What do you call a class not intended to be instantiated
Aaron "Castironpi" Brady
castironpi at gmail.com
Sun Sep 28 01:33:43 CEST 2008
On Sep 27, 6:16 pm, Terry Reedy <tjre... at udel.edu> wrote:
> Aaron "Castironpi" Brady wrote:
> >>>> class A(type):
> > ... def __call__( self, *ar ):
> > ... print 'call', self, ar
> > ...
> >>>> class B(object):
> > ... __metaclass__= A
> > ...
> >>>> B(3)
> > call <class '__main__.B'> (3,)
> > Overriding the __call__ method of 'type' has the effect of giving you
> > a static __call__ method on a class-- a method which doesn't need an
> > instance to call. Your behavior may be counterintuitive though, to
> > someone who wants to instantiate 'B', in this case, and proceed like a
> > normal object. That is, they want to call a generic class and use it,
> > and also expect instances of B to behave as B. You can't have both,
> > so either return B from B.__new__, or, to instantiate B, take the long
> > way and call B.__new__ directly.
> >>>> B.__new__(B)
> > <__main__.B object at 0x009FDB70>
> > Has anyone stepped through the C code to find out when the decision is
> > made to call which function, B.__new__ or A.__call__, when B is
> > called?
> For Python coded objects, ob(*args) in code translates to internal
> execution of type(ob).__call__(ob, *args) (without further
> translation!). The interpreter compiles a statement at a time, without
> looking back to do type inferencing, and so does not know what type is
> being called or if it is even callable.
> For B, B(*args) == type(B).__call__(B, *args) == A.__call__(B, *args).
> So there is no decision.
> For C coded objects, I believe ob(*args) in Python code translate to a C
> call of the C equivalent of type(ob).tp_call (or something like that).
> From observation, type.tp_call acts something like this:
> def __call__(cls, *args):
> if cls == type:
> if len(*args):
> return arg.__class__
> elif len(*args) == 3:
> return type.__new__(type, *args) # or maybe not pass type?
> raise TypeError('type() takes 1 or 3 arguments')
> return cls.__new__(cls, *args)
> So, for a normal class C (an instance of type), type.__call__ calls
> Terry Jan Reedy
Oh, I see. Then it's the class statement that calls type.__new__.
class A: ...
-> A= type( 'A', ... )
-> A= type.__call__( type, 'A', ... )
-> A= type.__new__( type, 'A', ... )
Plus an iteration over the contents of 'namespace', to search for
properties that themselves have a __get__ method. And returns an
unboundmethod instance "of" it, for a value of "of" that's hard to
concentrate on. I jest.
Perhaps what Steven is looking for is a subclass of 'type' that does
not give this default behavior of 'unboundmethoding' everything it
can. That is, defaulting to 'staticmethod' or 'classmethod', and
perhaps offering a 'boundmethod' decorator for the exceptions.
For the case of '__call__', which he does want to control, that could
merely call B.__call__, instead of B.__new__. Untested:
>>> class A(type):
... def __call__( self, *ar ):
... return self.__call__( *ar )
... return self.__call__( self, *ar )
Which would come through to 'B' as:
>>> class B(object):
... __metaclass__= A
... def __call__( cls, *ar ).
This is because self == B in the example. This makes me scowl. Very
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