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<!DOCTYPE html>

<head>

<title>contextlib --- Utilities for with-statement contexts — مستندات Python3.14.6</title><meta name="viewport" content="width=device-width, initial-scale=1.0">

title="جستجو در مستندات Python3.14.6"

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<h3><a href="../contents.html">فهرست عناوین</a></h3>

<ul>

<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate">contextlib</code> --- Utilities for <code class="xref std std-keyword docutils literal notranslate">with</code>-statement contexts</a><ul>

<li><a class="reference internal" href="#utilities">Utilities</a></li>

<li><a class="reference internal" href="#examples-and-recipes">Examples and Recipes</a><ul>

<li><a class="reference internal" href="#supporting-a-variable-number-of-context-managers">Supporting a variable number of context managers</a></li>

<li><a class="reference internal" href="#catching-exceptions-from-enter-methods">Catching exceptions from <code class="docutils literal notranslate">__enter__</code> methods</a></li>

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<li><a class="reference internal" href="#replacing-any-use-of-try-finally-and-flag-variables">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">Using a context manager as a function decorator</a></li>

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<li><a class="reference internal" href="#single-use-reusable-and-reentrant-context-managers">Single use, reusable and reentrant context managers</a><ul>

<li><a class="reference internal" href="#reentrant-context-managers">Reentrant context managers</a></li>

<li><a class="reference internal" href="#reusable-context-managers">Reusable context managers</a></li>

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<h4>موضوع قبلی</h4>

<a href="dataclasses.html"

title="فصل قبلی"><code class="xref py py-mod docutils literal notranslate">dataclasses</code> --- Data Classes</a>

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<li class="nav-item nav-item-this"><a href=""><code class="xref py py-mod docutils literal notranslate">contextlib</code> --- Utilities for <code class="xref std std-keyword docutils literal notranslate">with</code>-statement contexts</a></li>

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<h1><code class="xref py py-mod docutils literal notranslate">contextlib</code> --- Utilities for <code class="xref std std-keyword docutils literal notranslate">with</code>-statement contexts<a class="headerlink" href="#module-contextlib" title="Link to this heading">¶</a></h1>

Source code: <a class="extlink-source reference external" href="https://github.com/python/cpython/tree/3.14/Lib/contextlib.py">Lib/contextlib.py</a>

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>.

<h2>Utilities<a class="headerlink" href="#utilities" title="Link to this heading">¶</a></h2>

Functions and classes provided:

class contextlib.AbstractContextManager<a class="headerlink" href="#contextlib.AbstractContextManager" title="Link 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">__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>. A default

implementation for <code class="xref py py-meth docutils literal notranslate">__enter__()</code> is provided which returns

<code class="docutils literal notranslate">self</code> while <code class="xref py py-meth docutils literal notranslate">__exit__()</code> 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>.

Added in version 3.6.

</div>

</dd></dl>

class contextlib.AbstractAsyncContextManager<a class="headerlink" href="#contextlib.AbstractAsyncContextManager" title="Link 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.__aenter__" title="object.__aenter__"><code class="xref py py-meth docutils literal notranslate">__aenter__()</code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__aexit__" title="object.__aexit__"><code class="xref py py-meth docutils literal notranslate">__aexit__()</code></a>. A default

implementation for <code class="xref py py-meth docutils literal notranslate">__aenter__()</code> is provided which returns

<code class="docutils literal notranslate">self</code> while <code class="xref py py-meth docutils literal notranslate">__aexit__()</code> 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="../reference/datamodel.html#async-context-managers">Asynchronous Context Managers</a>.

Added in version 3.7.

</div>

</dd></dl>

@contextlib.contextmanager<a class="headerlink" href="#contextlib.contextmanager" title="Link 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:

@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)

</pre></div>

</div>

The function can then be used like this:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> 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 <code class="xref std std-keyword docutils literal notranslate">as</code> 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 <code class="xref std std-keyword docutils literal notranslate">with</code> statement that

the exception has been handled, and execution will resume with the statement

immediately following the <code class="xref std std-keyword docutils literal notranslate">with</code> statement.

