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# `contextlib` --- Utilities for `with`-statement contexts
**Source code:** [Lib/contextlib.py](https://github.com/python/cpython/tree/3.7/Lib/contextlib.py) \[https://github.com/python/cpython/tree/3.7/Lib/contextlib.py\]
- - - - - -
This module provides utilities for common tasks involving the [`with`](../reference/compound_stmts.xhtml#with)statement. For more information see also [上下文管理器类型](stdtypes.xhtml#typecontextmanager) and [with 语句上下文管理器](../reference/datamodel.xhtml#context-managers).
## 工具
Functions and classes provided:
*class* `contextlib.``AbstractContextManager`An [abstract base class](../glossary.xhtml#term-abstract-base-class) for classes that implement [`object.__enter__()`](../reference/datamodel.xhtml#object.__enter__ "object.__enter__") and [`object.__exit__()`](../reference/datamodel.xhtml#object.__exit__ "object.__exit__"). A default implementation for [`object.__enter__()`](../reference/datamodel.xhtml#object.__enter__ "object.__enter__") is provided which returns `self` while [`object.__exit__()`](../reference/datamodel.xhtml#object.__exit__ "object.__exit__") is an abstract method which by default returns `None`. See also the definition of [上下文管理器类型](stdtypes.xhtml#typecontextmanager).
3\.6 新版功能.
*class* `contextlib.``AbstractAsyncContextManager`An [abstract base class](../glossary.xhtml#term-abstract-base-class) for classes that implement [`object.__aenter__()`](../reference/datamodel.xhtml#object.__aenter__ "object.__aenter__") and [`object.__aexit__()`](../reference/datamodel.xhtml#object.__aexit__ "object.__aexit__"). A default implementation for [`object.__aenter__()`](../reference/datamodel.xhtml#object.__aenter__ "object.__aenter__") is provided which returns `self` while [`object.__aexit__()`](../reference/datamodel.xhtml#object.__aexit__ "object.__aexit__") is an abstract method which by default returns `None`. See also the definition of [异步上下文管理器](../reference/datamodel.xhtml#async-context-managers).
3\.7 新版功能.
`@``contextlib.``contextmanager`This function is a [decorator](../glossary.xhtml#term-decorator) that can be used to define a factory function for [`with`](../reference/compound_stmts.xhtml#with) statement context managers, without needing to create a class or separate [`__enter__()`](../reference/datamodel.xhtml#object.__enter__ "object.__enter__") and [`__exit__()`](../reference/datamodel.xhtml#object.__exit__ "object.__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)
>>> 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](../glossary.xhtml#term-generator)-iterator when called. This iterator must yield exactly one value, which will be bound to the targets in the [`with`](../reference/compound_stmts.xhtml#with) statement's `as` clause, if any.
At the point where the generator yields, the block nested in the [`with`](../reference/compound_stmts.xhtml#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`](../reference/compound_stmts.xhtml#try)...[`except`](../reference/compound_stmts.xhtml#except)...[`finally`](../reference/compound_stmts.xhtml#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()`](#contextlib.contextmanager "contextlib.contextmanager") uses [`ContextDecorator`](#contextlib.ContextDecorator "contextlib.ContextDecorator") so the context managers it creates can be used as decorators as well as in [`with`](../reference/compound_stmts.xhtml#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()`](#contextlib.contextmanager "contextlib.contextmanager") to meet the requirement that context managers support multiple invocations in order to be used as decorators).
在 3.2 版更改: Use of [`ContextDecorator`](#contextlib.ContextDecorator "contextlib.ContextDecorator").
`@``contextlib.``asynccontextmanager`Similar to [`contextmanager()`](#contextlib.contextmanager "contextlib.contextmanager"), but creates an [asynchronous context manager](../reference/datamodel.xhtml#async-context-managers).
This function is a [decorator](../glossary.xhtml#term-decorator) that can be used to define a factory function for [`async with`](../reference/compound_stmts.xhtml#async-with) statement asynchronous context managers, without needing to create a class or separate [`__aenter__()`](../reference/datamodel.xhtml#object.__aenter__ "object.__aenter__") and [`__aexit__()`](../reference/datamodel.xhtml#object.__aexit__ "object.__aexit__") methods. It must be applied to an [asynchronous generator](../glossary.xhtml#term-asynchronous-generator) function.
A simple example:
```
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 ...')
```
3\.7 新版功能.
`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('http://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`](../reference/compound_stmts.xhtml#with) block is exited.
`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
```
An example using *enter\_result*:
```
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
```
3\.7 新版功能.
`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.
例如:
```
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](#reentrant-cms).
3\.4 新版功能.
