section id="pronouncement">
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<h2><a class="toc-backref" href="#pronouncement" role="doc-backlink">Pronouncement</a></h2>
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<p>This PEP is rejected in favor of <a class="pep reference internal" href="../pep-0343/" title="PEP 343 – The “with” Statement">PEP 343</a>.</p>
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<dl>
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<dt>The ‘synchronize’ Keyword</dt><dd>The concept of code synchronization in Python is too low-level.
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To synchronize code a programmer must be aware of the details of
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the following pseudo-code pattern:<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">initialize_lock</span><span class="p">()</span>
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<span class="o">...</span>
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<span class="n">acquire_lock</span><span class="p">()</span>
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<span class="k">try</span><span class="p">:</span>
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<span class="n">change_shared_data</span><span class="p">()</span>
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<span class="k">finally</span><span class="p">:</span>
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<span class="n">release_lock</span><span class="p">()</span>
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</pre></div>
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</div>
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<p>This synchronized block pattern is not the only pattern (more
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discussed below) but it is very common. This PEP proposes
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replacing the above code with the following equivalent:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">synchronize</span><span class="p">:</span>
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<span class="n">change_shared_data</span><span class="p">()</span>
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</pre></div>
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</div>
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<p>The advantages of this scheme are simpler syntax and less room for
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user error. Currently users are required to write code about
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acquiring and releasing thread locks in ‘try/finally’ blocks;
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errors in this code can cause notoriously difficult concurrent
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thread locking issues.</p>
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</dd>
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<dt>The ‘asynchronize’ Keyword</dt><dd>While executing a ‘synchronize’ block of code a programmer may
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want to “drop back” to running asynchronously momentarily to run
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blocking input/output routines or something else that might take an
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indeterminate amount of time and does not require synchronization.
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This code usually follows the pattern:<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">initialize_lock</span><span class="p">()</span>
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<span class="o">...</span>
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<span class="n">acquire_lock</span><span class="p">()</span>
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<span class="k">try</span><span class="p">:</span>
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<span class="n">change_shared_data</span><span class="p">()</span>
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<span class="n">release_lock</span><span class="p">()</span> <span class="c1"># become async</span>
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<span class="n">do_blocking_io</span><span class="p">()</span>
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<span class="n">acquire_lock</span><span class="p">()</span> <span class="c1"># sync again</span>
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<span class="n">change_shared_data2</span><span class="p">()</span>
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<span class="k">finally</span><span class="p">:</span>
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||
<span class="n">release_lock</span><span class="p">()</span>
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</pre></div>
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</div>
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<p>The asynchronous section of the code is not very obvious visually,
|
||
so it is marked up with comments. Using the proposed
|
||
‘asynchronize’ keyword this code becomes much cleaner, easier to
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||
understand, and less prone to error:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">synchronize</span><span class="p">:</span>
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<span class="n">change_shared_data</span><span class="p">()</span>
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||
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<span class="n">asynchronize</span><span class="p">:</span>
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<span class="n">do_blocking_io</span><span class="p">()</span>
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||
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<span class="n">change_shared_data2</span><span class="p">()</span>
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||
</pre></div>
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</div>
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<p>Encountering an ‘asynchronize’ keyword inside a non-synchronized
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||
block can raise either an error or issue a warning (as all code
|
||
blocks are implicitly asynchronous anyway). It is important to
|
||
note that the above example is <strong>not</strong> the same as:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">synchronize</span><span class="p">:</span>
|
||
<span class="n">change_shared_data</span><span class="p">()</span>
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||
|
||
<span class="n">do_blocking_io</span><span class="p">()</span>
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||
|
||
<span class="n">synchronize</span><span class="p">:</span>
|
||
<span class="n">change_shared_data2</span><span class="p">()</span>
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||
</pre></div>
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||
</div>
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<p>Because both synchronized blocks of code may be running inside the
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same iteration of a loop, Consider:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">while</span> <span class="n">in_main_loop</span><span class="p">():</span>
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<span class="n">synchronize</span><span class="p">:</span>
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<span class="n">change_shared_data</span><span class="p">()</span>
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<span class="n">asynchronize</span><span class="p">:</span>
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<span class="n">do_blocking_io</span><span class="p">()</span>
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<span class="n">change_shared_data2</span><span class="p">()</span>
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</pre></div>
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</div>
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<p>Many threads may be looping through this code. Without the
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||
‘asynchronize’ keyword one thread cannot stay in the loop and
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release the lock at the same time while blocking IO is going on.
