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 <div class="section" id="module-contextlib">
<h1>29.6. <a class="reference internal" href="#module-contextlib" title="contextlib: Utilities for with-statement contexts."><code class="xref py py-mod docutils literal notranslate">contextlib</code></a> — Utilities for <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a>-statement contexts<a class="headerlink" href="#module-contextlib" title="Permalink to this headline">¶</a></h1>
Source code: <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/contextlib.py">Lib/contextlib.py</a>
<hr class="docutils" />
This module provides utilities for common tasks involving the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a>
statement. For more information see also <a class="reference internal" href="stdtypes.html#typecontextmanager">Context Manager Types</a> and
<a class="reference internal" href="../reference/datamodel.html#context-managers">With Statement Context Managers</a>.
<div class="section" id="utilities">
<h2>29.6.1. Utilities<a class="headerlink" href="#utilities" title="Permalink to this headline">¶</a></h2>
Functions and classes provided:
<dl class="class">
<dt id="contextlib.AbstractContextManager">
class <code class="descclassname">contextlib.</code><code class="descname">AbstractContextManager</code><a class="headerlink" href="#contextlib.AbstractContextManager" title="Permalink to this definition">¶</a></dt>
<dd>An <a class="reference internal" href="../glossary.html#term-abstract-base-class">abstract base class</a> for classes that implement
<a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate">object.__enter__()</code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate">object.__exit__()</code></a>. A default
implementation for <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate">object.__enter__()</code></a> is provided which returns
<code class="docutils literal notranslate">self</code> while <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate">object.__exit__()</code></a> is an abstract method which by default
returns <code class="docutils literal notranslate">None</code>. See also the definition of <a class="reference internal" href="stdtypes.html#typecontextmanager">Context Manager Types</a>.
<div class="versionadded">
New in version 3.6.
</div>
</dd></dl>

<dl class="function">
<dt id="contextlib.contextmanager">
<code class="descclassname">&#64;</code><code class="descclassname">contextlib.</code><code class="descname">contextmanager</code><a class="headerlink" href="#contextlib.contextmanager" title="Permalink to this definition">¶</a></dt>
<dd>This function is a <a class="reference internal" href="../glossary.html#term-decorator">decorator</a> that can be used to define a factory
function for <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement context managers, without needing to
create a class or separate <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate">__enter__()</code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate">__exit__()</code></a> methods.
While many objects natively support use in with statements, sometimes a
resource needs to be managed that isn’t a context manager in its own right,
and doesn’t implement a <code class="docutils literal notranslate">close()</code> method for use with <code class="docutils literal notranslate">contextlib.closing</code>
An abstract example would be the following to ensure correct resource
management:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from contextlib import contextmanager

@contextmanager
def managed_resource(*args, **kwds):
 # Code to acquire resource, e.g.:
 resource = acquire_resource(*args, **kwds)
 try:
 yield resource
 finally:
 # Code to release resource, e.g.:
 release_resource(resource)

&gt;&gt;&gt; with managed_resource(timeout=3600) as resource:
... # Resource is released at the end of this block,
... # even if code in the block raises an exception
</pre></div>
</div>
The function being decorated must return a <a class="reference internal" href="../glossary.html#term-generator">generator</a>-iterator when
called. This iterator must yield exactly one value, which will be bound to
the targets in the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement’s <a class="reference internal" href="../reference/compound_stmts.html#as"><code class="xref std std-keyword docutils literal notranslate">as</code></a> clause, if any.
At the point where the generator yields, the block nested in the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a>
statement is executed. The generator is then resumed after the block is exited.
If an unhandled exception occurs in the block, it is reraised inside the
generator at the point where the yield occurred. Thus, you can use a
<a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>…<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a>…<a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> statement to trap
the error (if any), or ensure that some cleanup takes place. If an exception is
trapped merely in order to log it or to perform some action (rather than to
suppress it entirely), the generator must reraise that exception. Otherwise the
generator context manager will indicate to the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement that
the exception has been handled, and execution will resume with the statement
immediately following the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement.
<a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate">contextmanager()</code></a> uses <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate">ContextDecorator</code></a> so the context managers
it creates can be used as decorators as well as in <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statements.
When used as a decorator, a new generator instance is implicitly created on
each function call (this allows the otherwise “one-shot” context managers
created by <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate">contextmanager()</code></a> to meet the requirement that context
managers support multiple invocations in order to be used as decorators).
<div class="versionchanged">
Changed in version 3.2: Use of <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate">ContextDecorator</code></a>.
</div>
</dd></dl>