`contextlib.``redirect_stdout`(*new\_target*)Context manager for temporarily redirecting [`sys.stdout`](sys.xhtml#sys.stdout "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()`](functions.xhtml#help "help") normally is sent to *sys.stdout*. You can capture that output in a string by redirecting the output to an [`io.StringIO`](io.xhtml#io.StringIO "io.StringIO") object:
```
f = io.StringIO()
with redirect_stdout(f):
help(pow)
s = f.getvalue()
```
To send the output of [`help()`](functions.xhtml#help "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()`](functions.xhtml#help "help") to *sys.stderr*:
```
with redirect_stdout(sys.stderr):
help(pow)
```
Note that the global side effect on [`sys.stdout`](sys.xhtml#sys.stdout "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](#reentrant-cms).
3\.4 新版功能.
`contextlib.``redirect_stderr`(*new\_target*)Similar to [`redirect_stdout()`](#contextlib.redirect_stdout "contextlib.redirect_stdout") but redirecting [`sys.stderr`](sys.xhtml#sys.stderr "sys.stderr") to another file or file-like object.
This context manager is [reentrant](#reentrant-cms).
3\.5 新版功能.
*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()`](#contextlib.contextmanager "contextlib.contextmanager"), so you get this functionality automatically.
Example of `ContextDecorator`:
```
from contextlib import ContextDecorator
class mycontext(ContextDecorator):
def __enter__(self):
print('Starting')
return self
def __exit__(self, *exc):
print('Finishing')
return False
>>> @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
```
This change is just syntactic sugar for any construct of the following form:
```
def f():
with cm():
# Do stuff
```
`ContextDecorator` lets you instead write:
```
@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
```
注解
As the decorated function must be able to be called multiple times, the underlying context manager must support use in multiple [`with`](../reference/compound_stmts.xhtml#with)statements. If this is not the case, then the original construct with the explicit `with` statement inside the function should be used.
3\.2 新版功能.
*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 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
```
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 [`with`](../reference/compound_stmts.xhtml#with) 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 `__init__` 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 [`with`](../reference/compound_stmts.xhtml#with)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.
3\.3 新版功能.
`enter_context`(*cm*)Enters a new context manager and adds its [`__exit__()`](../reference/datamodel.xhtml#object.__exit__ "object.__exit__") method to the callback stack. The return value is the result of the context manager's own [`__enter__()`](../reference/datamodel.xhtml#object.__enter__ "object.__enter__") method.
These context managers may suppress exceptions just as they normally would if used directly as part of a [`with`](../reference/compound_stmts.xhtml#with) statement.
`push`(*exit*)Adds a context manager's [`__exit__()`](../reference/datamodel.xhtml#object.__exit__ "object.__exit__") method to the callback stack.
As `__enter__` is *not* invoked, this method can be used to cover part of an [`__enter__()`](../reference/datamodel.xhtml#object.__enter__ "object.__enter__") implementation with a context manager's own [`__exit__()`](../reference/datamodel.xhtml#object.__exit__ "object.__exit__") 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 [`__exit__()`](../reference/datamodel.xhtml#object.__exit__ "object.__exit__") method and adds it directly to the callback stack.
By returning true values, these callbacks can suppress exceptions the same way context manager [`__exit__()`](../reference/datamodel.xhtml#object.__exit__ "object.__exit__") methods can.
The passed in object is returned from the function, allowing this method to be used as a function decorator.
`callback`(*callback*, *\*args*, *\*\*kwds*)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.
`pop_all`()Transfers the callback stack to a fresh [`ExitStack`](#contextlib.ExitStack "contextlib.ExitStack") 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 [`with`](../reference/compound_stmts.xhtml#with) statement).
For example, a group of files can be opened as an "all or nothing" operation as follows:
```
with ExitStack() as stack:
files = [stack.enter_context(open(fname)) for fname in filenames]
# 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.
```
`close`()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.
*class* `contextlib.``AsyncExitStack`An [asynchronous context manager](../reference/datamodel.xhtml#async-context-managers), similar to [`ExitStack`](#contextlib.ExitStack "contextlib.ExitStack"), that supports combining both synchronous and asynchronous context managers, as well as having coroutines for cleanup logic.
The `close()` method is not implemented, [`aclose()`](#contextlib.AsyncExitStack.aclose "contextlib.AsyncExitStack.aclose") must be used instead.
`enter_async_context`(*cm*)Similar to `enter_context()` but expects an asynchronous context manager.
`push_async_exit`(*exit*)Similar to `push()` but expects either an asynchronous context manager or a coroutine function.
`push_async_callback`(*callback*, *\*args*, *\*\*kwds*)Similar to `callback()` but expects a coroutine function.
`aclose`()Similar to `close()` but properly handles awaitables.
Continuing the example for [`asynccontextmanager()`](#contextlib.asynccontextmanager "contextlib.asynccontextmanager"):
```
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.
```
3\.7 新版功能.