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This pattern of releasing locks inside a main loop to do blocking
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IO is used extensively inside the CPython interpreter itself.</p>
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</dd>
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</dl>
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</section>
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<section id="synchronization-targets">
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<h2><a class="toc-backref" href="#synchronization-targets" role="doc-backlink">Synchronization Targets</a></h2>
|
||
<p>As proposed the ‘synchronize’ and ‘asynchronize’ keywords
|
||
synchronize a block of code. However programmers may want to
|
||
specify a target object that threads synchronize on. Any object
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||
can be a synchronization target.</p>
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||
<p>Consider a two-way queue object: two different objects are used by
|
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the same ‘synchronize’ code block to synchronize both queues
|
||
separately in the ‘get’ method:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">TwoWayQueue</span><span class="p">:</span>
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<span class="k">def</span><span class="w"> </span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">front</span> <span class="o">=</span> <span class="p">[]</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">rear</span> <span class="o">=</span> <span class="p">[]</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">putFront</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">item</span><span class="p">):</span>
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||
<span class="bp">self</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">front</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">getFront</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
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<span class="n">item</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">front</span><span class="p">)</span>
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<span class="k">return</span> <span class="n">item</span>
|
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|
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<span class="k">def</span><span class="w"> </span><span class="nf">putRear</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">item</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">rear</span><span class="p">)</span>
|
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|
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<span class="k">def</span><span class="w"> </span><span class="nf">getRear</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
|
||
<span class="n">item</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">rear</span><span class="p">)</span>
|
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<span class="k">return</span> <span class="n">item</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">put</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">item</span><span class="p">,</span> <span class="n">queue</span><span class="p">):</span>
|
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<span class="n">synchronize</span> <span class="n">queue</span><span class="p">:</span>
|
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<span class="n">queue</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">item</span><span class="p">)</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">get</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">queue</span><span class="p">):</span>
|
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<span class="n">synchronize</span> <span class="n">queue</span><span class="p">:</span>
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<span class="n">item</span> <span class="o">=</span> <span class="n">queue</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
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||
<span class="k">del</span> <span class="n">queue</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
|
||
<span class="k">return</span> <span class="n">item</span>
|
||
</pre></div>
|
||
</div>
|
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<p>Here is the equivalent code in Python as it is now without a
|
||
‘synchronize’ keyword:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">thread</span>
|
||
|
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<span class="k">class</span><span class="w"> </span><span class="nc">LockableQueue</span><span class="p">:</span>
|
||
|
||
<span class="k">def</span><span class="w"> </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">queue</span> <span class="o">=</span> <span class="p">[]</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">lock</span> <span class="o">=</span> <span class="n">thread</span><span class="o">.</span><span class="n">allocate_lock</span><span class="p">()</span>
|
||
|
||
<span class="k">class</span><span class="w"> </span><span class="nc">TwoWayQueue</span><span class="p">:</span>
|
||
