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<div class="section" id="module-gc">
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<span id="gc-garbage-collector-interface"></span><h1><a class="reference internal" href="#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> — Garbage Collector interface<a class="headerlink" href="#module-gc" title="Permalink to this headline">¶</a></h1>
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<hr class="docutils" />
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<p>This module provides an interface to the optional garbage collector. It
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provides the ability to disable the collector, tune the collection frequency,
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and set debugging options. It also provides access to unreachable objects that
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the collector found but cannot free. Since the collector supplements the
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reference counting already used in Python, you can disable the collector if you
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are sure your program does not create reference cycles. Automatic collection
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can be disabled by calling <code class="docutils literal notranslate"><span class="pre">gc.disable()</span></code>. To debug a leaking program call
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<code class="docutils literal notranslate"><span class="pre">gc.set_debug(gc.DEBUG_LEAK)</span></code>. Notice that this includes
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<code class="docutils literal notranslate"><span class="pre">gc.DEBUG_SAVEALL</span></code>, causing garbage-collected objects to be saved in
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gc.garbage for inspection.</p>
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<p>The <a class="reference internal" href="#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> module provides the following functions:</p>
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<dl class="function">
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<dt id="gc.enable">
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<code class="descclassname">gc.</code><code class="descname">enable</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.enable" title="Permalink to this definition">¶</a></dt>
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<dd><p>Enable automatic garbage collection.</p>
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</dd></dl>
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<dl class="function">
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<dt id="gc.disable">
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<code class="descclassname">gc.</code><code class="descname">disable</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.disable" title="Permalink to this definition">¶</a></dt>
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<dd><p>Disable automatic garbage collection.</p>
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</dd></dl>
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<dl class="function">
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<dt id="gc.isenabled">
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<code class="descclassname">gc.</code><code class="descname">isenabled</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.isenabled" title="Permalink to this definition">¶</a></dt>
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<dd><p>Returns true if automatic collection is enabled.</p>
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</dd></dl>
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<dl class="function">
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<dt id="gc.collect">
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<code class="descclassname">gc.</code><code class="descname">collect</code><span class="sig-paren">(</span><em>generation=2</em><span class="sig-paren">)</span><a class="headerlink" href="#gc.collect" title="Permalink to this definition">¶</a></dt>
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<dd><p>With no arguments, run a full collection. The optional argument <em>generation</em>
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may be an integer specifying which generation to collect (from 0 to 2). A
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<a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> is raised if the generation number is invalid. The number of
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unreachable objects found is returned.</p>
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<p>The free lists maintained for a number of built-in types are cleared
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whenever a full collection or collection of the highest generation (2)
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is run. Not all items in some free lists may be freed due to the
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particular implementation, in particular <a class="reference internal" href="functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a>.</p>
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</dd></dl>
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<dl class="function">
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<dt id="gc.set_debug">
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<code class="descclassname">gc.</code><code class="descname">set_debug</code><span class="sig-paren">(</span><em>flags</em><span class="sig-paren">)</span><a class="headerlink" href="#gc.set_debug" title="Permalink to this definition">¶</a></dt>
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<dd><p>Set the garbage collection debugging flags. Debugging information will be
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written to <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code>. See below for a list of debugging flags which can be
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combined using bit operations to control debugging.</p>
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</dd></dl>
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<dl class="function">
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<dt id="gc.get_debug">
