Steven D'Aprano wrote: > On Wed, 10 Mar 2010 08:12:14 -0500, Neal Becker wrote: > >> Want to switch __call__ behavior. Why doesn't this work? What is the >> correct way to write this? >> >> class X (object): >> def __init__(self, i): >> if i == 0: >> def __call__ (self): >> return 0 >> else: >> def __call_ (self): >> return 1 > > > Others have already pointed out that there are two reasons that won't > work: > > (1) you define __call__ as a local variable of the __init__ method which > then disappears as soon as the __init__ method completes; and > > (2) special methods like __call__ are only called on the class, not the > instance, so you can't give each instance its own method. > > > Perhaps the best way to solve this is to use delegation: > > > def zero_returner(): > return 0 > > def one_returner(): > return 1 > > > class X (object): > def __init__(self, i): > if i == 0: > self.func = zero_returner > else: > self.func = one_returner > def __call__(self, *args, **kwargs): > return self.func(*args, **kwargs) > > > zero_returner and one_returner can be any callable object, not > necessarily a function. > > Of course, all this assumes that your solution isn't even simpler: > > class X (object): > def __init__(self, i): > self.i = i > def __call__(self): > return self.i > > but I assume if it was that simple, you would have done that already. > > > The example I showed was just a toy problem. The real problem is I expect to call a function many times, and I want to avoid the overhead of the 'if blah' everytime.
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