<dl class="function">
<dt id="contextlib.closing">
<code class="descclassname">contextlib.</code><code class="descname">closing</code>(thing)<a class="headerlink" href="#contextlib.closing" title="Permalink to this definition">¶</a></dt>
<dd>Return a context manager that closes thing upon completion of the block. This
is basically equivalent to:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from contextlib import contextmanager

@contextmanager
def closing(thing):
 try:
 yield thing
 finally:
 thing.close()
</pre></div>
</div>
And lets you write code like this:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from contextlib import closing
from urllib.request import urlopen

with closing(urlopen(&#39;http://www.python.org&#39;)) as page:
 for line in page:
 print(line)
</pre></div>
</div>
without needing to explicitly close <code class="docutils literal notranslate">page</code>. Even if an error occurs,
<code class="docutils literal notranslate">page.close()</code> will be called when the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> block is exited.
</dd></dl>

<dl class="function">
<dt id="contextlib.suppress">
<code class="descclassname">contextlib.</code><code class="descname">suppress</code>(*exceptions)<a class="headerlink" href="#contextlib.suppress" title="Permalink to this definition">¶</a></dt>
<dd>Return a context manager that suppresses any of the specified exceptions
if they occur in the body of a with statement and then resumes execution
with the first statement following the end of the with statement.
As with any other mechanism that completely suppresses exceptions, this
context manager should be used only to cover very specific errors where
silently continuing with program execution is known to be the right
thing to do.
For example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from contextlib import suppress

with suppress(FileNotFoundError):
 os.remove(&#39;someotherfile.tmp&#39;)
</pre></div>
</div>
This code is equivalent to:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>try:
 os.remove(&#39;somefile.tmp&#39;)
except FileNotFoundError:
 pass

try:
 os.remove(&#39;someotherfile.tmp&#39;)
except FileNotFoundError:
 pass
</pre></div>
</div>
This context manager is <a class="reference internal" href="#reentrant-cms">reentrant</a>.
<div class="versionadded">
New in version 3.4.
</div>
</dd></dl>

<dl class="function">
<dt id="contextlib.redirect_stdout">
<code class="descclassname">contextlib.</code><code class="descname">redirect_stdout</code>(new_target)<a class="headerlink" href="#contextlib.redirect_stdout" title="Permalink to this definition">¶</a></dt>
<dd>Context manager for temporarily redirecting <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate">sys.stdout</code></a> to
another file or file-like object.
This tool adds flexibility to existing functions or classes whose output
is hardwired to stdout.
For example, the output of <a class="reference internal" href="functions.html#help" title="help"><code class="xref py py-func docutils literal notranslate">help()</code></a> normally is sent to sys.stdout.
You can capture that output in a string by redirecting the output to an
<a class="reference internal" href="io.html#io.StringIO" title="io.StringIO"><code class="xref py py-class docutils literal notranslate">io.StringIO</code></a> object:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>f = io.StringIO()
with redirect_stdout(f):
 help(pow)
s = f.getvalue()
</pre></div>
</div>
To send the output of <a class="reference internal" href="functions.html#help" title="help"><code class="xref py py-func docutils literal notranslate">help()</code></a> to a file on disk, redirect the output
to a regular file:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>with open(&#39;help.txt&#39;, &#39;w&#39;) as f:
 with redirect_stdout(f):
 help(pow)
</pre></div>
</div>
To send the output of <a class="reference internal" href="functions.html#help" title="help"><code class="xref py py-func docutils literal notranslate">help()</code></a> to sys.stderr:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>with redirect_stdout(sys.stderr):
 help(pow)
</pre></div>
</div>
Note that the global side effect on <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate">sys.stdout</code></a> means that this
context manager is not suitable for use in library code and most threaded
applications. It also has no effect on the output of subprocesses.
However, it is still a useful approach for many utility scripts.
This context manager is <a class="reference internal" href="#reentrant-cms">reentrant</a>.
<div class="versionadded">
New in version 3.4.
</div>
</dd></dl>