<code class="xref py py-func docutils literal notranslate">contextmanager()</code> 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 <code class="xref py py-func docutils literal notranslate">contextmanager()</code> to meet the requirement that context

managers support multiple invocations in order to be used as decorators).

تغییر داده شده در نسخه 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>

@contextlib.asynccontextmanager<a class="headerlink" href="#contextlib.asynccontextmanager" title="Link to this definition">¶</a></dt>

<dd>Similar to <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate">contextmanager()</code></a>, but creates an

<a class="reference internal" href="../reference/datamodel.html#async-context-managers">asynchronous context manager</a>.

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#async-with"><code class="xref std std-keyword docutils literal notranslate">async with</code></a> statement asynchronous context managers,

without needing to create a class or separate <a class="reference internal" href="../reference/datamodel.html#object.__aenter__" title="object.__aenter__"><code class="xref py py-meth docutils literal notranslate">__aenter__()</code></a> and

<a class="reference internal" href="../reference/datamodel.html#object.__aexit__" title="object.__aexit__"><code class="xref py py-meth docutils literal notranslate">__aexit__()</code></a> methods. It must be applied to an <a class="reference internal" href="../glossary.html#term-asynchronous-generator">asynchronous

generator</a> function.

A simple example:

@asynccontextmanager

async def get_connection():

conn = await acquire_db_connection()

try:

yield conn

finally:

await release_db_connection(conn)

async def get_all_users():

async with get_connection() as conn:

return conn.query('SELECT ...')

</pre></div>

</div>

Added in version 3.7.

</div>

Context managers defined with <code class="xref py py-func docutils literal notranslate">asynccontextmanager()</code> can be used

either as decorators or with <a class="reference internal" href="../reference/compound_stmts.html#async-with"><code class="xref std std-keyword docutils literal notranslate">async with</code></a> statements:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>import time

from contextlib import asynccontextmanager

@asynccontextmanager

async def timeit():

now = time.monotonic()

try:

yield

finally:

print(f'it took {time.monotonic() - now}s to run')

@timeit()

async def main():

# ... async code ...

</pre></div>

</div>

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 <code class="xref py py-func docutils literal notranslate">asynccontextmanager()</code> to meet the requirement that context

managers support multiple invocations in order to be used as decorators.

تغییر داده شده در نسخه 3.10: Async context managers created with <code class="xref py py-func docutils literal notranslate">asynccontextmanager()</code> can

be used as decorators.

</div>

</dd></dl>

contextlib.closing(thing)<a class="headerlink" href="#contextlib.closing" title="Link to this definition">¶</a></dt>

<dd>Return a context manager that closes thing upon completion of the block. This

is basically equivalent to:

@contextmanager

def closing(thing):

try:

yield thing

finally:

thing.close()

</pre></div>

</div>

And lets you write code like this:

from urllib.request import urlopen

with closing(urlopen('https://www.python.org')) 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.

توجه

Most types managing resources support the <a class="reference internal" href="../glossary.html#term-context-manager">context manager</a> protocol,

which closes thing on leaving the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement.

As such, <code class="xref py py-func docutils literal notranslate">closing()</code> is most useful for third party types that don't

support context managers.

This example is purely for illustration purposes,

as <a class="reference internal" href="urllib.request.html#urllib.request.urlopen" title="urllib.request.urlopen"><code class="xref py py-func docutils literal notranslate">urlopen()</code></a> would normally be used in a context manager.

</div>

</dd></dl>

contextlib.aclosing(thing)<a class="headerlink" href="#contextlib.aclosing" title="Link to this definition">¶</a></dt>

<dd>Return an async context manager that calls the <code class="docutils literal notranslate">aclose()</code> method of thing

upon completion of the block. This is basically equivalent to:

@asynccontextmanager

async def aclosing(thing):

try:

yield thing

finally:

await thing.aclose()

</pre></div>

</div>

Significantly, <code class="docutils literal notranslate">aclosing()</code> supports deterministic cleanup of async

generators when they happen to exit early by <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> or an

exception. For example:

async with aclosing(my_generator()) as values:

async for value in values:

if value == 42:

break

</pre></div>

</div>

This pattern ensures that the generator's async exit code is executed in

the same context as its iterations (so that exceptions and context

variables work as expected, and the exit code isn't run after the

lifetime of some task it depends on).