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<code class="descclassname">gc.</code><code class="descname">get_debug</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.get_debug" title="Permalink to this definition">¶</a></dt>
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<dd><p>Return the debugging flags currently set.</p>
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</dd></dl>
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<dl class="function">
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<dt id="gc.get_objects">
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<code class="descclassname">gc.</code><code class="descname">get_objects</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.get_objects" title="Permalink to this definition">¶</a></dt>
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<dd><p>Returns a list of all objects tracked by the collector, excluding the list
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returned.</p>
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</dd></dl>
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<dl class="function">
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<dt id="gc.get_stats">
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<code class="descclassname">gc.</code><code class="descname">get_stats</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.get_stats" title="Permalink to this definition">¶</a></dt>
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<dd><p>Return a list of three per-generation dictionaries containing collection
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statistics since interpreter start. The number of keys may change
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in the future, but currently each dictionary will contain the following
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items:</p>
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<ul class="simple">
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<li><p><code class="docutils literal notranslate"><span class="pre">collections</span></code> is the number of times this generation was collected;</p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">collected</span></code> is the total number of objects collected inside this
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generation;</p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">uncollectable</span></code> is the total number of objects which were found
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to be uncollectable (and were therefore moved to the <a class="reference internal" href="#gc.garbage" title="gc.garbage"><code class="xref py py-data docutils literal notranslate"><span class="pre">garbage</span></code></a>
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list) inside this generation.</p></li>
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</ul>
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<div class="versionadded">
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<p><span class="versionmodified added">New in version 3.4.</span></p>
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</div>
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</dd></dl>
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<dl class="function">
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<dt id="gc.set_threshold">
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<code class="descclassname">gc.</code><code class="descname">set_threshold</code><span class="sig-paren">(</span><em>threshold0</em><span class="optional">[</span>, <em>threshold1</em><span class="optional">[</span>, <em>threshold2</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.set_threshold" title="Permalink to this definition">¶</a></dt>
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<dd><p>Set the garbage collection thresholds (the collection frequency). Setting
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<em>threshold0</em> to zero disables collection.</p>
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<p>The GC classifies objects into three generations depending on how many
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collection sweeps they have survived. New objects are placed in the youngest
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||
generation (generation <code class="docutils literal notranslate"><span class="pre">0</span></code>). If an object survives a collection it is moved
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||
into the next older generation. Since generation <code class="docutils literal notranslate"><span class="pre">2</span></code> is the oldest
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||
generation, objects in that generation remain there after a collection. In
|
||
order to decide when to run, the collector keeps track of the number object
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||
allocations and deallocations since the last collection. When the number of
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||
allocations minus the number of deallocations exceeds <em>threshold0</em>, collection
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||
starts. Initially only generation <code class="docutils literal notranslate"><span class="pre">0</span></code> is examined. If generation <code class="docutils literal notranslate"><span class="pre">0</span></code> has
|
||
been examined more than <em>threshold1</em> times since generation <code class="docutils literal notranslate"><span class="pre">1</span></code> has been
|
||
examined, then generation <code class="docutils literal notranslate"><span class="pre">1</span></code> is examined as well. Similarly, <em>threshold2</em>
|
||
controls the number of collections of generation <code class="docutils literal notranslate"><span class="pre">1</span></code> before collecting
|
||
generation <code class="docutils literal notranslate"><span class="pre">2</span></code>.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="function">
|
||
<dt id="gc.get_count">
|
||
<code class="descclassname">gc.</code><code class="descname">get_count</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.get_count" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Return the current collection counts as a tuple of <code class="docutils literal notranslate"><span class="pre">(count0,</span> <span class="pre">count1,</span>
|
||
<span class="pre">count2)</span></code>.</p>