<dl class="function">
<dt id="contextlib.redirect_stderr">
<code class="descclassname">contextlib.</code><code class="descname">redirect_stderr</code>(new_target)<a class="headerlink" href="#contextlib.redirect_stderr" title="Permalink to this definition">¶</a></dt>
<dd>Similar to <a class="reference internal" href="#contextlib.redirect_stdout" title="contextlib.redirect_stdout"><code class="xref py py-func docutils literal notranslate">redirect_stdout()</code></a> but redirecting
<a class="reference internal" href="sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate">sys.stderr</code></a> to another file or file-like object.
This context manager is <a class="reference internal" href="#reentrant-cms">reentrant</a>.
<div class="versionadded">
New in version 3.5.
</div>
</dd></dl>

<dl class="class">
<dt id="contextlib.ContextDecorator">
class <code class="descclassname">contextlib.</code><code class="descname">ContextDecorator</code><a class="headerlink" href="#contextlib.ContextDecorator" title="Permalink to this definition">¶</a></dt>
<dd>A base class that enables a context manager to also be used as a decorator.
Context managers inheriting from <code class="docutils literal notranslate">ContextDecorator</code> have to implement
<code class="docutils literal notranslate">__enter__</code> and <code class="docutils literal notranslate">__exit__</code> as normal. <code class="docutils literal notranslate">__exit__</code> retains its optional
exception handling even when used as a decorator.
<code class="docutils literal notranslate">ContextDecorator</code> is used by <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate">contextmanager()</code></a>, so you get this
functionality automatically.
Example of <code class="docutils literal notranslate">ContextDecorator</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from contextlib import ContextDecorator

class mycontext(ContextDecorator):
 def __enter__(self):
 print(&#39;Starting&#39;)
 return self

def __exit__(self, *exc):
 print(&#39;Finishing&#39;)
 return False

&gt;&gt;&gt; @mycontext()
... def function():
... print(&#39;The bit in the middle&#39;)
...
&gt;&gt;&gt; function()
Starting
The bit in the middle
Finishing

&gt;&gt;&gt; with mycontext():
... print(&#39;The bit in the middle&#39;)
...
Starting
The bit in the middle
Finishing
</pre></div>
</div>
This change is just syntactic sugar for any construct of the following form:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def f():
 with cm():
 # Do stuff
</pre></div>
</div>
<code class="docutils literal notranslate">ContextDecorator</code> lets you instead write:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@cm()
def f():
 # Do stuff
</pre></div>
</div>
It makes it clear that the <code class="docutils literal notranslate">cm</code> applies to the whole function, rather than
just a piece of it (and saving an indentation level is nice, too).
Existing context managers that already have a base class can be extended by
using <code class="docutils literal notranslate">ContextDecorator</code> as a mixin class:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from contextlib import ContextDecorator

class mycontext(ContextBaseClass, ContextDecorator):
 def __enter__(self):
 return self

def __exit__(self, *exc):
 return False
</pre></div>
</div>
<div class="admonition note">
Note
As the decorated function must be able to be called multiple times, the
underlying context manager must support use in multiple <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a>
statements. If this is not the case, then the original construct with the
explicit <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement inside the function should be used.
</div>
<div class="versionadded">
New in version 3.2.
</div>
</dd></dl>