## 例子和配方
This section describes some examples and recipes for making effective use of the tools provided by [`contextlib`](#module-contextlib "contextlib: Utilities for with-statement contexts.").
### Supporting a variable number of context managers
The primary use case for [`ExitStack`](#contextlib.ExitStack "contextlib.ExitStack") is the one given in the class documentation: supporting a variable number of context managers and other cleanup operations in a single [`with`](../reference/compound_stmts.xhtml#with) 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:
```
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
```
As shown, [`ExitStack`](#contextlib.ExitStack "contextlib.ExitStack") also makes it quite easy to use [`with`](../reference/compound_stmts.xhtml#with)statements to manage arbitrary resources that don't natively support the context management protocol.
### Catching exceptions from `__enter__` methods
It is occasionally desirable to catch exceptions from an `__enter__`method implementation, *without* inadvertently catching exceptions from the [`with`](../reference/compound_stmts.xhtml#with) statement body or the context manager's `__exit__`method. By using [`ExitStack`](#contextlib.ExitStack "contextlib.ExitStack") the steps in the context management protocol can be separated slightly in order to allow this:
```
stack = ExitStack()
try:
x = stack.enter_context(cm)
except Exception:
# handle __enter__ exception
else:
with stack:
# Handle normal case
```
Actually needing to do this is likely to indicate that the underlying API should be providing a direct resource management interface for use with [`try`](../reference/compound_stmts.xhtml#try)/[`except`](../reference/compound_stmts.xhtml#except)/[`finally`](../reference/compound_stmts.xhtml#finally) statements, but not all APIs are well designed in that regard. When a context manager is the only resource management API provided, then [`ExitStack`](#contextlib.ExitStack "contextlib.ExitStack") can make it easier to handle various situations that can't be handled directly in a [`with`](../reference/compound_stmts.xhtml#with) statement.
### Cleaning up in an `__enter__` implementation
As noted in the documentation of [`ExitStack.push()`](#contextlib.ExitStack.push "contextlib.ExitStack.push"), this method can be useful in cleaning up an already allocated resource if later steps in the [`__enter__()`](../reference/datamodel.xhtml#object.__enter__ "object.__enter__") 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:
```
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
def __exit__(self, *exc_details):
# We don't need to duplicate any of our resource release logic
self.release_resource()
```
### Replacing any use of `try-finally` and flag variables
A pattern you will sometimes see is a `try-finally` statement with a flag variable to indicate whether or not the body of the `finally` clause should be executed. In its simplest form (that can't already be handled just by using an `except` clause instead), it looks something like this:
```
cleanup_needed = True
try:
result = perform_operation()
if result:
cleanup_needed = False
finally:
if cleanup_needed:
cleanup_resources()
```
As with any `try` 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.
[`ExitStack`](#contextlib.ExitStack "contextlib.ExitStack") makes it possible to instead register a callback for execution at the end of a `with` statement, and then later decide to skip executing that callback:
```
from contextlib import ExitStack
with ExitStack() as stack:
stack.callback(cleanup_resources)
result = perform_operation()
if result:
stack.pop_all()
```
This allows the intended cleanup up behaviour to be made explicit up front, rather than requiring a separate flag variable.
If a particular application uses this pattern a lot, it can be simplified even further by means of a small helper class:
```
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()
```
If the resource cleanup isn't already neatly bundled into a standalone function, then it is still possible to use the decorator form of [`ExitStack.callback()`](#contextlib.ExitStack.callback "contextlib.ExitStack.callback") to declare the resource cleanup in advance:
```
from contextlib import ExitStack
with ExitStack() as stack:
@stack.callback
def cleanup_resources():
...
result = perform_operation()
if result:
stack.pop_all()
```
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.
### Using a context manager as a function decorator
[`ContextDecorator`](#contextlib.ContextDecorator "contextlib.ContextDecorator") makes it possible to use a context manager in both an ordinary `with` 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 [`ContextDecorator`](#contextlib.ContextDecorator "contextlib.ContextDecorator") provides both capabilities in a single definition:
```
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('Entering: %s', self.name)
def __exit__(self, exc_type, exc, exc_tb):
logging.info('Exiting: %s', self.name)
```
Instances of this class can be used as both a context manager:
```
with track_entry_and_exit('widget loader'):
print('Some time consuming activity goes here')
load_widget()
```
And also as a function decorator:
```
@track_entry_and_exit('widget loader')
def activity():
print('Some time consuming activity goes here')
load_widget()
```
Note that there is one additional limitation when using context managers as function decorators: there's no way to access the return value of [`__enter__()`](../reference/datamodel.xhtml#object.__enter__ "object.__enter__"). If that value is needed, then it is still necessary to use an explicit `with` statement.
