Context manager for optionally redirected I/O

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I'm often faced with the situation that depending on some command line argument, input might either come from a file or standard input. The same goes for output. I really like how context managers in python 3 work, and therefore try to make all my open calls part of some with statement. But in this case, I'm having trouble.

if args.infile:
    with open(args.infile, "r") as f:
        process(f)
else:
    process(sys.stdin)

is already clumsy, and with both input and output I'd have to cater for four combinations. I would like something easier, e.g.

with (open(args.infile, "r") if args.infile
      else DummyManager(sys.stdin)) as f:
    process(f)

Is there something like this DummyManager in the python standard libraries? Something which implements the context manager protocol, but only to return a fixed value from its __enter__ method? I guess the most likely location for such a class would be contextlib, and since I didn't find anything like this there, perhaps there is no such thing. Are there other elegant solutions you can suggest?

asked 2 mins ago

MvG

20.9k

1 Answer

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0

It's trivial to create one with contextlib.contextmanager:

from contextlib import contextmanager

@contextmanager
def dummy_manager(ob):
    yield ob

That's it; this creates a context manager that does nothing but hand you ob back, and the __exit__ handler does exactly nothing.

answered 40 secs ago

Martijn Pieters

236k