Handling instances of a context manager inside another context manager

Question

How should a context manager created inside another context manager be handled in Python?

Example: suppose you have class A that acts as a context manager, and class B that also acts as a context manager. But class B instances will have to instantiate and use an instance of class A. I've gone through PEP 343 and this is the solution I thought of:

class A(object):     def __enter__(self):         # Acquire some resources here         return self      def __exit__(seplf, exception_type, exception, traceback):         # Release the resources and clean up         pass   class B(object):     def __init__(self):         self.a = A()      def __enter__(self):         # Acquire some resources, but also need to "start" our instance of A         self.a.__enter__()         return self      def __exit__(self, exception_type, exception, traceback):         # Release the resources, and make our instance of A clean up as well         self.a.__exit__(exception_type, exception, traceback) 

Is this the correct approach? Or am I missing some gotchas?

Answer 1

If you can use the @contextlib.contextmanager decorator your life gets a lot easier:

import contextlib  @contextlib.contextmanager def internal_cm():     try:         print "Entering internal_cm"         yield None         print "Exiting cleanly from internal_cm"     finally:         print "Finally internal_cm"   @contextlib.contextmanager def external_cm():     with internal_cm() as c:         try:             print "In external_cm_f", c             yield [c]             print "Exiting cleanly from external_cm_f", c         finally:             print "Finally external_cm_f", c   if "__main__" == __name__:     with external_cm():         print "Location A"     print     with external_cm():         print "Location B"         raise Exception("Some exception occurs!!")