参见
[**PEP 343**](https://www.python.org/dev/peps/pep-0343) \[https://www.python.org/dev/peps/pep-0343\] - "with" 语句Python [`with`](../reference/compound_stmts.xhtml#with) 语句的规范描述、背景和示例。
## Single use, reusable and reentrant context managers
Most context managers are written in a way that means they can only be used effectively in a [`with`](../reference/compound_stmts.xhtml#with) 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 [`with`](../reference/compound_stmts.xhtml#with) 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 [`with`](../reference/compound_stmts.xhtml#with) statement will close the file, preventing any further IO operations using that file object.
Context managers created using [`contextmanager()`](#contextlib.contextmanager "contextlib.contextmanager") 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:
```
>>> from contextlib import contextmanager
>>> @contextmanager
... def singleuse():
... print("Before")
... yield
... print("After")
...
>>> cm = singleuse()
>>> with cm:
... pass
...
Before
After
>>> with cm:
... pass
...
Traceback (most recent call last):
...
RuntimeError: generator didn't yield
```
### Reentrant context managers
More sophisticated context managers may be "reentrant". These context managers can not only be used in multiple [`with`](../reference/compound_stmts.xhtml#with) statements, but may also be used *inside* a `with` statement that is already using the same context manager.
[`threading.RLock`](threading.xhtml#threading.RLock "threading.RLock") is an example of a reentrant context manager, as are [`suppress()`](#contextlib.suppress "contextlib.suppress") and [`redirect_stdout()`](#contextlib.redirect_stdout "contextlib.redirect_stdout"). Here's a very simple example of reentrant use:
```
>>> from contextlib import redirect_stdout
>>> from io import StringIO
>>> stream = StringIO()
>>> write_to_stream = redirect_stdout(stream)
>>> with write_to_stream:
... print("This is written to the stream rather than stdout")
... with write_to_stream:
... print("This is also written to the stream")
...
>>> print("This is written directly to stdout")
This is written directly to stdout
>>> print(stream.getvalue())
This is written to the stream rather than stdout
This is also written to the stream
```
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. [`redirect_stdout()`](#contextlib.redirect_stdout "contextlib.redirect_stdout"), for example, is definitely not thread safe, as it makes a global modification to the system state by binding [`sys.stdout`](sys.xhtml#sys.stdout "sys.stdout")to a different stream.
### Reusable context managers
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.
[`threading.Lock`](threading.xhtml#threading.Lock "threading.Lock") is an example of a reusable, but not reentrant, context manager (for a reentrant lock, it is necessary to use [`threading.RLock`](threading.xhtml#threading.RLock "threading.RLock") instead).
Another example of a reusable, but not reentrant, context manager is [`ExitStack`](#contextlib.ExitStack "contextlib.ExitStack"), as it invokes *all* currently registered callbacks when leaving any with statement, regardless of where those callbacks were added:
```
>>> from contextlib import ExitStack
>>> stack = ExitStack()
>>> with stack:
... stack.callback(print, "Callback: from first context")
... print("Leaving first context")
...
Leaving first context
Callback: from first context
>>> with stack:
... stack.callback(print, "Callback: from second context")
... print("Leaving second context")
...
Leaving second context
Callback: from second context
>>> with stack:
... stack.callback(print, "Callback: from outer context")
... with stack:
... stack.callback(print, "Callback: from inner context")
... print("Leaving inner context")
... print("Leaving outer context")
...
Leaving inner context
Callback: from inner context
Callback: from outer context
Leaving outer context
```
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 [`ExitStack`](#contextlib.ExitStack "contextlib.ExitStack") instances instead of reusing a single instance avoids that problem:
```
>>> from contextlib import ExitStack
>>> with ExitStack() as outer_stack:
... outer_stack.callback(print, "Callback: from outer context")
... with ExitStack() as inner_stack:
... inner_stack.callback(print, "Callback: from inner context")
... print("Leaving inner context")
... print("Leaving outer context")
...