|
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</dd></dl>
|
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|
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<dl class="function">
|
||
<dt id="gc.get_threshold">
|
||
<code class="descclassname">gc.</code><code class="descname">get_threshold</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.get_threshold" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Return the current collection thresholds as a tuple of <code class="docutils literal notranslate"><span class="pre">(threshold0,</span>
|
||
<span class="pre">threshold1,</span> <span class="pre">threshold2)</span></code>.</p>
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</dd></dl>
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<dl class="function">
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<dt id="gc.get_referrers">
|
||
<code class="descclassname">gc.</code><code class="descname">get_referrers</code><span class="sig-paren">(</span><em>*objs</em><span class="sig-paren">)</span><a class="headerlink" href="#gc.get_referrers" title="Permalink to this definition">¶</a></dt>
|
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<dd><p>Return the list of objects that directly refer to any of objs. This function
|
||
will only locate those containers which support garbage collection; extension
|
||
types which do refer to other objects but do not support garbage collection will
|
||
not be found.</p>
|
||
<p>Note that objects which have already been dereferenced, but which live in cycles
|
||
and have not yet been collected by the garbage collector can be listed among the
|
||
resulting referrers. To get only currently live objects, call <a class="reference internal" href="#gc.collect" title="gc.collect"><code class="xref py py-func docutils literal notranslate"><span class="pre">collect()</span></code></a>
|
||
before calling <a class="reference internal" href="#gc.get_referrers" title="gc.get_referrers"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_referrers()</span></code></a>.</p>
|
||
<p>Care must be taken when using objects returned by <a class="reference internal" href="#gc.get_referrers" title="gc.get_referrers"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_referrers()</span></code></a> because
|
||
some of them could still be under construction and hence in a temporarily
|
||
invalid state. Avoid using <a class="reference internal" href="#gc.get_referrers" title="gc.get_referrers"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_referrers()</span></code></a> for any purpose other than
|
||
debugging.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="function">
|
||
<dt id="gc.get_referents">
|
||
<code class="descclassname">gc.</code><code class="descname">get_referents</code><span class="sig-paren">(</span><em>*objs</em><span class="sig-paren">)</span><a class="headerlink" href="#gc.get_referents" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Return a list of objects directly referred to by any of the arguments. The
|
||
referents returned are those objects visited by the arguments’ C-level
|
||
<a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_traverse</span></code></a> methods (if any), and may not be all objects actually
|
||
directly reachable. <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_traverse</span></code></a> methods are supported only by objects
|
||
that support garbage collection, and are only required to visit objects that may
|
||
be involved in a cycle. So, for example, if an integer is directly reachable
|
||
from an argument, that integer object may or may not appear in the result list.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="function">
|
||
<dt id="gc.is_tracked">
|
||
<code class="descclassname">gc.</code><code class="descname">is_tracked</code><span class="sig-paren">(</span><em>obj</em><span class="sig-paren">)</span><a class="headerlink" href="#gc.is_tracked" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Returns <code class="docutils literal notranslate"><span class="pre">True</span></code> if the object is currently tracked by the garbage collector,
|
||
<code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise. As a general rule, instances of atomic types aren’t
|
||
tracked and instances of non-atomic types (containers, user-defined
|
||
objects…) are. However, some type-specific optimizations can be present
|
||
in order to suppress the garbage collector footprint of simple instances
|
||
(e.g. dicts containing only atomic keys and values):</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">gc</span><span class="o">.</span><span class="n">is_tracked</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
|
||
<span class="go">False</span>
|
||
<span class="gp">>>> </span><span class="n">gc</span><span class="o">.</span><span class="n">is_tracked</span><span class="p">(</span><span class="s2">"a"</span><span class="p">)</span>
|
||
<span class="go">False</span>
|
||
<span class="gp">>>> </span><span class="n">gc</span><span class="o">.</span><span class="n">is_tracked</span><span class="p">([])</span>
|
||
<span class="go">True</span>
|
||
<span class="gp">>>> </span><span class="n">gc</span><span class="o">.</span><span class="n">is_tracked</span><span class="p">({})</span>
|
||
<span class="go">False</span>
|
||
<span class="gp">>>> </span><span class="n">gc</span><span class="o">.</span><span class="n">is_tracked</span><span class="p">({</span><span class="s2">"a"</span><span class="p">:</span> <span class="mi">1</span><span class="p">})</span>
|
||
<span class="go">False</span>
|
||
<span class="gp">>>> </span><span class="n">gc</span><span class="o">.</span><span class="n">is_tracked</span><span class="p">({</span><span class="s2">"a"</span><span class="p">:</span> <span class="p">[]})</span>
|
||
<span class="go">True</span>
|
||
</pre></div>
|
||
</div>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">New in version 3.1.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="function">
|
||
<dt id="gc.freeze">
|
||
<code class="descclassname">gc.</code><code class="descname">freeze</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.freeze" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Freeze all the objects tracked by gc - move them to a permanent generation
|
||
and ignore all the future collections. This can be used before a POSIX
|
||
fork() call to make the gc copy-on-write friendly or to speed up collection.