Added in version 3.10.

</div>

</dd></dl>

contextlib.nullcontext(enter_result=None)<a class="headerlink" href="#contextlib.nullcontext" title="Link to this definition">¶</a></dt>

<dd>Return a context manager that returns enter_result from <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>, but

otherwise does nothing. It is intended to be used as a stand-in for an

optional context manager, for example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>def myfunction(arg, ignore_exceptions=False):

if ignore_exceptions:

# Use suppress to ignore all exceptions.

cm = contextlib.suppress(Exception)

else:

# Do not ignore any exceptions, cm has no effect.

cm = contextlib.nullcontext()

with cm:

# Do something

</pre></div>

</div>

An example using enter_result:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>def process_file(file_or_path):

if isinstance(file_or_path, str):

# If string, open file

cm = open(file_or_path)

else:

# Caller is responsible for closing file

cm = nullcontext(file_or_path)

with cm as file:

# Perform processing on the file

</pre></div>

</div>

It can also be used as a stand-in for

<a class="reference internal" href="../reference/datamodel.html#async-context-managers">asynchronous context managers</a>:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>async def send_http(session=None):

if not session:

# If no http session, create it with aiohttp

cm = aiohttp.ClientSession()

else:

# Caller is responsible for closing the session

cm = nullcontext(session)

async with cm as session:

# Send http requests with session

</pre></div>

</div>

Added in version 3.7.

</div>

تغییر داده شده در نسخه 3.10: <a class="reference internal" href="../glossary.html#term-asynchronous-context-manager">asynchronous context manager</a> support was added.

</div>

</dd></dl>

contextlib.suppress(*exceptions)<a class="headerlink" href="#contextlib.suppress" title="Link 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 <code class="xref std std-keyword docutils literal notranslate">with</code> statement and then

resumes execution with the first statement following the end of the

<code class="xref std std-keyword docutils literal notranslate">with</code> 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:

with suppress(FileNotFoundError):

os.remove('somefile.tmp')

with suppress(FileNotFoundError):

os.remove('someotherfile.tmp')

</pre></div>

</div>

This code is equivalent to:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>try:

os.remove('somefile.tmp')

except FileNotFoundError:

pass

try:

os.remove('someotherfile.tmp')

except FileNotFoundError:

pass

</pre></div>

</div>

This context manager is <a class="reference internal" href="#reentrant-cms">reentrant</a>.

If the code within the <code class="xref std std-keyword docutils literal notranslate">with</code> block raises a

<a class="reference internal" href="exceptions.html#BaseExceptionGroup" title="BaseExceptionGroup"><code class="xref py py-exc docutils literal notranslate">BaseExceptionGroup</code></a>, suppressed exceptions are removed from the

group. Any exceptions of the group which are not suppressed are re-raised in

a new group which is created using the original group's <a class="reference internal" href="exceptions.html#BaseExceptionGroup.derive" title="BaseExceptionGroup.derive"><code class="xref py py-meth docutils literal notranslate">derive()</code></a>

method.

Added in version 3.4.

</div>

تغییر داده شده در نسخه 3.12: <code class="docutils literal notranslate">suppress</code> now supports suppressing exceptions raised as

part of a <a class="reference internal" href="exceptions.html#BaseExceptionGroup" title="BaseExceptionGroup"><code class="xref py py-exc docutils literal notranslate">BaseExceptionGroup</code></a>.

</div>

</dd></dl>

contextlib.redirect_stdout(new_target)<a class="headerlink" href="#contextlib.redirect_stdout" title="Link 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. The replacement stream is returned 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:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>with redirect_stdout(io.StringIO()) as 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('help.txt', 'w') as f:

with redirect_stdout(f):

help(pow)

</pre></div>

</div>

<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>.