<dl class="class">
<dt id="contextlib.ExitStack">
class <code class="descclassname">contextlib.</code><code class="descname">ExitStack</code><a class="headerlink" href="#contextlib.ExitStack" title="Permalink to this definition">¶</a></dt>
<dd>A context manager that is designed to make it easy to programmatically
combine other context managers and cleanup functions, especially those
that are optional or otherwise driven by input data.
For example, a set of files may easily be handled in a single with
statement as follows:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>with ExitStack() as stack:
 files = [stack.enter_context(open(fname)) for fname in filenames]
 # All opened files will automatically be closed at the end of
 # the with statement, even if attempts to open files later
 # in the list raise an exception
</pre></div>
</div>
Each instance maintains a stack of registered callbacks that are called in
reverse order when the instance is closed (either explicitly or implicitly
at the end of a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement). Note that callbacks are not
invoked implicitly when the context stack instance is garbage collected.
This stack model is used so that context managers that acquire their
resources in their <code class="docutils literal notranslate">__init__</code> method (such as file objects) can be
handled correctly.
Since registered callbacks are invoked in the reverse order of
registration, this ends up behaving as if multiple nested <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a>
statements had been used with the registered set of callbacks. This even
extends to exception handling - if an inner callback suppresses or replaces
an exception, then outer callbacks will be passed arguments based on that
updated state.
This is a relatively low level API that takes care of the details of
correctly unwinding the stack of exit callbacks. It provides a suitable
foundation for higher level context managers that manipulate the exit
stack in application specific ways.
<div class="versionadded">
New in version 3.3.
</div>
<dl class="method">
<dt id="contextlib.ExitStack.enter_context">
<code class="descname">enter_context</code>(cm)<a class="headerlink" href="#contextlib.ExitStack.enter_context" title="Permalink to this definition">¶</a></dt>
<dd>Enters a new context manager and adds its <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate">__exit__()</code></a> method to
the callback stack. The return value is the result of the context
manager’s own <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate">__enter__()</code></a> method.
These context managers may suppress exceptions just as they normally
would if used directly as part of a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement.
</dd></dl>

<dl class="method">
<dt id="contextlib.ExitStack.push">
<code class="descname">push</code>(exit)<a class="headerlink" href="#contextlib.ExitStack.push" title="Permalink to this definition">¶</a></dt>
<dd>Adds a context manager’s <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate">__exit__()</code></a> method to the callback stack.
As <code class="docutils literal notranslate">__enter__</code> is not invoked, this method can be used to cover
part of an <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate">__enter__()</code></a> implementation with a context manager’s own
<a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate">__exit__()</code></a> method.
If passed an object that is not a context manager, this method assumes
it is a callback with the same signature as a context manager’s
<a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate">__exit__()</code></a> method and adds it directly to the callback stack.
By returning true values, these callbacks can suppress exceptions the
same way context manager <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate">__exit__()</code></a> methods can.
The passed in object is returned from the function, allowing this
method to be used as a function decorator.
</dd></dl>

<dl class="method">
<dt id="contextlib.ExitStack.callback">
<code class="descname">callback</code>(callback, *args, **kwds)<a class="headerlink" href="#contextlib.ExitStack.callback" title="Permalink to this definition">¶</a></dt>
<dd>Accepts an arbitrary callback function and arguments and adds it to
the callback stack.
Unlike the other methods, callbacks added this way cannot suppress
exceptions (as they are never passed the exception details).
The passed in callback is returned from the function, allowing this
method to be used as a function decorator.
</dd></dl>

<dl class="method">
<dt id="contextlib.ExitStack.pop_all">
<code class="descname">pop_all</code>()<a class="headerlink" href="#contextlib.ExitStack.pop_all" title="Permalink to this definition">¶</a></dt>
<dd>Transfers the callback stack to a fresh <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate">ExitStack</code></a> instance
and returns it. No callbacks are invoked by this operation - instead,
they will now be invoked when the new stack is closed (either
explicitly or implicitly at the end of a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement).
For example, a group of files can be opened as an “all or nothing”
operation as follows:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>with ExitStack() as stack:
 files = [stack.enter_context(open(fname)) for fname in filenames]
 # Hold onto the close method, but don&#39;t call it yet.
 close_files = stack.pop_all().close
 # If opening any file fails, all previously opened files will be
 # closed automatically. If all files are opened successfully,
 # they will remain open even after the with statement ends.
 # close_files() can then be invoked explicitly to close them all.
</pre></div>
</div>
</dd></dl>

<dl class="method">
<dt id="contextlib.ExitStack.close">
<code class="descname">close</code>()<a class="headerlink" href="#contextlib.ExitStack.close" title="Permalink to this definition">¶</a></dt>
<dd>Immediately unwinds the callback stack, invoking callbacks in the
reverse order of registration. For any context managers and exit
callbacks registered, the arguments passed in will indicate that no
exception occurred.
</dd></dl>

