Source code: Lib/contextlib.py
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>
At the point where the generator yields, the block nested in the with statement is executed. The generator is then resumed after the block is exited. If an unhandled exception occurs in the block, it is reraised inside the generator at the point where the yield occurred. Thus, you can use a try...except...finally statement to trap the error (if any), or ensure that some cleanup takes place. If an exception is trapped merely in order to log it or to perform some action (rather than to suppress it entirely), the generator must reraise that exception. Otherwise the generator context manager will indicate to the with statement that the exception has been handled, and execution will resume with the statement immediately following the with statement.
contextmanager() uses ContextDecorator so the context managers it creates can be used as decorators as well as in with statements. When used as a decorator, a new generator instance is implicitly created on each function call (this allows the otherwise “one-shot” context managers created by contextmanager() to meet the requirement that context managers support multiple invocations in order to be used as decorators).
Changed in version 3.2: Use of ContextDecorator.
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 block is exited.
A base class that enables a context manager to also be used as a decorator.
Context managers inheriting from ContextDecorator have to implement __enter__ and __exit__ as normal. __exit__ retains its optional exception handling even when used as a decorator.
ContextDecorator is used by contextmanager(), so you get this functionality automatically.
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
As the decorated function must be able to be called multiple times, the underlying context manager must support use in multiple with statements. If this is not the case, then the original construct with the explicit with statement inside the function should be used.
New in version 3.2.