<span class="k">def</span><span class="w"> </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">front</span> <span class="o">=</span> <span class="n">LockableQueue</span><span class="p">()</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">rear</span> <span class="o">=</span> <span class="n">LockableQueue</span><span class="p">()</span>
|
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|
||
<span class="k">def</span><span class="w"> </span><span class="nf">putFront</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">item</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">front</span><span class="p">)</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">getFront</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
|
||
<span class="n">item</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">front</span><span class="p">)</span>
|
||
<span class="k">return</span> <span class="n">item</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">putRear</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">item</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">rear</span><span class="p">)</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">getRear</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
|
||
<span class="n">item</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">rear</span><span class="p">)</span>
|
||
<span class="k">return</span> <span class="n">item</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">put</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">item</span><span class="p">,</span> <span class="n">queue</span><span class="p">):</span>
|
||
<span class="n">queue</span><span class="o">.</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">queue</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">item</span><span class="p">)</span>
|
||
<span class="k">finally</span><span class="p">:</span>
|
||
<span class="n">queue</span><span class="o">.</span><span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">get</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">queue</span><span class="p">):</span>
|
||
<span class="n">queue</span><span class="o">.</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">item</span> <span class="o">=</span> <span class="n">queue</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
|
||
<span class="k">del</span> <span class="n">queue</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
|
||
<span class="k">return</span> <span class="n">item</span>
|
||
<span class="k">finally</span><span class="p">:</span>
|
||
<span class="n">queue</span><span class="o">.</span><span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>The last example had to define an extra class to associate a lock
|
||
with the queue where the first example the ‘synchronize’ keyword
|
||
does this association internally and transparently.</p>
|
||
</section>
|
||
<section id="other-patterns-that-synchronize">
|
||
<h2><a class="toc-backref" href="#other-patterns-that-synchronize" role="doc-backlink">Other Patterns that Synchronize</a></h2>
|
||
<p>There are some situations where the ‘synchronize’ and
|
||
‘asynchronize’ keywords cannot entirely replace the use of lock
|
||
methods like <code class="docutils literal notranslate"><span class="pre">acquire</span></code> and <code class="docutils literal notranslate"><span class="pre">release</span></code>. Some examples are if the
|
||
programmer wants to provide arguments for <code class="docutils literal notranslate"><span class="pre">acquire</span></code> or if a lock
|
||
is acquired in one code block but released in another, as shown
|
||
below.</p>
|
||
<p>Here is a class from Zope modified to use both the ‘synchronize’
|
||
and ‘asynchronize’ keywords and also uses a pool of explicit locks
|
||
that are acquired and released in different code blocks and thus
|
||
don’t use ‘synchronize’:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">thread</span>
|
||
<span class="kn">from</span><span class="w"> </span><span class="nn">ZServerPublisher</span><span class="w"> </span><span class="kn">import</span> <span class="n">ZServerPublisher</span>
|
||
|
||
<span class="k">class</span><span class="w"> </span><span class="nc">ZRendevous</span><span class="p">:</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">n</span><span class="o">=</span><span class="mi">1</span><span class="p">):</span>
|
||
<span class="n">pool</span><span class="o">=</span><span class="p">[]</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_lists</span><span class="o">=</span><span class="n">pool</span><span class="p">,</span> <span class="p">[],</span> <span class="p">[]</span>
|
||
|
||
<span class="n">synchronize</span><span class="p">:</span>
|
||
<span class="k">while</span> <span class="n">n</span> <span class="o">></span> <span class="mi">0</span><span class="p">:</span>
|
||
<span class="n">l</span><span class="o">=</span><span class="n">thread</span><span class="o">.</span><span class="n">allocate_lock</span><span class="p">()</span>
|
||
<span class="n">l</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
|
||
<span class="n">pool</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">l</span><span class="p">)</span>
|
||
<span class="n">thread</span><span class="o">.</span><span class="n">start_new_thread</span><span class="p">(</span><span class="n">ZServerPublisher</span><span class="p">,</span>
|
||
<span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">accept</span><span class="p">,))</span>