</dd></dl>

</div>
<div class="section" id="examples-and-recipes">
<h2>29.6.2. Examples and Recipes<a class="headerlink" href="#examples-and-recipes" title="Permalink to this headline">¶</a></h2>
This section describes some examples and recipes for making effective use of
the tools provided by <a class="reference internal" href="#module-contextlib" title="contextlib: Utilities for with-statement contexts."><code class="xref py py-mod docutils literal notranslate">contextlib</code></a>.
<div class="section" id="supporting-a-variable-number-of-context-managers">
<h3>29.6.2.1. Supporting a variable number of context managers<a class="headerlink" href="#supporting-a-variable-number-of-context-managers" title="Permalink to this headline">¶</a></h3>
The primary use case for <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate">ExitStack</code></a> is the one given in the class
documentation: supporting a variable number of context managers and other
cleanup operations in a single <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement. The variability
may come from the number of context managers needed being driven by user
input (such as opening a user specified collection of files), or from
some of the context managers being optional:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>with ExitStack() as stack:
 for resource in resources:
 stack.enter_context(resource)
 if need_special_resource():
 special = acquire_special_resource()
 stack.callback(release_special_resource, special)
 # Perform operations that use the acquired resources
</pre></div>
</div>
As shown, <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate">ExitStack</code></a> also makes it quite easy to use <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a>
statements to manage arbitrary resources that don’t natively support the
context management protocol.
</div>
<div class="section" id="simplifying-support-for-single-optional-context-managers">
<h3>29.6.2.2. Simplifying support for single optional context managers<a class="headerlink" href="#simplifying-support-for-single-optional-context-managers" title="Permalink to this headline">¶</a></h3>
In the specific case of a single optional context manager, <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate">ExitStack</code></a>
instances can be used as a “do nothing” context manager, allowing a context
manager to easily be omitted without affecting the overall structure of
the source code:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def debug_trace(details):
 if __debug__:
 return TraceContext(details)
 # Don&#39;t do anything special with the context in release mode
 return ExitStack()

with debug_trace():
 # Suite is traced in debug mode, but runs normally otherwise
</pre></div>
</div>
</div>
<div class="section" id="catching-exceptions-from-enter-methods">
<h3>29.6.2.3. Catching exceptions from <code class="docutils literal notranslate">__enter__</code> methods<a class="headerlink" href="#catching-exceptions-from-enter-methods" title="Permalink to this headline">¶</a></h3>
It is occasionally desirable to catch exceptions from an <code class="docutils literal notranslate">__enter__</code>
method implementation, without inadvertently catching exceptions from
the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement body or the context manager’s <code class="docutils literal notranslate">__exit__</code>
method. By using <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate">ExitStack</code></a> the steps in the context management
protocol can be separated slightly in order to allow this:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>stack = ExitStack()
try:
 x = stack.enter_context(cm)
except Exception:
 # handle __enter__ exception
else:
 with stack:
 # Handle normal case
</pre></div>
</div>
Actually needing to do this is likely to indicate that the underlying API
should be providing a direct resource management interface for use with
<a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>/<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a>/<a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> statements, but not
all APIs are well designed in that regard. When a context manager is the
only resource management API provided, then <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate">ExitStack</code></a> can make it
easier to handle various situations that can’t be handled directly in a
<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement.
</div>
<div class="section" id="cleaning-up-in-an-enter-implementation">
<h3>29.6.2.4. Cleaning up in an <code class="docutils literal notranslate">__enter__</code> implementation<a class="headerlink" href="#cleaning-up-in-an-enter-implementation" title="Permalink to this headline">¶</a></h3>
As noted in the documentation of <a class="reference internal" href="#contextlib.ExitStack.push" title="contextlib.ExitStack.push"><code class="xref py py-meth docutils literal notranslate">ExitStack.push()</code></a>, this
method can be useful in cleaning up an already allocated resource if later
steps in the <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate">__enter__()</code></a> implementation fail.
Here’s an example of doing this for a context manager that accepts resource
acquisition and release functions, along with an optional validation function,
and maps them to the context management protocol:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from contextlib import contextmanager, AbstractContextManager, ExitStack

class ResourceManager(AbstractContextManager):

def __init__(self, acquire_resource, release_resource, check_resource_ok=None):
 self.acquire_resource = acquire_resource
 self.release_resource = release_resource
 if check_resource_ok is None:
 def check_resource_ok(resource):
 return True
 self.check_resource_ok = check_resource_ok

