Source code:
———
This module provides utilities for common tasks involving the
statement. For more information see also
and
With Statement Context Managers
.
Utilities
Functions and classes provided:
classcontextlib.AbstractContextManager
An
for classes that implement
and
. 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
.
Added in version 3.6.
classcontextlib.AbstractAsyncContextManager
An
for classes that implement
and
. 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
.
Added in version 3.7.
@contextlib.contextmanager
This function is a
that can be used to define a factory function for
statement context managers, without needing to create a class or separate
and
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:
fromcontextlibimportcontextmanager@contextmanagerdefmanaged_resource(*args,**kwds):# Code to acquire resource, e.g.:resource=acquire_resource(*args,**kwds)try:yieldresourcefinally:# Code to release resource, e.g.:release_resource(resource)The function can then be used like this:
>>> withmanaged_resource(timeout=3600)asresource:... # Resource is released at the end of this block,... # even if code in the block raises an exceptionThe function being decorated must return a
-iterator when called. This iterator must yield exactly one value, which will be bound to the targets in the
statement’s as clause, if any.
At the point where the generator yields, the block nested in the
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
…
…
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.
uses
so the context managers it creates can be used as decorators as well as in
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).
Changed in version 3.2: Use of
.
@contextlib.asynccontextmanager
Similar to
, but creates an
.
This function is a
that can be used to define a factory function for
statement asynchronous context managers, without needing to create a class or separate
and
methods. It must be applied to an
function.
A simple example:
fromcontextlibimportasynccontextmanager@asynccontextmanagerasyncdefget_connection():conn=awaitacquire_db_connection()try:yieldconnfinally:awaitrelease_db_connection(conn)asyncdefget_all_users():asyncwithget_connection()asconn:returnconn.query('SELECT ...')Added in version 3.7.
Context managers defined with
can be used either as decorators or with
statements:
importtimefromcontextlibimportasynccontextmanager@asynccontextmanagerasyncdeftimeit():now=time.monotonic()try:yieldfinally:print(f'it took {time.monotonic()-now}s to run')@timeit()asyncdefmain():# ... 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
to meet the requirement that context managers support multiple invocations in order to be used as decorators.
Changed in version 3.10: Async context managers created with
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:
fromcontextlibimportcontextmanager@contextmanagerdefclosing(thing):try:yieldthingfinally:thing.close()And lets you write code like this:
fromcontextlibimportclosingfromurllib.requestimporturlopenwithclosing(urlopen('https://www.python.org'))aspage:forlineinpage:print(line)without needing to explicitly close page. Even if an error occurs, page.close() will be called when the
block is exited.
Note
Most types managing resources support the
protocol, which closes thing on leaving the
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
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:
fromcontextlibimportasynccontextmanager@asynccontextmanagerasyncdefaclosing(thing):try:yieldthingfinally:awaitthing.aclose()Significantly, aclosing() supports deterministic cleanup of async generators when they happen to exit early by
or an exception. For example:
fromcontextlibimportaclosingasyncwithaclosing(my_generator())asvalues:asyncforvalueinvalues:ifvalue==42:breakThis 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.
contextlib.nullcontext(enter_result=None)
Return a context manager that returns enter_result from
, but otherwise does nothing. It is intended to be used as a stand-in for an optional context manager, for example:
defmyfunction(arg,ignore_exceptions=False):ifignore_exceptions:# Use suppress to ignore all exceptions.cm=contextlib.suppress(Exception)else:# Do not ignore any exceptions, cm has no effect.cm=contextlib.nullcontext()withcm:# Do somethingAn example using enter_result:
defprocess_file(file_or_path):ifisinstance(file_or_path,str):# If string, open filecm=open(file_or_path)else:# Caller is responsible for closing filecm=nullcontext(file_or_path)withcmasfile:# Perform processing on the fileIt can also be used as a stand-in for
:
asyncdefsend_http(session=None):ifnotsession:# If no http session, create it with aiohttpcm=aiohttp.ClientSession()else:# Caller is responsible for closing the sessioncm=nullcontext(session)asyncwithcmassession:# Send http requests with sessionAdded in version 3.7.
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.
For example:
fromcontextlibimportsuppresswithsuppress(FileNotFoundError):os.remove('somefile.tmp')withsuppress(FileNotFoundError):os.remove('someotherfile.tmp')This code is equivalent to:
try:os.remove('somefile.tmp')exceptFileNotFoundError:passtry:os.remove('someotherfile.tmp')exceptFileNotFoundError:passThis context manager is
.
If the code within the with block raises 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
method.
Added in version 3.4.
