In Python, if I return inside a "with" block, will the file still close?

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.

Answer 2

Yes.

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

..is pretty much equivalent to:

def example(path, mode):     f = open(path, mode)      try:         return [line for line in f if condition]     finally:         f.close() 

More accurately, the __exit__ method in a context manager is always called when exiting the block (regardless of exceptions, returns etc). The file object's __exit__ method just calls f.close() (e.g here in CPython)