|
||
<span class="n">n</span><span class="o">=</span><span class="n">n</span><span class="o">-</span><span class="mi">1</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">accept</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
|
||
<span class="n">synchronize</span><span class="p">:</span>
|
||
<span class="n">pool</span><span class="p">,</span> <span class="n">requests</span><span class="p">,</span> <span class="n">ready</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_lists</span>
|
||
<span class="k">while</span> <span class="ow">not</span> <span class="n">requests</span><span class="p">:</span>
|
||
<span class="n">l</span><span class="o">=</span><span class="n">pool</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
|
||
<span class="k">del</span> <span class="n">pool</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
|
||
<span class="n">ready</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">l</span><span class="p">)</span>
|
||
|
||
<span class="n">asynchronize</span><span class="p">:</span>
|
||
<span class="n">l</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
|
||
|
||
<span class="n">pool</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">l</span><span class="p">)</span>
|
||
|
||
<span class="n">r</span><span class="o">=</span><span class="n">requests</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
|
||
<span class="k">del</span> <span class="n">requests</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
|
||
<span class="k">return</span> <span class="n">r</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">handle</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">request</span><span class="p">,</span> <span class="n">response</span><span class="p">):</span>
|
||
<span class="n">synchronize</span><span class="p">:</span>
|
||
<span class="n">pool</span><span class="p">,</span> <span class="n">requests</span><span class="p">,</span> <span class="n">ready</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_lists</span>
|
||
<span class="n">requests</span><span class="o">.</span><span class="n">append</span><span class="p">((</span><span class="n">name</span><span class="p">,</span> <span class="n">request</span><span class="p">,</span> <span class="n">response</span><span class="p">))</span>
|
||
<span class="k">if</span> <span class="n">ready</span><span class="p">:</span>
|
||
<span class="n">l</span><span class="o">=</span><span class="n">ready</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
|
||
<span class="k">del</span> <span class="n">ready</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
|
||
<span class="n">l</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Here is the original class as found in the
|
||
‘Zope/ZServer/PubCore/ZRendevous.py’ module. The “convenience” of
|
||
the ‘_a’ and ‘_r’ shortcut names obscure the code:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">thread</span>
|
||
<span class="kn">from</span><span class="w"> </span><span class="nn">ZServerPublisher</span><span class="w"> </span><span class="kn">import</span> <span class="n">ZServerPublisher</span>
|
||
|
||
<span class="k">class</span><span class="w"> </span><span class="nc">ZRendevous</span><span class="p">:</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">n</span><span class="o">=</span><span class="mi">1</span><span class="p">):</span>
|
||
<span class="n">sync</span><span class="o">=</span><span class="n">thread</span><span class="o">.</span><span class="n">allocate_lock</span><span class="p">()</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_a</span><span class="o">=</span><span class="n">sync</span><span class="o">.</span><span class="n">acquire</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_r</span><span class="o">=</span><span class="n">sync</span><span class="o">.</span><span class="n">release</span>
|
||
<span class="n">pool</span><span class="o">=</span><span class="p">[]</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_lists</span><span class="o">=</span><span class="n">pool</span><span class="p">,</span> <span class="p">[],</span> <span class="p">[]</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_a</span><span class="p">()</span>
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="k">while</span> <span class="n">n</span> <span class="o">></span> <span class="mi">0</span><span class="p">:</span>
|
||
<span class="n">l</span><span class="o">=</span><span class="n">thread</span><span class="o">.</span><span class="n">allocate_lock</span><span class="p">()</span>
|
||
<span class="n">l</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
|
||
<span class="n">pool</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">l</span><span class="p">)</span>
|
||
<span class="n">thread</span><span class="o">.</span><span class="n">start_new_thread</span><span class="p">(</span><span class="n">ZServerPublisher</span><span class="p">,</span>
|
||
<span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">accept</span><span class="p">,))</span>
|
||
<span class="n">n</span><span class="o">=</span><span class="n">n</span><span class="o">-</span><span class="mi">1</span>
|
||
<span class="k">finally</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">_r</span><span class="p">()</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">accept</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">_a</span><span class="p">()</span>