|
||
Also collection before a POSIX fork() call may free pages for future
|
||
allocation which can cause copy-on-write too so it’s advised to disable gc
|
||
in master process and freeze before fork and enable gc in child process.</p>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">New in version 3.7.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="function">
|
||
<dt id="gc.unfreeze">
|
||
<code class="descclassname">gc.</code><code class="descname">unfreeze</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.unfreeze" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Unfreeze the objects in the permanent generation, put them back into the
|
||
oldest generation.</p>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">New in version 3.7.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="function">
|
||
<dt id="gc.get_freeze_count">
|
||
<code class="descclassname">gc.</code><code class="descname">get_freeze_count</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gc.get_freeze_count" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Return the number of objects in the permanent generation.</p>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">New in version 3.7.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<p>The following variables are provided for read-only access (you can mutate the
|
||
values but should not rebind them):</p>
|
||
<dl class="data">
|
||
<dt id="gc.garbage">
|
||
<code class="descclassname">gc.</code><code class="descname">garbage</code><a class="headerlink" href="#gc.garbage" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>A list of objects which the collector found to be unreachable but could
|
||
not be freed (uncollectable objects). Starting with Python 3.4, this
|
||
list should be empty most of the time, except when using instances of
|
||
C extension types with a non-NULL <code class="docutils literal notranslate"><span class="pre">tp_del</span></code> slot.</p>
|
||
<p>If <a class="reference internal" href="#gc.DEBUG_SAVEALL" title="gc.DEBUG_SAVEALL"><code class="xref py py-const docutils literal notranslate"><span class="pre">DEBUG_SAVEALL</span></code></a> is set, then all unreachable objects will be
|
||
added to this list rather than freed.</p>
|
||
<div class="versionchanged">
|
||
<p><span class="versionmodified changed">Changed in version 3.2: </span>If this list is non-empty at <a class="reference internal" href="../glossary.html#term-interpreter-shutdown"><span class="xref std std-term">interpreter shutdown</span></a>, a
|
||
<a class="reference internal" href="exceptions.html#ResourceWarning" title="ResourceWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ResourceWarning</span></code></a> is emitted, which is silent by default. If
|
||
<a class="reference internal" href="#gc.DEBUG_UNCOLLECTABLE" title="gc.DEBUG_UNCOLLECTABLE"><code class="xref py py-const docutils literal notranslate"><span class="pre">DEBUG_UNCOLLECTABLE</span></code></a> is set, in addition all uncollectable objects
|
||
are printed.</p>
|
||
</div>
|
||
<div class="versionchanged">
|
||
<p><span class="versionmodified changed">Changed in version 3.4: </span>Following <span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0442"><strong>PEP 442</strong></a>, objects with a <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method don’t end
|
||
up in <a class="reference internal" href="#gc.garbage" title="gc.garbage"><code class="xref py py-attr docutils literal notranslate"><span class="pre">gc.garbage</span></code></a> anymore.</p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="data">
|
||
<dt id="gc.callbacks">
|
||
<code class="descclassname">gc.</code><code class="descname">callbacks</code><a class="headerlink" href="#gc.callbacks" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>A list of callbacks that will be invoked by the garbage collector before and
|
||
after collection. The callbacks will be called with two arguments,
|
||
<em>phase</em> and <em>info</em>.</p>
|
||
<p><em>phase</em> can be one of two values:</p>
|
||
<blockquote>
|
||
<div><p>“start”: The garbage collection is about to start.</p>
|
||
<p>“stop”: The garbage collection has finished.</p>
|
||
</div></blockquote>
|
||
<p><em>info</em> is a dict providing more information for the callback. The following
|
||
keys are currently defined:</p>
|
||
<blockquote>
|
||
<div><p>“generation”: The oldest generation being collected.</p>
|
||