@contextmanager
 def _cleanup_on_error(self):
 with ExitStack() as stack:
 stack.push(self)
 yield
 # The validation check passed and didn&#39;t raise an exception
 # Accordingly, we want to keep the resource, and pass it
 # back to our caller
 stack.pop_all()

def __enter__(self):
 resource = self.acquire_resource()
 with self._cleanup_on_error():
 if not self.check_resource_ok(resource):
 msg = &quot;Failed validation for {!r}&quot;
 raise RuntimeError(msg.format(resource))
 return resource

def __exit__(self, *exc_details):
 # We don&#39;t need to duplicate any of our resource release logic
 self.release_resource()
</pre></div>
</div>
</div>
<div class="section" id="replacing-any-use-of-try-finally-and-flag-variables">
<h3>29.6.2.5. Replacing any use of <code class="docutils literal notranslate">try-finally</code> and flag variables<a class="headerlink" href="#replacing-any-use-of-try-finally-and-flag-variables" title="Permalink to this headline">¶</a></h3>
A pattern you will sometimes see is a <code class="docutils literal notranslate">try-finally</code> statement with a flag
variable to indicate whether or not the body of the <code class="docutils literal notranslate">finally</code> clause should
be executed. In its simplest form (that can’t already be handled just by
using an <code class="docutils literal notranslate">except</code> clause instead), it looks something like this:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>cleanup_needed = True
try:
 result = perform_operation()
 if result:
 cleanup_needed = False
finally:
 if cleanup_needed:
 cleanup_resources()
</pre></div>
</div>
As with any <code class="docutils literal notranslate">try</code> statement based code, this can cause problems for
development and review, because the setup code and the cleanup code can end
up being separated by arbitrarily long sections of code.
<a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate">ExitStack</code></a> makes it possible to instead register a callback for
execution at the end of a <code class="docutils literal notranslate">with</code> statement, and then later decide to skip
executing that callback:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from contextlib import ExitStack

class Callback(ExitStack):
 def __init__(self, callback, *args, **kwds):
 super(Callback, self).__init__()
 self.callback(callback, *args, **kwds)

def cancel(self):
 self.pop_all()

with Callback(cleanup_resources) as cb:
 result = perform_operation()
 if result:
 cb.cancel()
</pre></div>
</div>
If the resource cleanup isn’t already neatly bundled into a standalone
function, then it is still possible to use the decorator form of
<a class="reference internal" href="#contextlib.ExitStack.callback" title="contextlib.ExitStack.callback"><code class="xref py py-meth docutils literal notranslate">ExitStack.callback()</code></a> to declare the resource cleanup in
advance:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from contextlib import ExitStack

with ExitStack() as stack:
 @stack.callback
 def cleanup_resources():
 ...
 result = perform_operation()
 if result:
 stack.pop_all()
</pre></div>
</div>
Due to the way the decorator protocol works, a callback function
declared this way cannot take any parameters. Instead, any resources to
be released must be accessed as closure variables.
</div>
<div class="section" id="using-a-context-manager-as-a-function-decorator">
<h3>29.6.2.6. Using a context manager as a function decorator<a class="headerlink" href="#using-a-context-manager-as-a-function-decorator" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate">ContextDecorator</code></a> makes it possible to use a context manager in
both an ordinary <code class="docutils literal notranslate">with</code> statement and also as a function decorator.
For example, it is sometimes useful to wrap functions or groups of statements
with a logger that can track the time of entry and time of exit. Rather than
writing both a function decorator and a context manager for the task,
inheriting from <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate">ContextDecorator</code></a> provides both capabilities in a
single definition:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from contextlib import ContextDecorator
import logging

logging.basicConfig(level=logging.INFO)

class track_entry_and_exit(ContextDecorator):
 def __init__(self, name):
 self.name = name

def __enter__(self):
 logging.info(&#39;Entering: %s&#39;, self.name)

