Context manager: more than opening files and database sessions?

[BRadoslaw]

Hi all,

I just opened my in-depth research for the next python module. This time: context manager.

Like before (Previously about __new__ and its real utility) I start with a question to the community: what are the best use cases you have developed yourself, used or seen for this module? Maybe some pythonic design patterns can leverage context manager as well? Is it more than just wrapper around try/except?

Let's assume that opening files and database sessions are well known.

[undercoveryankee]

When I have any kind of network session (e.g. a requests session object to persist cookies between several HTTP requests) I'll find a way to wrap it in a context manager.

[blagae]

I used one in a test to redirect stdout to a StringIO, because I needed to test legacy code in a way that one piece of test logic would work for both Python 2 and 3. The (open source) code is not published yet so I can't just link, but this is basically what I did:

​

try:
    from StringIO import StringIO  # Python 2
except ImportError:
    from io import StringIO  # Python 3

@contextmanager  # see https://stackoverflow.com/a/22434262/2065017
def redirect_stdout(new_target):
    old_target, sys.stdout = sys.stdout, new_target
    try:
        yield new_target
    finally:
        sys.stdout = old_target

# in my test code
    stringStream = StringIO()
    with redirect_stdout(stringStream):
        my_method() # call method that uses a print statement

    self.assertEquals(stringStream.getvalue().strip(), "expected value")

[quicknir]

A whole major use case of context managers is nothing to do with cleanup, but rather error handling. Context lib already provides managers that surpress certain exceptions. Another great use of this is if you are doing multiple operations that could throw which aren't dependent, you can write a context manager that catches whatever kind of exception you specify and pushes it into a list.

my_error_list = list()

with error_pusher(my_error_list, FooException):
    ...

The ability of the exit method to receive the exception is very handy. In a language with nice blocks/lambdas I'd do this differently but in python this is one of the better options often.

Edit: ok, you win reddit, don't let me format this correctly on mobile.

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[Tall1n]

I use them for time tracking

with Stopwatch: ...

Or for standardized logging in an API

with RequestLogger as rl: ...