Leaving inner context
Callback: from inner context
Leaving outer context
Callback: from outer context
```
### 导航
- [索引](../genindex.xhtml "总目录")
- [模块](../py-modindex.xhtml "Python 模块索引") |
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- 数据持久化
- pickle —— Python 对象序列化
- copyreg — Register pickle support functions
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- 数据压缩和存档
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- 加密服务
- hashlib — 安全哈希与消息摘要
- hmac — 基于密钥的消息验证
- secrets — Generate secure random numbers for managing secrets
- 通用操作系统服务
- os — 操作系统接口模块
- io — 处理流的核心工具
- time — 时间的访问和转换
- argparse — 命令行选项、参数和子命令解析器
- getopt — C-style parser for command line options
- 模块 logging — Python 的日志记录工具
- logging.config — 日志记录配置
- logging.handlers — Logging handlers
- getpass — 便携式密码输入工具
- curses — 终端字符单元显示的处理
- curses.textpad — Text input widget for curses programs
- curses.ascii — Utilities for ASCII characters
- curses.panel — A panel stack extension for curses
- platform — Access to underlying platform's identifying data
- errno — Standard errno system symbols
- ctypes — Python 的外部函数库
- 并发执行
- threading — 基于线程的并行
- multiprocessing — 基于进程的并行
- concurrent 包
- concurrent.futures — 启动并行任务
- subprocess — 子进程管理
- sched — 事件调度器
- queue — 一个同步的队列类
- _thread — 底层多线程 API
- _dummy_thread — _thread 的替代模块
- dummy_threading — 可直接替代 threading 模块。
- contextvars — Context Variables
- Context Variables
- Manual Context Management
- asyncio support
- 网络和进程间通信
- asyncio — 异步 I/O
- socket — 底层网络接口
- ssl — TLS/SSL wrapper for socket objects
- select — Waiting for I/O completion
- selectors — 高级 I/O 复用库
- asyncore — 异步socket处理器
- asynchat — 异步 socket 指令/响应 处理器
- signal — Set handlers for asynchronous events
- mmap — Memory-mapped file support
- 互联网数据处理
- email — 电子邮件与 MIME 处理包
- json — JSON 编码和解码器
- mailcap — Mailcap file handling
- mailbox — Manipulate mailboxes in various formats
- mimetypes — Map filenames to MIME types
- base64 — Base16, Base32, Base64, Base85 数据编码
- binhex — 对binhex4文件进行编码和解码
- binascii — 二进制和 ASCII 码互转
- quopri — Encode and decode MIME quoted-printable data
- uu — Encode and decode uuencode files
- 结构化标记处理工具
- html — 超文本标记语言支持
- html.parser — 简单的 HTML 和 XHTML 解析器
- html.entities — HTML 一般实体的定义
- XML处理模块
- xml.etree.ElementTree — The ElementTree XML API
- xml.dom — The Document Object Model API
- xml.dom.minidom — Minimal DOM implementation
- xml.dom.pulldom — Support for building partial DOM trees
- xml.sax — Support for SAX2 parsers
- xml.sax.handler — Base classes for SAX handlers
- xml.sax.saxutils — SAX Utilities
- xml.sax.xmlreader — Interface for XML parsers
- xml.parsers.expat — Fast XML parsing using Expat
- 互联网协议和支持
- webbrowser — 方便的Web浏览器控制器
- cgi — Common Gateway Interface support
- cgitb — Traceback manager for CGI scripts
- wsgiref — WSGI Utilities and Reference Implementation
- urllib — URL 处理模块
- urllib.request — 用于打开 URL 的可扩展库
- urllib.response — Response classes used by urllib
- urllib.parse — Parse URLs into components
- urllib.error — Exception classes raised by urllib.request
- urllib.robotparser — Parser for robots.txt
- http — HTTP 模块
- http.client — HTTP协议客户端
- ftplib — FTP protocol client
- poplib — POP3 protocol client
- imaplib — IMAP4 protocol client
- nntplib — NNTP protocol client
- smtplib —SMTP协议客户端
- smtpd — SMTP Server
- telnetlib — Telnet client
- uuid — UUID objects according to RFC 4122
- socketserver — A framework for network servers
- http.server — HTTP 服务器
- http.cookies — HTTP state management
- http.cookiejar — Cookie handling for HTTP clients
- xmlrpc — XMLRPC 服务端与客户端模块
- xmlrpc.client — XML-RPC client access
- xmlrpc.server — Basic XML-RPC servers
- ipaddress — IPv4/IPv6 manipulation library
- 多媒体服务
- audioop — Manipulate raw audio data
- aifc — Read and write AIFF and AIFC files
- sunau — 读写 Sun AU 文件
- wave — 读写WAV格式文件
- chunk — Read IFF chunked data
- colorsys — Conversions between color systems
- imghdr — 推测图像类型
- sndhdr — 推测声音文件的类型
- ossaudiodev — Access to OSS-compatible audio devices
- 国际化
- gettext — 多语种国际化服务
- locale — 国际化服务
- 程序框架
- turtle — 海龟绘图
- cmd — 支持面向行的命令解释器
- shlex — Simple lexical analysis
- Tk图形用户界面(GUI)
- tkinter — Tcl/Tk的Python接口
- tkinter.ttk — Tk themed widgets
- tkinter.tix — Extension widgets for Tk
- tkinter.scrolledtext — 滚动文字控件
- IDLE
- 其他图形用户界面（GUI）包
- 开发工具
- typing — 类型标注支持
- pydoc — Documentation generator and online help system
- doctest — Test interactive Python examples
- unittest — 单元测试框架
- unittest.mock — mock object library
- unittest.mock 上手指南
- 2to3 - 自动将 Python 2 代码转为 Python 3 代码