def __exit__(self, exc_type, exc, exc_tb):
 logging.info(&#39;Exiting: %s&#39;, self.name)
</pre></div>
</div>
Instances of this class can be used as both a context manager:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>with track_entry_and_exit(&#39;widget loader&#39;):
 print(&#39;Some time consuming activity goes here&#39;)
 load_widget()
</pre></div>
</div>
And also as a function decorator:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@track_entry_and_exit(&#39;widget loader&#39;)
def activity():
 print(&#39;Some time consuming activity goes here&#39;)
 load_widget()
</pre></div>
</div>
Note that there is one additional limitation when using context managers
as function decorators: there’s no way to access the return value of
<a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate">__enter__()</code></a>. If that value is needed, then it is still necessary to use
an explicit <code class="docutils literal notranslate">with</code> statement.
<div class="admonition seealso">
See also
<dl class="last docutils">
<dt><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0343">PEP 343</a> - The “with” statement</dt>
<dd>The specification, background, and examples for the Python <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a>
statement.</dd>
</dl>
</div>
</div>
</div>
<div class="section" id="single-use-reusable-and-reentrant-context-managers">
<h2>29.6.3. Single use, reusable and reentrant context managers<a class="headerlink" href="#single-use-reusable-and-reentrant-context-managers" title="Permalink to this headline">¶</a></h2>
Most context managers are written in a way that means they can only be
used effectively in a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement once. These single use
context managers must be created afresh each time they’re used -
attempting to use them a second time will trigger an exception or
otherwise not work correctly.
This common limitation means that it is generally advisable to create
context managers directly in the header of the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement
where they are used (as shown in all of the usage examples above).
Files are an example of effectively single use context managers, since
the first <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement will close the file, preventing any
further IO operations using that file object.
Context managers created using <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate">contextmanager()</code></a> are also single use
context managers, and will complain about the underlying generator failing
to yield if an attempt is made to use them a second time:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from contextlib import contextmanager
&gt;&gt;&gt; @contextmanager
... def singleuse():
... print(&quot;Before&quot;)
... yield
... print(&quot;After&quot;)
...
&gt;&gt;&gt; cm = singleuse()
&gt;&gt;&gt; with cm:
... pass
...
Before
After
&gt;&gt;&gt; with cm:
... pass
...
Traceback (most recent call last):
 ...
RuntimeError: generator didn&#39;t yield
</pre></div>
</div>
<div class="section" id="reentrant-context-managers">
<h3>29.6.3.1. Reentrant context managers<a class="headerlink" href="#reentrant-context-managers" title="Permalink to this headline">¶</a></h3>
More sophisticated context managers may be “reentrant”. These context
managers can not only be used in multiple <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statements,
but may also be used inside a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement that is already
using the same context manager.
<a class="reference internal" href="threading.html#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate">threading.RLock</code></a> is an example of a reentrant context manager, as are
<a class="reference internal" href="#contextlib.suppress" title="contextlib.suppress"><code class="xref py py-func docutils literal notranslate">suppress()</code></a> and <a class="reference internal" href="#contextlib.redirect_stdout" title="contextlib.redirect_stdout"><code class="xref py py-func docutils literal notranslate">redirect_stdout()</code></a>. Here’s a very simple example of
reentrant use:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from contextlib import redirect_stdout
&gt;&gt;&gt; from io import StringIO
&gt;&gt;&gt; stream = StringIO()
&gt;&gt;&gt; write_to_stream = redirect_stdout(stream)
&gt;&gt;&gt; with write_to_stream:
... print(&quot;This is written to the stream rather than stdout&quot;)
... with write_to_stream:
... print(&quot;This is also written to the stream&quot;)
...
&gt;&gt;&gt; print(&quot;This is written directly to stdout&quot;)
This is written directly to stdout
&gt;&gt;&gt; print(stream.getvalue())
This is written to the stream rather than stdout
This is also written to the stream
</pre></div>
</div>
Real world examples of reentrancy are more likely to involve multiple
functions calling each other and hence be far more complicated than this
example.
Note also that being reentrant is not the same thing as being thread safe.
<a class="reference internal" href="#contextlib.redirect_stdout" title="contextlib.redirect_stdout"><code class="xref py py-func docutils literal notranslate">redirect_stdout()</code></a>, for example, is definitely not thread safe, as it
makes a global modification to the system state by binding <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate">sys.stdout</code></a>
to a different stream.
</div>
<div class="section" id="reusable-context-managers">
<h3>29.6.3.2. Reusable context managers<a class="headerlink" href="#reusable-context-managers" title="Permalink to this headline">¶</a></h3>
Distinct from both single use and reentrant context managers are “reusable”
context managers (or, to be completely explicit, “reusable, but not
reentrant” context managers, since reentrant context managers are also
reusable). These context managers support being used multiple times, but
will fail (or otherwise not work correctly) if the specific context manager
instance has already been used in a containing with statement.
<a class="reference internal" href="threading.html#threading.Lock" title="threading.Lock"><code class="xref py py-class docutils literal notranslate">threading.Lock</code></a> is an example of a reusable, but not reentrant,
context manager (for a reentrant lock, it is necessary to use
<a class="reference internal" href="threading.html#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate">threading.RLock</code></a> instead).
Another example of a reusable, but not reentrant, context manager is
<a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate">ExitStack</code></a>, as it invokes all currently registered callbacks
when leaving any with statement, regardless of where those callbacks
were added:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from contextlib import ExitStack
&gt;&gt;&gt; stack = ExitStack()
&gt;&gt;&gt; with stack:
... stack.callback(print, &quot;Callback: from first context&quot;)
... print(&quot;Leaving first context&quot;)
...
Leaving first context
Callback: from first context
&gt;&gt;&gt; with stack:
... stack.callback(print, &quot;Callback: from second context&quot;)
... print(&quot;Leaving second context&quot;)
...
Leaving second context
Callback: from second context
&gt;&gt;&gt; with stack:
... stack.callback(print, &quot;Callback: from outer context&quot;)
... with stack:
... stack.callback(print, &quot;Callback: from inner context&quot;)
... print(&quot;Leaving inner context&quot;)
... print(&quot;Leaving outer context&quot;)
...
Leaving inner context
Callback: from inner context
Callback: from outer context
Leaving outer context
</pre></div>
</div>
As the output from the example shows, reusing a single stack object across
multiple with statements works correctly, but attempting to nest them
will cause the stack to be cleared at the end of the innermost with
statement, which is unlikely to be desirable behaviour.
Using separate <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate">ExitStack</code></a> instances instead of reusing a single
instance avoids that problem:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from contextlib import ExitStack
&gt;&gt;&gt; with ExitStack() as outer_stack:
... outer_stack.callback(print, &quot;Callback: from outer context&quot;)
... with ExitStack() as inner_stack:
... inner_stack.callback(print, &quot;Callback: from inner context&quot;)
... print(&quot;Leaving inner context&quot;)
... print(&quot;Leaving outer context&quot;)
...
Leaving inner context
Callback: from inner context
Leaving outer context
Callback: from outer context
</pre></div>
</div>
</div>
</div>
</div>