Changed in version 3.12: suppress now supports suppressing exceptions raised as part of a
.
contextlib.redirect_stdout(new_target)
Context manager for temporarily redirecting
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
normally is sent to sys.stdout. You can capture that output in a string by redirecting the output to an
object. The replacement stream is returned from the
method and so is available as the target of the
statement:
withredirect_stdout(io.StringIO())asf:help(pow)s=f.getvalue()To send the output of
to a file on disk, redirect the output to a regular file:
withopen('help.txt','w')asf:withredirect_stdout(f):help(pow)To send the output of
to sys.stderr:
withredirect_stdout(sys.stderr):help(pow)Note that the global side effect on
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
.
Added in version 3.4.
contextlib.redirect_stderr(new_target)
Similar to
but redirecting
to another file or file-like object.
This context manager is
.
Added in version 3.5.
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
, it changes the current working directory upon entering and restores the old one on exit.
This context manager is
.
Added in version 3.11.
classcontextlib.ContextDecorator
A base class that enables a context manager to also be used as a decorator.
Context managers inheriting from ContextDecorator have to implement
and
as normal. __exit__ retains its optional exception handling even when used as a decorator.
ContextDecorator is used by
, so you get this functionality automatically.
Example of ContextDecorator:
fromcontextlibimportContextDecoratorclassmycontext(ContextDecorator):def__enter__(self):print('Starting')returnselfdef__exit__(self,*exc):print('Finishing')returnFalseThe class can then be used like this:
>>> @mycontext()... deffunction():... print('The bit in the middle')...>>> function()StartingThe bit in the middleFinishing>>> withmycontext():... print('The bit in the middle')...StartingThe bit in the middleFinishingThis change is just syntactic sugar for any construct of the following form:
deff():withcm():# Do stuffContextDecorator lets you instead write:
@cm()deff():# Do stuffIt 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:
fromcontextlibimportContextDecoratorclassmycontext(ContextBaseClass,ContextDecorator):def__enter__(self):returnselfdef__exit__(self,*exc):returnFalseNote
As the decorated function must be able to be called multiple times, the underlying context manager must support use in multiple
statements. If this is not the case, then the original construct with the explicit with statement inside the function should be used.
Added in version 3.2.
classcontextlib.AsyncContextDecorator
Similar to
but only for asynchronous functions.
Example of AsyncContextDecorator:
fromasyncioimportrunfromcontextlibimportAsyncContextDecoratorclassmycontext(AsyncContextDecorator):asyncdef__aenter__(self):print('Starting')returnselfasyncdef__aexit__(self,*exc):print('Finishing')returnFalseThe class can then be used like this:
>>> @mycontext()... asyncdeffunction():... print('The bit in the middle')...>>> run(function())StartingThe bit in the middleFinishing>>> asyncdeffunction():... asyncwithmycontext():... print('The bit in the middle')...>>> run(function())StartingThe bit in the middleFinishingAdded in version 3.10.
classcontextlib.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:
withExitStack()asstack:files=[stack.enter_context(open(fname))forfnameinfilenames]# 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 exceptionThe
method returns the ExitStack 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
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
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.
enter_context(cm)
Enters a new context manager and adds its
method to the callback stack. The return value is the result of the context manager’s own
method.
These context managers may suppress exceptions just as they normally would if used directly as part of a
statement.
Changed in version 3.11: Raises
instead of
if cm is not a context manager.
push(exit)
Adds a context manager’s
method to the callback stack.
As __enter__ is not invoked, this method can be used to cover part of an
implementation with a context manager’s own
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
method and adds it directly to the callback stack.
By returning true values, these callbacks can suppress exceptions the same way context manager
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 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
statement).
For example, a group of files can be opened as an “all or nothing” operation as follows:
withExitStack()asstack:files=[stack.enter_context(open(fname))forfnameinfilenames]# 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.
classcontextlib.AsyncExitStack
An
, similar to
, that supports combining both synchronous and asynchronous context managers, as well as having coroutines for cleanup logic.
The
method is not implemented;
must be used instead.
asyncenter_async_context(cm)
Similar to
but expects an asynchronous context manager.
Changed in version 3.11: Raises
instead of
if cm is not an asynchronous context manager.
push_async_exit(exit)
Similar to
but expects either an asynchronous context manager or a coroutine function.
push_async_callback(callback, /, *args, **kwds)
Similar to
but expects a coroutine function.
asyncaclose()
Similar to
but properly handles awaitables.
Continuing the example for
:
asyncwithAsyncExitStack()asstack:connections=[awaitstack.enter_async_context(get_connection())foriinrange(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.Added in version 3.7.
Examples and Recipes
This section describes some examples and recipes for making effective use of the tools provided by contextlib.
Supporting a variable number of context managers
The primary use case for
is the one given in the class documentation: supporting a variable number of context managers and other cleanup operations in a single
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:
withExitStack()asstack:forresourceinresources:stack.enter_context(resource)ifneed_special_resource():special=acquire_special_resource()stack.callback(release_special_resource,special)# Perform operations that use the acquired resourcesAs shown,
also makes it quite easy to use
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
method implementation, without inadvertently catching exceptions from the
statement body or the context manager’s
method. By using
the steps in the context management protocol can be separated slightly in order to allow this:
stack=ExitStack()try:x=stack.enter_context(cm)exceptException:# handle __enter__ exceptionelse:withstack:# Handle normal caseActually needing to do this is likely to indicate that the underlying API should be providing a direct resource management interface for use with
/
/
statements, but not all APIs are well designed in that regard. When a context manager is the only resource management API provided, then
can make it easier to handle various situations that can’t be handled directly in a
statement.