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="n">pool</span><span class="p">,</span> <span class="n">requests</span><span class="p">,</span> <span class="n">ready</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_lists</span>
|
||
<span class="k">while</span> <span class="ow">not</span> <span class="n">requests</span><span class="p">:</span>
|
||
<span class="n">l</span><span class="o">=</span><span class="n">pool</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
|
||
<span class="k">del</span> <span class="n">pool</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
|
||
<span class="n">ready</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">l</span><span class="p">)</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_r</span><span class="p">()</span>
|
||
<span class="n">l</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_a</span><span class="p">()</span>
|
||
<span class="n">pool</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">l</span><span class="p">)</span>
|
||
|
||
<span class="n">r</span><span class="o">=</span><span class="n">requests</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
|
||
<span class="k">del</span> <span class="n">requests</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
|
||
<span class="k">return</span> <span class="n">r</span>
|
||
<span class="k">finally</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">_r</span><span class="p">()</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">handle</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">request</span><span class="p">,</span> <span class="n">response</span><span class="p">):</span>
|
||
<span class="bp">self</span><span class="o">.</span><span class="n">_a</span><span class="p">()</span>
|
||
<span class="k">try</span><span class="p">:</span>
|
||
<span class="n">pool</span><span class="p">,</span> <span class="n">requests</span><span class="p">,</span> <span class="n">ready</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_lists</span>
|
||
<span class="n">requests</span><span class="o">.</span><span class="n">append</span><span class="p">((</span><span class="n">name</span><span class="p">,</span> <span class="n">request</span><span class="p">,</span> <span class="n">response</span><span class="p">))</span>
|
||
<span class="k">if</span> <span class="n">ready</span><span class="p">:</span>
|
||
<span class="n">l</span><span class="o">=</span><span class="n">ready</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
|
||
<span class="k">del</span> <span class="n">ready</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
|
||
<span class="n">l</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
|
||
<span class="k">finally</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">_r</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>In particular the asynchronize section of the <code class="docutils literal notranslate"><span class="pre">accept</span></code> method is
|
||
not very obvious. To beginner programmers, ‘synchronize’ and
|
||
‘asynchronize’ remove many of the problems encountered when
|
||
juggling multiple <code class="docutils literal notranslate"><span class="pre">acquire</span></code> and <code class="docutils literal notranslate"><span class="pre">release</span></code> methods on different
|
||
locks in different <code class="docutils literal notranslate"><span class="pre">try/finally</span></code> blocks.</p>
|
||
</section>
|
||
<section id="formal-syntax">
|
||
<h2><a class="toc-backref" href="#formal-syntax" role="doc-backlink">Formal Syntax</a></h2>
|
||
<p>Python syntax is defined in a modified BNF grammar notation
|
||
described in the Python Language Reference <a class="footnote-reference brackets" href="#id2" id="id1">[1]</a>. This section
|
||
describes the proposed synchronization syntax using this grammar:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">synchronize_stmt</span><span class="p">:</span> <span class="s1">'synchronize'</span> <span class="p">[</span><span class="n">test</span><span class="p">]</span> <span class="s1">':'</span> <span class="n">suite</span>
|
||
<span class="n">asynchronize_stmt</span><span class="p">:</span> <span class="s1">'asynchronize'</span> <span class="p">[</span><span class="n">test</span><span class="p">]</span> <span class="s1">':'</span> <span class="n">suite</span>
|
||
<span class="n">compound_stmt</span><span class="p">:</span> <span class="o">...</span> <span class="o">|</span> <span class="n">synchronized_stmt</span> <span class="o">|</span> <span class="n">asynchronize_stmt</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>(The ‘…’ indicates other compound statements elided).</p>
|
||
</section>
|
||
<section id="proposed-implementation">
|
||
<h2><a class="toc-backref" href="#proposed-implementation" role="doc-backlink">Proposed Implementation</a></h2>
|
||
<p>The author of this PEP has not explored an implementation yet.
|
||
There are several implementation issues that must be resolved.
|
||
The main implementation issue is what exactly gets locked and
|
||
unlocked during a synchronized block.</p>
|
||
<p>During an unqualified synchronized block (the use of the
|
||
‘synchronize’ keyword without a target argument) a lock could be
|
||
created and associated with the synchronized code block object.
