Question

Consider the following:

with open(path, mode) as f:
    return [line for line in f if condition]

Will the file be closed properly, or does using return somehow bypass the context manager?

Answer 1

Yes, it acts like the finally block after a try block, i.e. it always executes (unless the python process terminates in an unusual way of course).

It is also mentioned in one of the examples of PEP-343 which is the specification for the with statement:

with locked(myLock):
    # Code here executes with myLock held.  The lock is
    # guaranteed to be released when the block is left (even
    # if via return or by an uncaught exception).

Something worth mentioning is however, that you cannot easily catch exceptions thrown by the open() call without putting the whole with block inside a try..except block which is usually not what one wants.