Steven W. Orr a écrit : > I saw this and tried to use it: > > ------------------><8------------------- const.py------------- > class _const: > class ConstError(TypeError): pass > def __setattr__(self,name,value): > if self.__dict__.has_key(name): > raise self.ConstError, "Can't rebind const(%s)"%name > self.__dict__[name]=value > > import sys > sys.modules[__name__]=_const() > ------------------><8------------------- const.py------------- > > Then when I go to try to use it I'm supposed to say: > > const.pi = 3.14159 > const.e = 2.7178 > > > Two questions: > > 1. Why do I not have to say > > _const.pi = 3.14159 > _const.e = 2.7178
Because of the last two lines of module const. sys.modules is a dict of already imported modules (yes, Python's modules are objects too), which avoids modules being imported twice or more. __name__ is a magic variable that is set either to the name of the module - when it's imported - or to '__main__' - when it's being directly executed as the main program. Here, when you first do 'import const', the module's code itself sets sys.modules['const'] to an instance of _const, so what you import in your namespace as 'const' is not the const module instance but a _const instance. > and is this in the tutorial? Hmmm... Not sure. FWIW, it's mostly a hack !-) > 2. Can I make this behave in such a way that I can create my constants > with a classname s/classname/name/ > that is different for different uses? e.g., > > irrational_const.pi = 3.14159 > irrational_const.e = 2.7178 > > even_const.first = 2 > even_const.firstPlus = 4 irrational_const = const.__class__() even_const = const.__class__() Now while I find this hack interesting, it's also totally unpythonic IMHO. The usual convention is to use ALL_UPPER names for (pseudo) symbolic constants, and I don't see any reason to forcefit B&D languages concepts into Python. My 2 cents... -- http://mail.python.org/mailman/listinfo/python-list
