28.6. contextlib
— Utilities for with
-statement contexts¶
Source code: Lib/contextlib.py
This module provides utilities for common tasks involving the with
statement. For more information see also Context Manager Types and
With Statement Context Managers.
28.6.1. Utilities¶
Functions and classes provided:
-
@
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.A simple example (this is not recommended as a real way of generating HTML!):
from contextlib import contextmanager @contextmanager def tag(name): print("<%s>" % name) yield print("</%s>" % name) >>> with tag("h1"): ... print("foo") ... <h1> foo </h1>
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’sas
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 atry
...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 thewith
statement that the exception has been handled, and execution will resume with the statement immediately following thewith
statement.contextmanager()
usesContextDecorator
so the context managers it creates can be used as decorators as well as inwith
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 bycontextmanager()
to meet the requirement that context managers support multiple invocations in order to be used as decorators).Changed in version 3.2: Use of
ContextDecorator
.
-
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 thewith
block is exited.
-
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 bycontextmanager()
, 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
Note
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 explicitwith
statement inside the function should be used.New in version 3.2.
-
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
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
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.
New in version 3.3.
-
enter_context
(cm)¶ Enters a new context manager and adds its
__exit__()
method to the callback stack. The return value is the result of the context manager’s own__enter__()
method.These context managers may suppress exceptions just as they normally would if used directly as part of a
with
statement.
-
push
(exit)¶ Adds a context manager’s
__exit__()
method to the callback stack.As
__enter__
is not invoked, this method can be used to cover part of an__enter__()
implementation with a context manager’s own__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__()
method and adds it directly to the callback stack.By returning true values, these callbacks can suppress exceptions the same way context manager
__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
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 awith
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.
-
28.6.2. Examples and Recipes¶
This section describes some examples and recipes for making effective use of
the tools provided by contextlib
.
28.6.2.1. Supporting a variable number of context managers¶
The primary use case for ExitStack
is the one given in the class
documentation: supporting a variable number of context managers and other
cleanup operations in a single 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
also makes it quite easy to use with
statements to manage arbitrary resources that don’t natively support the
context management protocol.
28.6.2.2. Simplifying support for single optional context managers¶
In the specific case of a single optional context manager, ExitStack
instances can be used as a “do nothing” context manager, allowing a context
manager to easily be omitted without affecting the overall structure of
the source code:
def debug_trace(details):
if __debug__:
return TraceContext(details)
# Don't do anything special with the context in release mode
return ExitStack()
with debug_trace():
# Suite is traced in debug mode, but runs normally otherwise
28.6.2.3. Catching exceptions from __enter__
methods¶
It is occasionally desirable to catch exceptions from an __enter__
method implementation, without inadvertently catching exceptions from
the with
statement body or the context manager’s __exit__
method. By using 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
/except
/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
can make it
easier to handle various situations that can’t be handled directly in a
with
statement.
28.6.2.4. Cleaning up in an __enter__
implementation¶
As noted in the documentation of ExitStack.push()
, this
method can be useful in cleaning up an already allocated resource if later
steps in the __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, ExitStack
class ResourceManager:
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()
28.6.2.5. 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
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()
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
28.6.2.6. Using a context manager as a function decorator¶
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
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: {}'.format(name))
def __exit__(self, exc_type, exc, exc_tb):
logging.info('Exiting: {}'.format(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__()
. If that value is needed, then it is still necessary to use
an explicit with
statement.