`contextlib` --- `with` 陳述式工具程式¶

原始碼：Lib/contextlib.py

This module provides utilities for common tasks involving the with statement. For more information see also 情境管理器型別 and With 陳述式的情境管理器.

Utilities¶

Functions and classes provided:

class contextlib.AbstractContextManager¶: An abstract base class for classes that implement __enter__() and __exit__(). A default implementation for __enter__() is provided which returns self while __exit__() is an abstract method which by default returns None. See also the definition of 情境管理器型別.

在 3.6 版被加入.

class contextlib.AbstractAsyncContextManager¶: An abstract base class for classes that implement __aenter__() and __aexit__(). A default implementation for __aenter__() is provided which returns self while __aexit__() is an abstract method which by default returns None. See also the definition of Asynchronous Context Managers.

在 3.7 版被加入.

@contextlib.contextmanager¶

This function is a decorator that can be used to define a factory function for with statement context managers, without needing to create a class or separate __enter__() and __exit__() 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 close() method for use with contextlib.closing.

An abstract example would be the following to ensure correct resource management:

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)

The function can then be used like this:

>>> 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

The function being decorated must return a generator-iterator when called. This iterator must yield exactly one value, which will be bound to the targets in the with statement's as clause, if any.

At the point where the generator yields, the block nested in the with 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 try...except...finally 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 with statement that the exception has been handled, and execution will resume with the statement immediately following the with statement.

contextmanager() uses ContextDecorator so the context managers it creates can be used as decorators as well as in with 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 contextmanager() to meet the requirement that context managers support multiple invocations in order to be used as decorators).

在 3.2 版的變更: Use of ContextDecorator.

@contextlib.asynccontextmanager¶

Similar to contextmanager(), but creates an asynchronous context manager.

This function is a decorator that can be used to define a factory function for async with statement asynchronous context managers, without needing to create a class or separate __aenter__() and __aexit__() methods. It must be applied to an asynchronous generator function.

一個簡單範例：

from contextlib import asynccontextmanager

@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 ...')

在 3.7 版被加入.

Context managers defined with asynccontextmanager() can be used either as decorators or with async with statements:

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 ...

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 asynccontextmanager() to meet the requirement that context managers support multiple invocations in order to be used as decorators.

在 3.10 版的變更: Async context managers created with asynccontextmanager() can be used as decorators.

contextlib.closing(thing)¶

Return a context manager that closes thing upon completion of the block. This is basically equivalent to:

from contextlib import contextmanager

@contextmanager
def closing(thing):
    try:
        yield thing
    finally:
        thing.close()

And lets you write code like this:

from contextlib import closing
from urllib.request import urlopen

with closing(urlopen('https://www.python.org')) as page:
    for line in page:
        print(line)

without needing to explicitly close page. Even if an error occurs, page.close() will be called when the with block is exited.

備註

Most types managing resources support the context manager protocol, which closes thing on leaving the with statement. As such, closing() is most useful for third party types that don't support context managers. This example is purely for illustration purposes, as urlopen() would normally be used in a context manager.

contextlib.aclosing(thing)¶

Return an async context manager that calls the aclose() method of thing upon completion of the block. This is basically equivalent to:

from contextlib import asynccontextmanager

@asynccontextmanager
async def aclosing(thing):
    try:
        yield thing
    finally:
        await thing.aclose()

Significantly, aclosing() supports deterministic cleanup of async generators when they happen to exit early by break or an exception. For example:

from contextlib import aclosing

async with aclosing(my_generator()) as values:
    async for value in values:
        if value == 42:
            break

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).

在 3.10 版被加入.

contextlib.nullcontext(enter_result=None)¶

Return a context manager that returns enter_result from __enter__(), but otherwise does nothing. It is intended to be used as a stand-in for an optional context manager, for example:

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

一個使用 enter_result 的範例：

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

It can also be used as a stand-in for asynchronous context managers:

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

在 3.7 版被加入.

在 3.10 版的變更: asynchronous context manager support was added.

contextlib.suppress(*exceptions)¶

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.

舉例來說：

from contextlib import suppress

with suppress(FileNotFoundError):
    os.remove('somefile.tmp')

with suppress(FileNotFoundError):
    os.remove('someotherfile.tmp')

This code is equivalent to:

try:
    os.remove('somefile.tmp')
except FileNotFoundError:
    pass

try:
    os.remove('someotherfile.tmp')
except FileNotFoundError:
    pass

This context manager is reentrant.

If the code within the with block raises a BaseExceptionGroup, 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 derive() method.

在 3.4 版被加入.

在 3.12 版的變更: suppress now supports suppressing exceptions raised as part of a BaseExceptionGroup.

contextlib.redirect_stdout(new_target)¶

Context manager for temporarily redirecting sys.stdout 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 help() normally is sent to sys.stdout. You can capture that output in a string by redirecting the output to an io.StringIO object. The replacement stream is returned from the __enter__() method and so is available as the target of the with statement:

with redirect_stdout(io.StringIO()) as f:
    help(pow)
s = f.getvalue()

To send the output of help() to a file on disk, redirect the output to a regular file:

with open('help.txt', 'w') as f:
    with redirect_stdout(f):
        help(pow)

To send the output of help() to sys.stderr:

with redirect_stdout(sys.stderr):
    help(pow)

Note that the global side effect on sys.stdout 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 reentrant.

在 3.4 版被加入.

contextlib.redirect_stderr(new_target)¶

Similar to redirect_stdout() but redirecting sys.stderr to another file or file-like object.

This context manager is reentrant.

在 3.5 版被加入.

contextlib.chdir(path)¶

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 chdir(), it changes the current working directory upon entering and restores the old one on exit.

This context manager is reentrant.

在 3.11 版被加入.

class contextlib.ContextDecorator¶

A base class that enables a context manager to also be used as a decorator.

Context managers inheriting from ContextDecorator have to implement __enter__() and __exit__() as normal. __exit__ retains its optional exception handling even when used as a decorator.

ContextDecorator is used by contextmanager(), so you get this functionality automatically.

ContextDecorator 範例：

from contextlib import ContextDecorator

class mycontext(ContextDecorator):
    def __enter__(self):
        print('Starting')
        return self

    def __exit__(self, *exc):
        print('Finishing')
        return False

這個類別可以這樣使用：

>>> @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

這個變更只是以下形式的語法糖：

def f():
    with cm():
        # Do stuff

ContextDecorator 讓你可以改寫成：

@cm()
def f():
    # Do stuff

It makes it clear that the cm 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 ContextDecorator as a mixin class:

from contextlib import ContextDecorator

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

    def __exit__(self, *exc):
        return False

備註

As the decorated function must be able to be called multiple times, the underlying context manager must support use in multiple with statements. If this is not the case, then the original construct with the explicit with statement inside the function should be used.

在 3.2 版被加入.

class contextlib.AsyncContextDecorator¶

和 ContextDecorator 類似，但僅用於非同步函式。

AsyncContextDecorator 範例：

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

這個類別可以這樣使用：

>>> @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

在 3.10 版被加入.

class contextlib.ExitStack¶

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:

with ExitStack() as stack:
    files = [stack.enter_context(open(fname)) for 

contextlib --- with 陳述式工具程式¶

Utilities¶

`contextlib` --- `with` 陳述式工具程式¶