<p>“collected”: When <em>phase</em> is “stop”, the number of objects
|
||
successfully collected.</p>
|
||
<p>“uncollectable”: When <em>phase</em> is “stop”, the number of objects
|
||
that could not be collected and were put in <a class="reference internal" href="#gc.garbage" title="gc.garbage"><code class="xref py py-data docutils literal notranslate"><span class="pre">garbage</span></code></a>.</p>
|
||
</div></blockquote>
|
||
<p>Applications can add their own callbacks to this list. The primary
|
||
use cases are:</p>
|
||
<blockquote>
|
||
<div><p>Gathering statistics about garbage collection, such as how often
|
||
various generations are collected, and how long the collection
|
||
takes.</p>
|
||
<p>Allowing applications to identify and clear their own uncollectable
|
||
types when they appear in <a class="reference internal" href="#gc.garbage" title="gc.garbage"><code class="xref py py-data docutils literal notranslate"><span class="pre">garbage</span></code></a>.</p>
|
||
</div></blockquote>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">New in version 3.3.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<p>The following constants are provided for use with <a class="reference internal" href="#gc.set_debug" title="gc.set_debug"><code class="xref py py-func docutils literal notranslate"><span class="pre">set_debug()</span></code></a>:</p>
|
||
<dl class="data">
|
||
<dt id="gc.DEBUG_STATS">
|
||
<code class="descclassname">gc.</code><code class="descname">DEBUG_STATS</code><a class="headerlink" href="#gc.DEBUG_STATS" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Print statistics during collection. This information can be useful when tuning
|
||
the collection frequency.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="data">
|
||
<dt id="gc.DEBUG_COLLECTABLE">
|
||
<code class="descclassname">gc.</code><code class="descname">DEBUG_COLLECTABLE</code><a class="headerlink" href="#gc.DEBUG_COLLECTABLE" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Print information on collectable objects found.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="data">
|
||
<dt id="gc.DEBUG_UNCOLLECTABLE">
|
||
<code class="descclassname">gc.</code><code class="descname">DEBUG_UNCOLLECTABLE</code><a class="headerlink" href="#gc.DEBUG_UNCOLLECTABLE" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Print information of uncollectable objects found (objects which are not
|
||
reachable but cannot be freed by the collector). These objects will be added
|
||
to the <code class="docutils literal notranslate"><span class="pre">garbage</span></code> list.</p>
|
||
<div class="versionchanged">
|
||
<p><span class="versionmodified changed">Changed in version 3.2: </span>Also print the contents of the <a class="reference internal" href="#gc.garbage" title="gc.garbage"><code class="xref py py-data docutils literal notranslate"><span class="pre">garbage</span></code></a> list at
|
||
<a class="reference internal" href="../glossary.html#term-interpreter-shutdown"><span class="xref std std-term">interpreter shutdown</span></a>, if it isn’t empty.</p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="data">
|
||
<dt id="gc.DEBUG_SAVEALL">
|
||
<code class="descclassname">gc.</code><code class="descname">DEBUG_SAVEALL</code><a class="headerlink" href="#gc.DEBUG_SAVEALL" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>When set, all unreachable objects found will be appended to <em>garbage</em> rather
|
||
than being freed. This can be useful for debugging a leaking program.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="data">
|
||
<dt id="gc.DEBUG_LEAK">
|
||
<code class="descclassname">gc.</code><code class="descname">DEBUG_LEAK</code><a class="headerlink" href="#gc.DEBUG_LEAK" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>The debugging flags necessary for the collector to print information about a
|
||
leaking program (equal to <code class="docutils literal notranslate"><span class="pre">DEBUG_COLLECTABLE</span> <span class="pre">|</span> <span class="pre">DEBUG_UNCOLLECTABLE</span> <span class="pre">|</span>
|
||
<span class="pre">DEBUG_SAVEALL</span></code>).</p>
|
||
</dd></dl>
|
||
|
||
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|
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|
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|
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|
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