Files should be explicitly closed

[Craig Rodrigues]

Across Twisted's code base you'll see this pattern:

data = open('somefile').read()

CPython's reference counting memory manager closes the file object created by open('somefile') almost immediately. But other implementations (namely PyPy) don't use reference counting, so this leaks a file descriptor for an indeterminate amount of time.

This pattern is especially bad in the case of writing a file:

open('somefile', 'w').write('data')

...because the code that immediately follows often expects somefile to exist and contain data. But there's no guarantee on PyPy when both will be true. This responsible for at least some of conch's test failures under PyPy.

The solution in this modern world is to replace all instances of this pattern with a ​context manager instead:

with open('somefile') as f:
    data = f.read()

with open('somefile', 'w') as f:
    f.write('data')

[GitHub <noreply@…>]

In 56e1c4a:

Merge pull request #309 from twisted/8580-scop-files

Author: scop
Reviewer: rodrigc
Fixes: #8580

More file close related fixes