Jens Thoms Toerring wrote: > Hi, > > I hope that this isn't a stupid question, asked already a > hundred times, but I haven't found anything definitive on > the problem I got bitten by. I have two Python files like > this: > > -------- S1.py ------ > import random > import S2 > > class R( object ) : > r = random.random( ) > > if __name__ == "__main__" : > print R.r > S2.p( ) > > -------- S2.py ------ > import S1 > > def p( ) : > print S1.R.r > > and my expectation was that the static variable 'r' of class > R would be identical when accessed from S1.py and S2.py. > Unfortunately, that isn't the case, the output is different > (and R seems to get instantiated twice). > > But when I define R in S2.py instead > > -------- S1.py ------ > import S2 > > print S2.R.r > S2.p( ) > > -------- S2.py ------ > import random > > class R( object ) : > r = random.random( ) > > def p( ) : > print R.r > > or, alternatively, if I put the defintion of class R into > a third file which I then import from the other 2 files, > things suddenly start to work as expected/
That's the correct approach. > Can someone > explain what's going one here? I found this a bit sur- > prising. You should never import your program's main module anywhere else in the program. When Python imports a module it looks it up by the module's name in the sys.modules cache. For the main script that name will be "__main__" regardless of the file's actual name, so a subsequent "import S2" will result in a cache miss and a new module instance. Similar problems occur when there is a PYTHONPATH pointing into a package and you have both import package.module and import module Again you will end up with two module instances, one called "package.module", the other just "module". > This is, of course, not my "real" code - it would be much > more sensible to pass the number to the function in the > second file as an argument - but is the smallest possinle > program I could come up with that demonstrate the prob- > lem. In my "real" code it's unfortunately not possible > to pass that number to whatever is going to use it in the > other file, I have to simulate a kind of global variable > shared between different files. -- http://mail.python.org/mailman/listinfo/python-list
