Type unification and modules

[Guido van Rossum]

Pekka Pessi <Pekka.Pessi at nokia.com> writes:

> 	However, I the __call__ attribute does not work (or then I have
> 	an old alpha):
> 
> Python 2.2a2 (#1, Sep  3 2001, 23:06:29) 
> [GCC 2.96 20000731 (Red Hat Linux 7.1 2.96-85)] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
> >>> import types
> >>> class Module(types.ModuleType):
> ...  pass
> ... 
> >>> m = Module()
> >>> def modulecall():
> ...  print "called a module"
> ... 
> >>> m.__call__ = modulecall
> >>> m()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> TypeError: object is not callable: <module '?' (built-in)>
> >>> class Class:
> ...   pass
> ... 
> >>> c = Class()
> >>> def instancecall():print "called an instance"
> ... 
> >>> c.__call__ = instancecall
> >>> c()
> called an instance
> >>> 

You are abusing a shortcut that doesn't work for new classes for a
different reason.  It works if you write this the normal way:

    >>> class Module(types.ModuleType):
	    def __call__(self):
		print "hello world"
    >>> m = Module()
    >>> m()
    hello world
    >>> 

> 	I thought that modules were exception to the rule, but only pure
> 	instances seem to support __call__ now.  There seems to be an
> 	IsInstance() test somewhere preventing calling other objects.

Not exactly.  (Guessing about the implementation rarely gets you where
you want to be in your understanding. :-)

> >Do you want to be able to call a module? Why?
> 
> 	Actually, I want to call *any* object with a __call__ attribute.

Well, being callable and having a __call__ attribute are still not
entirely the same.  That's just how it is.  The __call__ method has to
be implemented by the type.

> 	Modules are only way to access other languages; if I have a
> 	module which just provides access to an object implemented in C,
> 	I'd like to create that object just by calling the module.

You can't do that in Python right now, and you must be aware of the
common workaround: define a function that you call to create the
object.

> 	On the other hand, now that type objects can be are used as
> 	constructors, I should call the type object instead. Hmmm.

Exactly.

--Guido van Rossum (home page: http://www.python.org/~guido/)