</div>
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 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">29.6. <code class="docutils literal notranslate">contextlib</code> — Utilities for <code class="docutils literal notranslate">with</code>-statement contexts</a><ul>
<li><a class="reference internal" href="#utilities">29.6.1. Utilities</a></li>
<li><a class="reference internal" href="#examples-and-recipes">29.6.2. Examples and Recipes</a><ul>
<li><a class="reference internal" href="#supporting-a-variable-number-of-context-managers">29.6.2.1. Supporting a variable number of context managers</a></li>
<li><a class="reference internal" href="#simplifying-support-for-single-optional-context-managers">29.6.2.2. Simplifying support for single optional context managers</a></li>
<li><a class="reference internal" href="#catching-exceptions-from-enter-methods">29.6.2.3. Catching exceptions from <code class="docutils literal notranslate">__enter__</code> methods</a></li>
<li><a class="reference internal" href="#cleaning-up-in-an-enter-implementation">29.6.2.4. Cleaning up in an <code class="docutils literal notranslate">__enter__</code> implementation</a></li>
<li><a class="reference internal" href="#replacing-any-use-of-try-finally-and-flag-variables">29.6.2.5. Replacing any use of <code class="docutils literal notranslate">try-finally</code> and flag variables</a></li>
<li><a class="reference internal" href="#using-a-context-manager-as-a-function-decorator">29.6.2.6. Using a context manager as a function decorator</a></li>
</ul>
</li>
<li><a class="reference internal" href="#single-use-reusable-and-reentrant-context-managers">29.6.3. Single use, reusable and reentrant context managers</a><ul>
<li><a class="reference internal" href="#reentrant-context-managers">29.6.3.1. Reentrant context managers</a></li>
<li><a class="reference internal" href="#reusable-context-managers">29.6.3.2. Reusable context managers</a></li>
</ul>
</li>
</ul>
</li>
</ul>

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