Conditional with statement in Python

Question

Is there a way to begin a block of code with a with statement, but conditionally?

Something like:

if needs_with():
    with get_stuff() as gs:

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

To clarify, one scenario would have a block encased in the with statement, while another possibility would be the same block, but not encased (i.e., as if it wasn't indented)

Initial experiments of course give indentation errors..

Answer 1

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.