I have a function like:
def func(filename):
with open(filename) as f:
return [line.split('\t')[0] for line in f]
Is the "with" statement closing the file even when there is a 'sudden' function return? Can I ignore the "with" statement? i.e. is it safe and equivalent (from a memory leak perspective) to do,
def func(filename):
return [line.split('\t')[0] for line in open(filename)]
?
It's safe. The context manager's __exit__ is called even if you return while inside of the context, so the file handle is properly closed.
Here's a simple test:
class ContextTest(object):
def __enter__(self):
print('Enter')
def __exit__(self, type, value, traceback):
print('Exit')
def test():
with ContextTest() as foo:
print('Inside')
return
When you call test(), you get:
Enter
Inside
Exit
... as test to something else, it stopped me up a bit since your function name is also test.Guaranteeing this safety is actually the entire purpose of the with...as... syntax; it replaces try/finally blocks that would otherwise be fairly awkward. So yes, it's guaranteed to be safe, which is why I far prefer with open as f to f = open.
See http://effbot.org/zone/python-with-statement.htm for a good explanation of why the syntax exists and how it works. Note that you can write your own classes with __enter__ and __exit__ methods to really take advantage of this syntax.
Also see the PEP for this feature: http://www.python.org/dev/peps/pep-0343/
