Function acting as both decorator and context manager in Python?

Question

This might be pushing things a little too far, but mostly out of curiosity..

Would it be possible to have a callable object (function/class) that acts as both a Context Manager and a decorator at the same time:

def xxx(*args, **kw):     # or as a class  @xxx(foo, bar) def im_decorated(a, b):     print('do the stuff')  with xxx(foo, bar):     print('do the stuff') 

Answer 1

Starting in Python 3.2, support for this is even included in the standard library. Deriving from the class contextlib.ContextDecorator makes it easy to write classes that can be used as both, a decorator or a context manager. This functionality could be easily backported to Python 2.x -- here is a basic implementation:

class ContextDecorator(object):     def __call__(self, f):         @functools.wraps(f)         def decorated(*args, **kwds):             with self:                 return f(*args, **kwds)         return decorated 

Derive your context manager from this class and define the __enter__() and __exit__() methods as usual.

Answer 2

In Python 3.2+, you can define a context manager that is also a decorator using @contextlib.contextmanager.

From the docs:

contextmanager() uses ContextDecorator so the context managers it creates can be used as decorators as well as in with statements

Example usage:

>>> from contextlib import contextmanager >>> @contextmanager ... def example_manager(message): ...     print('Starting', message) ...     try: ...         yield ...     finally: ...         print('Done', message) ...  >>> with example_manager('printing Hello World'): ...     print('Hello, World!') ...  Starting printing Hello World Hello, World! Done printing Hello World >>>  >>> @example_manager('running my function') ... def some_function(): ...     print('Inside my function') ...  >>> some_function() Starting running my function Inside my function Done running my function