- test — Regression tests package for Python
- test.support — Utilities for the Python test suite
- test.support.script_helper — Utilities for the Python execution tests
- 调试和分析
- bdb — Debugger framework
- faulthandler — Dump the Python traceback
- pdb — The Python Debugger
- The Python Profilers
- timeit — 测量小代码片段的执行时间
- trace — Trace or track Python statement execution
- tracemalloc — Trace memory allocations
- 软件打包和分发
- distutils — 构建和安装 Python 模块
- ensurepip — Bootstrapping the pip installer
- venv — 创建虚拟环境
- zipapp — Manage executable Python zip archives
- Python运行时服务
- sys — 系统相关的参数和函数
- sysconfig — Provide access to Python's configuration information
- builtins — 内建对象
- main — 顶层脚本环境
- warnings — Warning control
- dataclasses — 数据类
- contextlib — Utilities for with-statement contexts
- abc — 抽象基类
- atexit — 退出处理器
- traceback — Print or retrieve a stack traceback
- future — Future 语句定义
- gc — 垃圾回收器接口
- inspect — 检查对象
- site — Site-specific configuration hook
- 自定义 Python 解释器
- code — Interpreter base classes
- codeop — Compile Python code
- 导入模块
- zipimport — Import modules from Zip archives
- pkgutil — Package extension utility
- modulefinder — 查找脚本使用的模块
- runpy — Locating and executing Python modules
- importlib — The implementation of import
- Python 语言服务
- parser — Access Python parse trees
- ast — 抽象语法树
- symtable — Access to the compiler's symbol tables
- symbol — 与 Python 解析树一起使用的常量
- token — 与Python解析树一起使用的常量
- keyword — 检验Python关键字
- tokenize — Tokenizer for Python source
- tabnanny — 模糊缩进检测
- pyclbr — Python class browser support
- py_compile — Compile Python source files
- compileall — Byte-compile Python libraries
- dis — Python 字节码反汇编器
- pickletools — Tools for pickle developers
- 杂项服务
- formatter — Generic output formatting
- Windows系统相关模块
- msilib — Read and write Microsoft Installer files
- msvcrt — Useful routines from the MS VC++ runtime
- winreg — Windows 注册表访问
- winsound — Sound-playing interface for Windows
- Unix 专有服务
- posix — The most common POSIX system calls
- pwd — 用户密码数据库
- spwd — The shadow password database
- grp — The group database
- crypt — Function to check Unix passwords
- termios — POSIX style tty control
- tty — 终端控制功能
- pty — Pseudo-terminal utilities
- fcntl — The fcntl and ioctl system calls
- pipes — Interface to shell pipelines
- resource — Resource usage information
- nis — Interface to Sun's NIS (Yellow Pages)
- Unix syslog 库例程
- 被取代的模块
- optparse — Parser for command line options
- imp — Access the import internals
- 未创建文档的模块
- 平台特定模块
- 扩展和嵌入 Python 解释器
- 推荐的第三方工具
- 不使用第三方工具创建扩展
- 使用 C 或 C++ 扩展 Python
- 自定义扩展类型：教程
- 定义扩展类型：已分类主题
- 构建C/C++扩展
- 在Windows平台编译C和C++扩展
- 在更大的应用程序中嵌入 CPython 运行时
- Embedding Python in Another Application
- Python/C API 参考手册
- 概述
- 代码标准
- 包含文件
- 有用的宏
- 对象、类型和引用计数
- 异常
- 嵌入Python
- 调试构建
- 稳定的应用程序二进制接口
- The Very High Level Layer
- Reference Counting
- 异常处理
- Printing and clearing
- 抛出异常
- Issuing warnings
- Querying the error indicator
- Signal Handling
- Exception Classes
- Exception Objects
- Unicode Exception Objects
- Recursion Control
- 标准异常
- 标准警告类别
- 工具
- 操作系统实用程序
- 系统功能
- 过程控制
- 导入模块
- Data marshalling support
- 语句解释及变量编译
- 字符串转换与格式化
- 反射
- 编解码器注册与支持功能
- 抽象对象层
- Object Protocol
- 数字协议
- Sequence Protocol
- Mapping Protocol
- 迭代器协议
- 缓冲协议
- Old Buffer Protocol
- 具体的对象层
- 基本对象
- 数值对象
- 序列对象
- 容器对象
- 函数对象
- 其他对象
- Initialization, Finalization, and Threads
- 在Python初始化之前
- 全局配置变量
- Initializing and finalizing the interpreter
- Process-wide parameters
- Thread State and the Global Interpreter Lock
- Sub-interpreter support
- Asynchronous Notifications
- Profiling and Tracing
- Advanced Debugger Support
- Thread Local Storage Support
- 内存管理
- 概述
- 原始内存接口
- Memory Interface
- 对象分配器
- 默认内存分配器
- Customize Memory Allocators
- The pymalloc allocator
- tracemalloc C API
- 示例
- 对象实现支持
- 在堆中分配对象
- Common Object Structures
- Type 对象
- Number Object Structures
- Mapping Object Structures
- Sequence Object Structures
- Buffer Object Structures
- Async Object Structures
- 使对象类型支持循环垃圾回收
- API 和 ABI 版本管理
- 分发 Python 模块
- 关键术语
- 开源许可与协作
- 安装工具
- 阅读指南
- 我该如何...？
- ...为我的项目选择一个名字？
- ...创建和分发二进制扩展？
- 安装 Python 模块
- 关键术语
- 基本使用
- 我应如何 ...？
- ... 在 Python 3.4 之前的 Python 版本中安装 pip ？
- ... 只为当前用户安装软件包？
- ... 安装科学计算类 Python 软件包？
- ... 使用并行安装的多个 Python 版本？
- 常见的安装问题
- 在 Linux 的系统 Python 版本上安装
- 未安装 pip
- 安装二进制编译扩展
- Python 常用指引
- 将 Python 2 代码迁移到 Python 3
- 简要说明
- 详情
- 将扩展模块移植到 Python 3
- 条件编译
- 对象API的更改
- 模块初始化和状态
- CObject 替换为 Capsule
- 其他选项
- Curses Programming with Python
- What is curses?
