[Python-Dev] Creating APIs that work as both decorators and context managers

[Michael Foord]

Hello all,

I've put a recipe up on the Python cookbook for creating APIs that work 
as both decorators and context managers and wonder if it would be 
considered a useful addition to the functools module.

http://code.activestate.com/recipes/577273-decorator-and-context-manager-from-a-single-api/

I wrote this after writing almost identical code the second time for 
"patch" in the mock module. (The patch decorator can be used as a 
decorator or as a context manager and I was writing a new variant.) Both 
py.test and django have similar code in places, so it is not an uncommon 
pattern.

It is only 40 odd lines (ignore the ugly Python 2 & 3 compatibility 
hack), so I'm fine with it living on the cookbook - but it is at least 
slightly fiddly to write and has the added niceness of providing the 
optional exception handling semantics of __exit__ for decorators as well.

Example use (really hope email doesn't swallow the whitespace - my 
apologies in advance if it does):

from context import Context

class mycontext(Context):
     def __init__(self, *args):
         """Normal initialiser"""

     def start(self):
         """
         Called on entering the with block or starting the decorated 
function.

         If used in a with statement whatever this method returns will 
be the
         context manager.
         """

     def finish(self, *exc):
         """
         Called on exit. Arguments and return value of this method have
         the same meaning as the __exit__ method of a normal context
         manager.
         """

@mycontext('some', 'args')
def function():
     pass

with mycontext('some', 'args') as something:
     pass

I'm not entirely happy with the name of the class or the start and 
finish methods, so open to suggestions there. start and finish *could* 
be __enter__ and __exit__ - but that would make the class you implement 
*look* like a normal context manager and I thought it was better to 
distinguish them. Perhaps before and after?

All the best,

Michael Foord

-- 
http://www.ironpythoninaction.com/
http://www.voidspace.org.uk/blog

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