|
||
Any threads that are to execute the block must first acquire the
|
||
code block lock.</p>
|
||
<p>When an ‘asynchronize’ keyword is encountered in a ‘synchronize’
|
||
block the code block lock is unlocked before the inner block is
|
||
executed and re-locked when the inner block terminates.</p>
|
||
<p>When a synchronized block target is specified the object is
|
||
associated with a lock. How this is implemented cleanly is
|
||
probably the highest risk of this proposal. Java Virtual Machines
|
||
typically associate a special hidden lock object with target
|
||
object and use it to synchronized the block around the target
|
||
only.</p>
|
||
</section>
|
||
<section id="backward-compatibility">
|
||
<h2><a class="toc-backref" href="#backward-compatibility" role="doc-backlink">Backward Compatibility</a></h2>
|
||
<p>Backward compatibility is solved with the new <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">__future__</span></code>
|
||
Python syntax (<a class="pep reference internal" href="../pep-0236/" title="PEP 236 – Back to the __future__">PEP 236</a>), and the new warning framework (<a class="pep reference internal" href="../pep-0230/" title="PEP 230 – Warning Framework">PEP 230</a>)
|
||
to evolve the
|
||
Python language into phasing out any conflicting names that use
|
||
the new keywords ‘synchronize’ and ‘asynchronize’. To use the
|
||
syntax now, a developer could use the statement:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">__future__</span><span class="w"> </span><span class="kn">import</span> <span class="n">threadsync</span> <span class="c1"># or whatever</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>In addition, any code that uses the keyword ‘synchronize’ or
|
||
‘asynchronize’ as an identifier will be issued a warning from
|
||
Python. After the appropriate period of time, the syntax would
|
||
become standard, the above import statement would do nothing, and
|
||
any identifiers named ‘synchronize’ or ‘asynchronize’ would raise
|
||
an exception.</p>
|
||
</section>
|
||
<section id="pep-310-reliable-acquisition-release-pairs">
|
||
<h2><a class="toc-backref" href="#pep-310-reliable-acquisition-release-pairs" role="doc-backlink">PEP 310 Reliable Acquisition/Release Pairs</a></h2>
|
||
<p><a class="pep reference internal" href="../pep-0310/" title="PEP 310 – Reliable Acquisition/Release Pairs">PEP 310</a> proposes the ‘with’ keyword that can serve the same
|
||
function as ‘synchronize’ (but no facility for ‘asynchronize’).
|
||
The pattern:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">initialize_lock</span><span class="p">()</span>
|
||
|
||
<span class="k">with</span> <span class="n">the_lock</span><span class="p">:</span>
|
||
<span class="n">change_shared_data</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>is equivalent to the proposed:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">synchronize</span> <span class="n">the_lock</span><span class="p">:</span>
|
||
<span class="n">change_shared_data</span><span class="p">()</span>
|
||
</pre></div>
|
||
</div>
|
||
<p><a class="pep reference internal" href="../pep-0310/" title="PEP 310 – Reliable Acquisition/Release Pairs">PEP 310</a> must synchronize on an existing lock, while this PEP
|
||
proposes that unqualified ‘synchronize’ statements synchronize on
|
||
a global, internal, transparent lock in addition to qualified
|
||
‘synchronize’ statements. The ‘with’ statement also requires lock
|
||
initialization, while the ‘synchronize’ statement can synchronize
|
||
on any target object <strong>including</strong> locks.</p>
|
||
<p>While limited in this fashion, the ‘with’ statement is more
|
||
abstract and serves more purposes than synchronization. For
|
||
example, transactions could be used with the ‘with’ keyword:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">initialize_transaction</span><span class="p">()</span>
|
||
|
||
<span class="k">with</span> <span class="n">my_transaction</span><span class="p">:</span>
|
||
<span class="n">do_in_transaction</span><span class="p">()</span>
|
||
|
||
<span class="c1"># when the block terminates, the transaction is committed.</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>The ‘synchronize’ and ‘asynchronize’ keywords cannot serve this or