Cleaning up in an __enter__ implementation
As noted in the documentation of
, this method can be useful in cleaning up an already allocated resource if later steps in the
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:
fromcontextlibimportcontextmanager,AbstractContextManager,ExitStackclassResourceManager(AbstractContextManager):def__init__(self,acquire_resource,release_resource,check_resource_ok=None):self.acquire_resource=acquire_resourceself.release_resource=release_resourceifcheck_resource_okisNone:defcheck_resource_ok(resource):returnTrueself.check_resource_ok=check_resource_ok@contextmanagerdef_cleanup_on_error(self):withExitStack()asstack: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 callerstack.pop_all()def__enter__(self):resource=self.acquire_resource()withself._cleanup_on_error():ifnotself.check_resource_ok(resource):msg="Failed validation for {!r}"raiseRuntimeError(msg.format(resource))returnresourcedef__exit__(self,*exc_details):# We don't need to duplicate any of our resource release logicself.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=Truetry:result=perform_operation()ifresult:cleanup_needed=Falsefinally:ifcleanup_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.
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:
fromcontextlibimportExitStackwithExitStack()asstack:stack.callback(cleanup_resources)result=perform_operation()ifresult:stack.pop_all()This allows the intended cleanup 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:
fromcontextlibimportExitStackclassCallback(ExitStack):def__init__(self,callback,/,*args,**kwds):super().__init__()self.callback(callback,*args,**kwds)defcancel(self):self.pop_all()withCallback(cleanup_resources)ascb:result=perform_operation()ifresult: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
to declare the resource cleanup in advance:
fromcontextlibimportExitStackwithExitStack()asstack:@stack.callbackdefcleanup_resources():...result=perform_operation()ifresult: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
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
provides both capabilities in a single definition:
fromcontextlibimportContextDecoratorimportlogginglogging.basicConfig(level=logging.INFO)classtrack_entry_and_exit(ContextDecorator):def__init__(self,name):self.name=namedef__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:
withtrack_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')defactivity():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
. If that value is needed, then it is still necessary to use an explicit with statement.
See also
- The “with” statementThe specification, background, and examples for the Python
statement.
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
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
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
statement will close the file, preventing any further IO operations using that file object.
Context managers created using
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:
>>> fromcontextlibimportcontextmanager>>> @contextmanager... defsingleuse():... print("Before")... yield... print("After")...>>> cm=singleuse()>>> withcm:... pass...BeforeAfter>>> withcm:... pass...Traceback (most recent call last):...RuntimeError: generator didn't yieldReentrant context managers
More sophisticated context managers may be “reentrant”. These context managers can not only be used in multiple
statements, but may also be used inside a with statement that is already using the same context manager.
is an example of a reentrant context manager, as are
,
, and
. Here’s a very simple example of reentrant use:
>>> fromcontextlibimportredirect_stdout>>> fromioimportStringIO>>> stream=StringIO()>>> write_to_stream=redirect_stdout(stream)>>> withwrite_to_stream:... print("This is written to the stream rather than stdout")... withwrite_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 stdoutThis is also written to the streamReal 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.
, for example, is definitely not thread safe, as it makes a global modification to the system state by binding
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.
is an example of a reusable, but not reentrant, context manager (for a reentrant lock, it is necessary to use
instead).
Another example of a reusable, but not reentrant, context manager is
, as it invokes all currently registered callbacks when leaving any with statement, regardless of where those callbacks were added:
>>> fromcontextlibimportExitStack>>> stack=ExitStack()>>> withstack:... stack.callback(print,"Callback: from first context")... print("Leaving first context")...Leaving first contextCallback: from first context>>> withstack:... stack.callback(print,"Callback: from second context")... print("Leaving second context")...Leaving second contextCallback: from second context>>> withstack:... stack.callback(print,"Callback: from outer context")... withstack:... stack.callback(print,"Callback: from inner context")... print("Leaving inner context")... print("Leaving outer context")...Leaving inner contextCallback: from inner contextCallback: from outer contextLeaving outer contextAs 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
instances instead of reusing a single instance avoids that problem:
>>> fromcontextlibimportExitStack>>> withExitStack()asouter_stack:... outer_stack.callback(print,"Callback: from outer context")... withExitStack()asinner_stack:... inner_stack.callback(print,"Callback: from inner context")... print("Leaving inner context")... print("Leaving outer context")...Leaving inner contextCallback: from inner contextLeaving outer contextCallback: from outer context