Conditional with statement in Python

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Python 3.3 and above

Python 3.3 introduced contextlib.ExitStack for just this kind of situation. It gives you a “stack”, to which you add context managers as necessary. In your case, you would do this:

from contextlib import ExitStack

with ExitStack() as stack:
    if needs_with():
        gs = stack.enter_context(get_stuff())

    # do nearly the same large block of stuff,
    # involving gs or not, depending on needs_with()

Anything that is entered to stack is automatically exited at the end of the with statement as usual. (If nothing is entered, that’s not a problem.) In this example, whatever is returned by get_stuff() is exited automatically.

If you have to use an earlier version of python, you might be able to use the contextlib2 module, although this is not standard. It backports this and other features to earlier versions of python. You could even do a conditional import, if you like this approach.

Python 3.7 and above

Python 3.7 further introduced contextlib.nullcontext (a couple years after this answer was originally posted, and since that time mentioned in several other answers). In the comments, @Kache points out the most elegant usage of this option:

from contextlib import nullcontext

with get_stuff() if needs_with() else nullcontext() as gs:
    # do nearly the same large block of stuff,
    # involving gs or not, depending on needs_with()

Note that if needs_with() is False, then gs will be None inside the context block. If you want gs to be something_else in that case, you just replace nullcontext() with nullcontext(something_else).

This approach is obviously not as flexible as ExitStack, because this is just a binary choice, whereas ExitStack allows you to add as many exiting things as you want, with complicated logic and so on. But this certainly answers the OP’s simple requirements.

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