Conditional with statement in Python

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.

If you want to avoid duplicating code and are using a version of Python prior to 3.7 (when contextlib.nullcontext was introduced) or even 3.3 (when contextlib.ExitStack was introduced), you could do something like:

class dummy_context_mgr():
    def __enter__(self):
        return None
    def __exit__(self, exc_type, exc_value, traceback):
        return False

or:

import contextlib

@contextlib.contextmanager
def dummy_context_mgr():
    yield None

and then use it as:

with get_stuff() if needs_with() else dummy_context_mgr() as gs:
   # do stuff involving gs or not

You alternatively could make get_stuff() return different things based on needs_with().

(See Mike's answer or Daniel's answer for what you can do in later versions.)

As of Python 3.7 you can use contextlib.nullcontext:

from contextlib import nullcontext

if needs_with():
    cm = get_stuff()
else:
    cm = nullcontext()

with cm as gs:
    # Do stuff

contextlib.nullcontext is pretty much just a no-op context manager. You can pass it an argument that it will yield, if you depend on something existing after the as:

>>> with nullcontext(5) as value:
...     print(value)
...
5

Otherwise it'll just return None:

>>> with nullcontext() as value:
...     print(value)
...
None

It's super neat, check out the docs for it here: https://docs.python.org/3/library/contextlib.html#contextlib.nullcontext

A third-party option to achieve exactly this:
https://pypi.python.org/pypi/conditional

from conditional import conditional

with conditional(needs_with(), get_stuff()):
    # do stuff

Conditional with statement in Python

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