Added in version 3.4.

</div>

</dd></dl>

contextlib.redirect_stderr(new_target)<a class="headerlink" href="#contextlib.redirect_stderr" title="Link 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>.

Added in version 3.5.

</div>

</dd></dl>

contextlib.chdir(path)<a class="headerlink" href="#contextlib.chdir" title="Link to this definition">¶</a></dt>

<dd>Non parallel-safe context manager to change the current working directory.

As this changes a global state, the working directory, it is not suitable

for use in most threaded or async contexts. It is also not suitable for most

non-linear code execution, like generators, where the program execution is

temporarily relinquished -- unless explicitly desired, you should not yield

when this context manager is active.

This is a simple wrapper around <a class="reference internal" href="os.html#os.chdir" title="os.chdir"><code class="xref py py-func docutils literal notranslate">chdir()</code></a>, it changes the current

working directory upon entering and restores the old one on exit.

This context manager is <a class="reference internal" href="#reentrant-cms">reentrant</a>.

Added in version 3.11.

</div>

</dd></dl>

class contextlib.ContextDecorator<a class="headerlink" href="#contextlib.ContextDecorator" title="Link 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">__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>:

class mycontext(ContextDecorator):

def __enter__(self):

print('Starting')

return self

def __exit__(self, *exc):

print('Finishing')

return False

</pre></div>

</div>

The class can then be used like this:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> @mycontext()

... def function():

... print('The bit in the middle')

...

>>> function()

Starting

The bit in the middle

Finishing

>>> with mycontext():

... print('The bit in the middle')

...

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:

class mycontext(ContextBaseClass, ContextDecorator):

def __enter__(self):

return self

return False

</pre></div>

</div>

توجه

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 <code class="xref std std-keyword docutils literal notranslate">with</code> statement inside the function should be used.

</div>

Added in version 3.2.

</div>

</dd></dl>

class contextlib.AsyncContextDecorator<a class="headerlink" href="#contextlib.AsyncContextDecorator" title="Link to this definition">¶</a></dt>

<dd>Similar to <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate">ContextDecorator</code></a> but only for asynchronous functions.

Example of <code class="docutils literal notranslate">AsyncContextDecorator</code>:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>from asyncio import run

from contextlib import AsyncContextDecorator

class mycontext(AsyncContextDecorator):

async def __aenter__(self):

print('Starting')

return self

async def __aexit__(self, *exc):

print('Finishing')

return False

</pre></div>

</div>

The class can then be used like this:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> @mycontext()

... async def function():

... print('The bit in the middle')

...

>>> run(function())

Starting

The bit in the middle

Finishing

>>> async def function():

... async with mycontext():

... print('The bit in the middle')

...

>>> run(function())

Starting

The bit in the middle

Finishing

</pre></div>

</div>

Added in version 3.10.

</div>

</dd></dl>

class contextlib.ExitStack<a class="headerlink" href="#contextlib.ExitStack" title="Link 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>

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> method returns the <code class="xref py py-class docutils literal notranslate">ExitStack</code> instance, and

performs no additional operations.

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.

Added in version 3.3.

</div>

enter_context(cm)<a class="headerlink" href="#contextlib.ExitStack.enter_context" title="Link 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.

تغییر داده شده در نسخه 3.11: Raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> instead of <a class="reference internal" href="exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a> if cm

is not a context manager.

</div>

</dd></dl>

push(exit)<a class="headerlink" href="#contextlib.ExitStack.push" title="Link 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

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>

callback(callback, <abbr title="Positional-only parameter separator (PEP 570)">/</abbr>, *args, **kwds)<a class="headerlink" href="#contextlib.ExitStack.callback" title="Link 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>

pop_all()<a class="headerlink" href="#contextlib.ExitStack.pop_all" title="Link to this definition">¶</a></dt>

<dd>Transfers the callback stack to a fresh <code class="xref py py-class docutils literal notranslate">ExitStack</code> 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:

# Hold onto the close method, but don'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>

close()<a class="headerlink" href="#contextlib.ExitStack.close" title="Link 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>

class contextlib.AsyncExitStack<a class="headerlink" href="#contextlib.AsyncExitStack" title="Link to this definition">¶</a></dt>

<dd>An <a class="reference internal" href="../reference/datamodel.html#async-context-managers">asynchronous context manager</a>, similar

to <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate">ExitStack</code></a>, that supports combining both synchronous and

asynchronous context managers, as well as having coroutines for

cleanup logic.

