Skipping execution of -with- block

by

According to PEP-343, a with statement translates from:

with EXPR as VAR:
    BLOCK

to:

mgr = (EXPR)
exit = type(mgr).__exit__  # Not calling it yet
value = type(mgr).__enter__(mgr)
exc = True
try:
    try:
        VAR = value  # Only if "as VAR" is present
        BLOCK
    except:
        # The exceptional case is handled here
        exc = False
        if not exit(mgr, *sys.exc_info()):
            raise
        # The exception is swallowed if exit() returns true
finally:
    # The normal and non-local-goto cases are handled here
    if exc:
        exit(mgr, None, None, None)

As you can see, there is nothing obvious you can do from the call to the __enter__() method of the context manager that can skip the body (“BLOCK“) of the with statement.

People have done Python-implementation-specific things, such as manipulating the call stack inside of the __enter__(), in projects such as withhacks. I recall Alex Martelli posting a very interesting with-hack on stackoverflow a year or two back (don’t recall enough of the post off-hand to search and find it).

But the simple answer to your question / problem is that you cannot do what you’re asking, skipping the body of the with statement, without resorting to so-called “deep magic” (which is not necessarily portable between python implementations). With deep magic, you might be able to do it, but I recommend only doing such things as an exercise in seeing how it might be done, never in “production code”.

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