522 lines
19 KiB
ReStructuredText
522 lines
19 KiB
ReStructuredText
:mod:`contextlib` --- Utilities for :keyword:`with`\ -statement contexts
|
|
========================================================================
|
|
|
|
.. module:: contextlib
|
|
:synopsis: Utilities for with-statement contexts.
|
|
|
|
**Source code:** :source:`Lib/contextlib.py`
|
|
|
|
--------------
|
|
|
|
This module provides utilities for common tasks involving the :keyword:`with`
|
|
statement. For more information see also :ref:`typecontextmanager` and
|
|
:ref:`context-managers`.
|
|
|
|
|
|
Utilities
|
|
---------
|
|
|
|
Functions and classes provided:
|
|
|
|
.. decorator:: contextmanager
|
|
|
|
This function is a :term:`decorator` that can be used to define a factory
|
|
function for :keyword:`with` statement context managers, without needing to
|
|
create a class or separate :meth:`__enter__` and :meth:`__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 :term:`generator`-iterator when
|
|
called. This iterator must yield exactly one value, which will be bound to
|
|
the targets in the :keyword:`with` statement's :keyword:`as` clause, if any.
|
|
|
|
At the point where the generator yields, the block nested in the :keyword:`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
|
|
:keyword:`try`...\ :keyword:`except`...\ :keyword:`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 :keyword:`with` statement that
|
|
the exception has been handled, and execution will resume with the statement
|
|
immediately following the :keyword:`with` statement.
|
|
|
|
:func:`contextmanager` uses :class:`ContextDecorator` so the context managers
|
|
it creates can be used as decorators as well as in :keyword:`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 :func:`contextmanager` to meet the requirement that context
|
|
managers support multiple invocations in order to be used as decorators).
|
|
|
|
.. versionchanged:: 3.2
|
|
Use of :class:`ContextDecorator`.
|
|
|
|
|
|
.. function:: 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 :keyword:`with` block is exited.
|
|
|
|
|
|
.. class:: 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 :func:`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
|
|
|
|
.. note::
|
|
As the decorated function must be able to be called multiple times, the
|
|
underlying context manager must support use in multiple :keyword:`with`
|
|
statements. If this is not the case, then the original construct with the
|
|
explicit :keyword:`with` statement inside the function should be used.
|
|
|
|
.. versionadded:: 3.2
|
|
|
|
|
|
.. class:: 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 :keyword:`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 :keyword:`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.
|
|
|
|
.. versionadded:: 3.3
|
|
|
|
.. method:: enter_context(cm)
|
|
|
|
Enters a new context manager and adds its :meth:`__exit__` method to
|
|
the callback stack. The return value is the result of the context
|
|
manager's own :meth:`__enter__` method.
|
|
|
|
These context managers may suppress exceptions just as they normally
|
|
would if used directly as part of a :keyword:`with` statement.
|
|
|
|
.. method:: push(exit)
|
|
|
|
Adds a context manager's :meth:`__exit__` method to the callback stack.
|
|
|
|
As ``__enter__`` is *not* invoked, this method can be used to cover
|
|
part of an :meth:`__enter__` implementation with a context manager's own
|
|
:meth:`__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
|
|
:meth:`__exit__` method and adds it directly to the callback stack.
|
|
|
|
By returning true values, these callbacks can suppress exceptions the
|
|
same way context manager :meth:`__exit__` methods can.
|
|
|
|
The passed in object is returned from the function, allowing this
|
|
method to be used as a function decorator.
|
|
|
|
.. method:: 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.
|
|
|
|
.. method:: pop_all()
|
|
|
|
Transfers the callback stack to a fresh :class:`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 :keyword:`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]
|
|
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
|
|
|
|
.. method:: 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.
|
|
|
|
|
|
Examples and Recipes
|
|
--------------------
|
|
|
|
This section describes some examples and recipes for making effective use of
|
|
the tools provided by :mod:`contextlib`.
|
|
|
|
|
|
Supporting a variable number of context managers
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The primary use case for :class:`ExitStack` is the one given in the class
|
|
documentation: supporting a variable number of context managers and other
|
|
cleanup operations in a single :keyword:`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, :class:`ExitStack` also makes it quite easy to use :keyword:`with`
|
|
statements to manage arbitrary resources that don't natively support the
|
|
context management protocol.
|
|
|
|
|
|
Simplifying support for single optional context managers
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
In the specific case of a single optional context manager, :class:`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
|
|
|
|
|
|
Catching exceptions from ``__enter__`` methods
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
It is occasionally desirable to catch exceptions from an ``__enter__``
|
|
method implementation, *without* inadvertently catching exceptions from
|
|
the :keyword:`with` statement body or the context manager's ``__exit__``
|
|
method. By using :class:`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
|
|
:keyword:`try`/:keyword:`except`/:keyword:`finally` statements, but not
|
|
all APIs are well designed in that regard. When a context manager is the
|
|
only resource management API provided, then :class:`ExitStack` can make it
|
|
easier to handle various situations that can't be handled directly in a
|
|
:keyword:`with` statement.
|
|
|
|
|
|
Cleaning up in an ``__enter__`` implementation
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
As noted in the documentation of :meth:`ExitStack.push`, this
|
|
method can be useful in cleaning up an already allocated resource if later
|
|
steps in the :meth:`__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()
|
|
|
|
|
|
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.
|
|
|
|
:class:`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
|
|
:meth:`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
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
:class:`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 :class:`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
|
|
:meth:`__enter__`. If that value is needed, then it is still necessary to use
|
|
an explicit ``with`` statement.
|
|
|
|
.. seealso::
|
|
|
|
:pep:`0343` - The "with" statement
|
|
The specification, background, and examples for the Python :keyword:`with`
|
|
statement.
|
|
|