- Starting and ending a curses application
- Windows and Pads
- Displaying Text
- User Input
- For More Information
- 实现描述器
- 摘要
- 定义和简介
- 描述器协议
- 发起调用描述符
- 描述符示例
- Properties
- 函数和方法
- Static Methods and Class Methods
- 函数式编程指引
- 概述
- 迭代器
- 生成器表达式和列表推导式
- 生成器
- 内置函数
- itertools 模块
- The functools module
- Small functions and the lambda expression
- Revision History and Acknowledgements
- 引用文献
- 日志 HOWTO
- 日志基础教程
- 进阶日志教程
- 日志级别
- 有用的处理程序
- 记录日志中引发的异常
- 使用任意对象作为消息
- 优化
- 日志操作手册
- 在多个模块中使用日志
- 在多线程中使用日志
- 使用多个日志处理器和多种格式化
- 在多个地方记录日志
- 日志服务器配置示例
- 处理日志处理器的阻塞
- Sending and receiving logging events across a network
- Adding contextual information to your logging output
- Logging to a single file from multiple processes
- Using file rotation
- Use of alternative formatting styles
- Customizing LogRecord
- Subclassing QueueHandler - a ZeroMQ example
- Subclassing QueueListener - a ZeroMQ example
- An example dictionary-based configuration
- Using a rotator and namer to customize log rotation processing
- A more elaborate multiprocessing example
- Inserting a BOM into messages sent to a SysLogHandler
- Implementing structured logging
- Customizing handlers with dictConfig()
- Using particular formatting styles throughout your application
- Configuring filters with dictConfig()
- Customized exception formatting
- Speaking logging messages
- Buffering logging messages and outputting them conditionally
- Formatting times using UTC (GMT) via configuration
- Using a context manager for selective logging
- 正则表达式HOWTO
- 概述
- 简单模式
- 使用正则表达式
- 更多模式能力
- 修改字符串
- 常见问题
- 反馈
- 套接字编程指南
- 套接字
- 创建套接字
- 使用一个套接字
- 断开连接
- 非阻塞的套接字
- 排序指南
- 基本排序
- 关键函数
- Operator 模块函数
- 升序和降序
- 排序稳定性和排序复杂度
- 使用装饰-排序-去装饰的旧方法
- 使用 cmp 参数的旧方法
- 其它
- Unicode 指南
- Unicode 概述
- Python's Unicode Support
- Reading and Writing Unicode Data
- Acknowledgements
- 如何使用urllib包获取网络资源
- 概述
- Fetching URLs
- 处理异常
- info and geturl
- Openers and Handlers
- Basic Authentication
- Proxies
- Sockets and Layers
- 脚注
- Argparse 教程
- 概念
- 基础
- 位置参数介绍
- Introducing Optional arguments
- Combining Positional and Optional arguments
- Getting a little more advanced
- Conclusion
- ipaddress模块介绍
- 创建 Address/Network/Interface 对象
- 审查 Address/Network/Interface 对象
- Network 作为 Address 列表
- 比较
- 将IP地址与其他模块一起使用
- 实例创建失败时获取更多详细信息
- Argument Clinic How-To
- The Goals Of Argument Clinic
- Basic Concepts And Usage
- Converting Your First Function
- Advanced Topics