|
||
any other general acquire/release pattern other than thread
|
||
synchronization.</p>
|
||
</section>
|
||
<section id="how-java-does-it">
|
||
<h2><a class="toc-backref" href="#how-java-does-it" role="doc-backlink">How Java Does It</a></h2>
|
||
<p>Java defines a ‘synchronized’ keyword (note the grammatical tense
|
||
different between the Java keyword and this PEP’s ‘synchronize’)
|
||
which must be qualified on any object. The syntax is:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">synchronized</span> <span class="p">(</span><span class="n">Expression</span><span class="p">)</span> <span class="n">Block</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Expression must yield a valid object (null raises an error and
|
||
exceptions during ‘Expression’ terminate the ‘synchronized’ block
|
||
for the same reason) upon which ‘Block’ is synchronized.</p>
|
||
</section>
|
||
<section id="how-jython-does-it">
|
||
<h2><a class="toc-backref" href="#how-jython-does-it" role="doc-backlink">How Jython Does It</a></h2>
|
||
<p>Jython uses a ‘synchronize’ class with the static method
|
||
‘make_synchronized’ that accepts one callable argument and returns
|
||
a newly created, synchronized, callable “wrapper” around the
|
||
argument.</p>
|
||
</section>
|
||
<section id="summary-of-proposed-changes-to-python">
|
||
<h2><a class="toc-backref" href="#summary-of-proposed-changes-to-python" role="doc-backlink">Summary of Proposed Changes to Python</a></h2>
|
||
<p>Adding new ‘synchronize’ and ‘asynchronize’ keywords to the
|
||
language.</p>
|
||
</section>
|
||
<section id="risks">
|
||
<h2><a class="toc-backref" href="#risks" role="doc-backlink">Risks</a></h2>
|
||
<p>This PEP proposes adding two keywords to the Python language. This
|
||
may break code.</p>
|
||
<p>There is no implementation to test.</p>
|
||
<p>It’s not the most important problem facing Python programmers
|
||
today (although it is a fairly notorious one).</p>
|
||
<p>The equivalent Java keyword is the past participle ‘synchronized’.
|
||
This PEP proposes the present tense, ‘synchronize’ as being more
|
||
in spirit with Python (there being less distinction between
|
||
compile-time and run-time in Python than Java).</p>
|
||
</section>
|
||
<section id="dissenting-opinion">
|
||
<h2><a class="toc-backref" href="#dissenting-opinion" role="doc-backlink">Dissenting Opinion</a></h2>
|
||
<p>This PEP has not been discussed on python-dev.</p>
|
||
</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>The Python Language Reference
|
||
<a class="reference external" href="http://docs.python.org/reference/">http://docs.python.org/reference/</a></aside>
|
||
</aside>
|
||
</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-0319.rst">https://github.com/python/peps/blob/main/peps/pep-0319.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0319.rst">2025-02-01 08:59:27 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="#pronouncement">Pronouncement</a></li>
|
||
<li><a class="reference internal" href="#synchronization-targets">Synchronization Targets</a></li>
|
||
<li><a class="reference internal" href="#other-patterns-that-synchronize">Other Patterns that Synchronize</a></li>
|
||
<li><a class="reference internal" href="#formal-syntax">Formal Syntax</a></li>
|
||
<li><a class="reference internal" href="#proposed-implementation">Proposed Implementation</a></li>
|
||
<li><a class="reference internal" href="#backward-compatibility">Backward Compatibility</a></li>
|
||
<li><a class="reference internal" href="#pep-310-reliable-acquisition-release-pairs">PEP 310 Reliable Acquisition/Release Pairs</a></li>
|
||
<li><a class="reference internal" href="#how-java-does-it">How Java Does It</a></li>
|
||
<li><a class="reference internal" href="#how-jython-does-it">How Jython Does It</a></li>
|
||
<li><a class="reference internal" href="#summary-of-proposed-changes-to-python">Summary of Proposed Changes to Python</a></li>
|
||
<li><a class="reference internal" href="#risks">Risks</a></li>
|
||
<li><a class="reference internal" href="#dissenting-opinion">Dissenting Opinion</a></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-0319.rst">Page Source (GitHub)</a>
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