withif.py

"""
Horrible hacks to allow context managers to exit early, *not* running
whatever code they're wrapping. I'm not sure if this is a reasonable
thing to do or not....
"""

import sys

class ExitEarlyException(Exception):
    """This is an exception that python
    will always ignore. It's part of a hack
    that allows context manager (`with` statements)
    to exit without ever running the code they're
    wrapping.
    """
    pass

oldhook = sys.excepthook
def manage_exit_early_hook(exctype, value, traceback):
    """Part of a hack that lets context managers (`with` statements)
    exit without ever running the code that they're wrapping
    """
    if exctype == ExitEarlyException: pass
    else: oldhook(exctype, value, traceback)
sys.excepthook = manage_exit_early_hook

from contextlib import contextmanager

@contextmanager
def exit_early(dont_run):
    if dont_run:
        raise ExitEarlyException
    yield

with exit_early(False):
    print("This runs")

with exit_early(True):
    print("This doesn't run")