Why does my contextmanager-function not work like my contextmanager class in python?

Question

In my code, I need to be able to open and close a device properly, and therefore see the need to use a context manager. While a context manager is usually defined as a class

Answer 1

The example in the documentation for contextmanager is somewhat misleading. The portion of the function after yield does not really correspond to the __exit__ of the context manager protocol. The key point in the documentation is this:

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.

So if you want to handle an exception in your contextmanager-decorated function, you need to write your own try that wraps the yield and handle the exceptions yourself, executing cleanup code in a finally (or just block the exception in except and execute your cleanup after the try/except). For example:

@contextlib.contextmanager
def cm():
    print "before"
    exc = None
    try:
        yield
    except Exception, exc:
        print "Exception was caught"
    print "after"
    if exc is not None:
        raise exc

>>> with cm():
...     print "Hi!"
before
Hi!
after

>>> with cm():
...     print "Hi!"
...     1/0
before
Hi!
Exception was caught
after

This page also shows an instructive example.