The <a class="reference internal" href="#contextlib.ExitStack.close" title="contextlib.ExitStack.close"><code class="xref py py-meth docutils literal notranslate">close()</code></a> method is not implemented; <a class="reference internal" href="#contextlib.AsyncExitStack.aclose" title="contextlib.AsyncExitStack.aclose"><code class="xref py py-meth docutils literal notranslate">aclose()</code></a> must be used

instead.

async enter_async_context(cm)<a class="headerlink" href="#contextlib.AsyncExitStack.enter_async_context" title="Link to this definition">¶</a></dt>

<dd>Similar to <a class="reference internal" href="#contextlib.ExitStack.enter_context" title="contextlib.ExitStack.enter_context"><code class="xref py py-meth docutils literal notranslate">ExitStack.enter_context()</code></a> but expects an asynchronous context

manager.

is not an asynchronous context manager.

</div>

</dd></dl>

push_async_exit(exit)<a class="headerlink" href="#contextlib.AsyncExitStack.push_async_exit" title="Link to this definition">¶</a></dt>

<dd>Similar to <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> but expects either an asynchronous context manager

or a coroutine function.

</dd></dl>

push_async_callback(callback, <abbr title="Positional-only parameter separator (PEP 570)">/</abbr>, *args, **kwds)<a class="headerlink" href="#contextlib.AsyncExitStack.push_async_callback" title="Link to this definition">¶</a></dt>

<dd>Similar to <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> but expects a coroutine function.

</dd></dl>

async aclose()<a class="headerlink" href="#contextlib.AsyncExitStack.aclose" title="Link to this definition">¶</a></dt>

<dd>Similar to <a class="reference internal" href="#contextlib.ExitStack.close" title="contextlib.ExitStack.close"><code class="xref py py-meth docutils literal notranslate">ExitStack.close()</code></a> but properly handles awaitables.

</dd></dl>

Continuing the example for <a class="reference internal" href="#contextlib.asynccontextmanager" title="contextlib.asynccontextmanager"><code class="xref py py-func docutils literal notranslate">asynccontextmanager()</code></a>:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>async with AsyncExitStack() as stack:

connections = [await stack.enter_async_context(get_connection())

for i in range(5)]

# All opened connections will automatically be released at the end of

# the async with statement, even if attempts to open a connection

# later in the list raise an exception.

</pre></div>

</div>

Added in version 3.7.

</div>

</dd></dl>

</section>

<h2>Examples and Recipes<a class="headerlink" href="#examples-and-recipes" title="Link to this heading">¶</a></h2>

This section describes some examples and recipes for making effective use of

the tools provided by <code class="xref py py-mod docutils literal notranslate">contextlib</code>.

<h3>Supporting a variable number of context managers<a class="headerlink" href="#supporting-a-variable-number-of-context-managers" title="Link to this heading">¶</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:

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.

</section>

<h3>Catching exceptions from <code class="docutils literal notranslate">__enter__</code> methods<a class="headerlink" href="#catching-exceptions-from-enter-methods" title="Link to this heading">¶</a></h3>

It is occasionally desirable to catch exceptions from 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>

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 <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. 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.

</section>

<h3>Cleaning up in an <code class="docutils literal notranslate">__enter__</code> implementation<a class="headerlink" href="#cleaning-up-in-an-enter-implementation" title="Link to this heading">¶</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'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 = "Failed validation for {!r}"

raise RuntimeError(msg.format(resource))

return resource

# We don't need to duplicate any of our resource release logic

self.release_resource()

</pre></div>

</div>

</section>

<h3>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="Link to this heading">¶</a></h3>

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