- 使用 DTrace 和 SystemTap 检测CPython
- Enabling the static markers
- Static DTrace probes
- Static SystemTap markers
- Available static markers
- SystemTap Tapsets
- 示例
- Python 常见问题
- Python常见问题
- 一般信息
- 现实世界中的 Python
- 编程常见问题
- 一般问题
- 核心语言
- 数字和字符串
- 性能
- 序列（元组/列表）
- 对象
- 模块
- 设计和历史常见问题
- 为什么Python使用缩进来分组语句？
- 为什么简单的算术运算得到奇怪的结果？
- 为什么浮点计算不准确？
- 为什么Python字符串是不可变的？
- 为什么必须在方法定义和调用中显式使用“self”？
- 为什么不能在表达式中赋值？
- 为什么Python对某些功能（例如list.index()）使用方法来实现，而其他功能（例如len(List)）使用函数实现？
- 为什么 join()是一个字符串方法而不是列表或元组方法？
- 异常有多快？
- 为什么Python中没有switch或case语句？
- 难道不能在解释器中模拟线程，而非得依赖特定于操作系统的线程实现吗？
- 为什么lambda表达式不能包含语句？
- 可以将Python编译为机器代码，C或其他语言吗？
- Python如何管理内存？
- 为什么CPython不使用更传统的垃圾回收方案？
- CPython退出时为什么不释放所有内存？
- 为什么有单独的元组和列表数据类型？
- 列表是如何在CPython中实现的？
- 字典是如何在CPython中实现的？
- 为什么字典key必须是不可变的？
- 为什么 list.sort() 没有返回排序列表？
- 如何在Python中指定和实施接口规范？
- 为什么没有goto？
- 为什么原始字符串（r-strings）不能以反斜杠结尾？
- 为什么Python没有属性赋值的“with”语句？
- 为什么 if/while/def/class语句需要冒号？
- 为什么Python在列表和元组的末尾允许使用逗号？
- 代码库和插件 FAQ
- 通用的代码库问题
- 通用任务
- 线程相关
- 输入输出
- 网络 / Internet 编程
- 数据库
- 数学和数字
- 扩展/嵌入常见问题
- 可以使用C语言中创建自己的函数吗？
- 可以使用C++语言中创建自己的函数吗？
- C很难写，有没有其他选择？
- 如何从C执行任意Python语句？
- 如何从C中评估任意Python表达式？
- 如何从Python对象中提取C的值？
- 如何使用Py_BuildValue()创建任意长度的元组？
- 如何从C调用对象的方法？
- 如何捕获PyErr_Print()（或打印到stdout / stderr的任何内容）的输出？
- 如何从C访问用Python编写的模块？
- 如何从Python接口到C ++对象？
- 我使用Setup文件添加了一个模块，为什么make失败了？
- 如何调试扩展？
- 我想在Linux系统上编译一个Python模块，但是缺少一些文件。为什么?
- 如何区分“输入不完整”和“输入无效”？
- 如何找到未定义的g++符号__builtin_new或__pure_virtual？
- 能否创建一个对象类，其中部分方法在C中实现，而其他方法在Python中实现（例如通过继承）？
- Python在Windows上的常见问题
- 我怎样在Windows下运行一个Python程序？
- 我怎么让 Python 脚本可执行？
- 为什么有时候 Python 程序会启动缓慢？
- 我怎样使用Python脚本制作可执行文件？
- *.pyd 文件和DLL文件相同吗？
- 我怎样将Python嵌入一个Windows程序？
- 如何让编辑器不要在我的 Python 源代码中插入 tab ？
- 如何在不阻塞的情况下检查按键？
- 图形用户界面（GUI）常见问题
- 图形界面常见问题
- Python 是否有平台无关的图形界面工具包？
- 有哪些Python的GUI工具是某个平台专用的？
- 有关Tkinter的问题
- “为什么我的电脑上安装了 Python ？”
- 什么是Python？
- 为什么我的电脑上安装了 Python ？
- 我能删除 Python 吗？
- 术语对照表
- 文档说明
- Python 文档贡献者
- 解决 Bug
- 文档错误
- 使用 Python 的错误追踪系统
- 开始为 Python 贡献您的知识
- 版权
- 历史和许可证
- 软件历史
- 访问Python或以其他方式使用Python的条款和条件
- Python 3.7.3 的 PSF 许可协议
- Python 2.0 的 BeOpen.com 许可协议
- Python 1.6.1 的 CNRI 许可协议
- Python 0.9.0 至 1.2 的 CWI 许可协议
- 集成软件的许可和认可
- Mersenne Twister
- 套接字
- Asynchronous socket services
- Cookie management
- Execution tracing
- UUencode and UUdecode functions
- XML Remote Procedure Calls
- test_epoll
- Select kqueue
- SipHash24
- strtod and dtoa
- OpenSSL
- expat
- libffi
- zlib
- cfuhash
- libmpdec