Hi Steven,
Steven D'Aprano wrote: > On Sat, 20 Mar 2010 13:16:08 +0100, News123 wrote: > >> Hi, >> >> >> I wondered about the best way, that a module's function could determine >> the existance and value of variables in the __main__ module. >> >> >> What I came up with is: >> ########### main.py ########## >> import mod >> A = 4 >> if __name__ == "__main__": mod.f() >> ########### mod.py ########## >> def f(): >> try: >> from __main__ import A >> except ImportError as e: >> A = "does not exist" >> print "__main__.A" ,A >> >> Is there anything better / more pythonic? > > > First problem: > > You try to import from "__main__", but the other module is called "main". > __main__ is a special name, which Python understands as meaning "this > module that you are in now". For example: My choice of names was perhaps not very smart. I could have called main.py also mytoplevel.py > > $ cat test_main.py > x = "something special" > import __main__ # magic alias for this module > print __main__.x > > $ python test_main.py > something special > > So inside "mod.py", the line "from __main__ import A" tries to import A > from mod.py, not from main.py. You need to say "from main import A" > instead. > I think you're wrong, my above code seems to work. __main__ refers not to the current module, but to the urrent 'top-level-module' so from __main__ import A tries to import from the top level module which is in my case main.py. > But if you do this, it creates a second problem: you have a circular > import, which is generally considered a Bad Thing. The problem is that: > > (1) module main tries to import module mod > (2) module mod tries to import module main > (3) but main is still waiting for mod to be imported > > Now, in your case you escape that trap, because the import is inside a > function, so it doesn't occur until you call the function. But it is > still considered poor practice: it is best to avoid circular imports > unless you really, really need them. > > > The question is, why does module mod.py care what is happening in > main.py? It is better for mod.py to be self-contained, and not care about > main.py at all. If it needs A, let the caller pass A to it: The reason is pure lazyness. I would like to 'try' something quickly. I have a module used by many different python programs. In case the __main__ module contains a certain object I'd like to extract information from this object if not not. This is for debug, not for 'production'. I'd prefer to change only one file and not many. > > ########### main.py ########## > import mod > A = 4 > if __name__ == "__main__": > mod.f(__name__, A) > > ########### mod.py ########## > def f(caller, A): > print "%s.A: %s" % (caller, A) > > > N -- http://mail.python.org/mailman/listinfo/python-list
