Understanding the Python with statement and context managers

The contextlib.contextmanager function decorator provides a handy way of providing a context manager without the need to write a full-fledged ContextManager class of your own (with __enter__ and __exit__ methods, so you don’t have to remember the arguments to the __exit__ method, or that the __exit__ method must return True in order to suppress the exception). Instead, you write a function with a single yield at the point you want the with block to run, and you trap any exceptions (that effectively come from the yield) as you normally would.

from contextlib import contextmanager
@contextmanager
def handler():
    # Put here what would ordinarily go in the `__enter__` method
    # In this case, there's nothing to do
    try:
        yield # You can return something if you want, that gets picked up in the 'as'
    except Exception as e:
        print "no not possible"
    finally:
        print "Ok I caught you"

with handler():
    name="rubicon"/2 #to raise an exception

Why go to the extra trouble of writing a context manager? Code re-use. You can use the same context manager in multiple places, without having to duplicate the exception handling. If the exception handling is unique to that situation, then don’t bother with a context manager. But if the same pattern crops up again and again (or if it might for your users, e.g., closing a file, unlocking a mutex), it’s worth the extra trouble. It’s also a neat pattern to use if the exception handling is a bit complicated, as it separates the exception handling from the main line of code flow.