python - Skipping execution of -with- block

Question

I am defining a context manager class and I would like to be able to skip the block of code without raising an exception if certain conditions are met during instantiation. For example,

class My_Context(object):
    def __init__(self,mode=0):
        """
        if mode = 0, proceed as normal
        if mode = 1, do not execute block
        """
        self.mode=mode
    def __enter__(self):
        if self.mode==1:
            print 'Exiting...'
            CODE TO EXIT PREMATURELY
    def __exit__(self, type, value, traceback):
        print 'Exiting...'

with My_Context(mode=1):
    print 'Executing block of codes...'

Answer 1

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".