python: "with" syntax for opening files with two functions

Question

Let's say I want to open a text file for reading using the following syntax:

with open(fname,'r') as f:
    # do something
    pass

But if I detect that it ends with .gz, I would call gzip.open().

if fname.endswith('.gz'):
    with gzip.open(fname,'rt') as f:
            # do something
            pass
else:
    with open(fname,'r') as f:
            # do something
            pass

If "do something" part is long and not convenient to write in a function (e.g. it would create a nested function, which cannot be serialized), what is the shortest way to call with either gzip.open or open based on the return of fname.endswith('.gz')?

Answer 1

The context manager helps close the object.

You don't have to create the object used as a context manager, at the same time you use with to enter the context, though. The open() and gzip.open() calls return a new object that happens to be a context manager, and you can create them before you enter the context:

if fname.endswith('.gz'):
    f = gzip.open(fname,'rt')
else:
    f = open(fname, 'r')

with f:
    # do something

In both cases, the object returns self on entering the context, so there is no need to use as f here.

Also, functions are first-class citizens, so you can also use a variable to store the function and then call that in the with statement to create the context manager and file object:

if fname.endswith('.gz'):
    opener = gzip.open
else:
    opener = open

with opener(fname, 'rt') as f:  # yes, both open and gzip.open support mode='rt'
    # do something

This doesn't really buy you anything over the other method here, but you could use a dictionary to map extensions to callables if you so desire.

The bottom line is that with calls context-manager hook methods, nothing less, nothing more. The expression after with is supposed to supply such a manager, but creating that object is not subject to the context management protocol.