Emanuele D'Arrigo wrote: > Hi everybody, > > I was unit testing some code today and I eventually stumbled on one of > those "is" issues quickly solved replacing the "is" with "==". Still, > I don't quite see the sense of why these two cases are different: > >>>> def aFunction(): > ... pass > ... >>>> f = aFunction >>>> f is aFunction
In fact, for any defined unqualified name x the assignment "n = x" guarantees that "n is x" is True. > True <--- Ok, this seems reasonable. Nevertheless, I suspect I > shouldn't quite rely on it. > You can take that to the bank on any working Python implementation. It's hardwired into the language's semantics. >>>> class MyClass(object): > ... def myMethod(self): > ... pass > ... >>>> c = MyClass() >>>> m = c.myMethod >>>> m is c.myMethod > False <--- What? Why is that? > > In my mind I was expecting that when the method is assigned to "m" all > that it happens is that its address is assigned to the name "m" so > that effectively the same address is now pointed to by two names, like > in the function case. I googled around for some hint but I wouldn't > exactly say I'm clear on the issue just yet... > > Can anybody shed some light? Or point to a resource to look at? Or > what's the bit of python's source code that is responsible for dealing > with those assignments? > Instance-relative references to class methods are a very special case. They become what are called "bound methods" - the interpreter creates a new bound method for each reference. This allows the bound method to provide the instance as a first argument when it is called. >>> class C(object): ... def MyMethod(self): ... pass ... >>> c = C() >>> a = c.MyMethod >>> b = c.MyMethod >>> a, b (<bound method C.MyMethod of <__main__.C object at 0x7ff33fcc>>, <bound method C.MyMethod of <__main__.C object at 0x7ff33fcc>>) >>> regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ Want to know? Come to PyCon - soon! http://us.pycon.org/ -- http://mail.python.org/mailman/listinfo/python-list
