module with __call__ defined is not callable?

[Delaney, Timothy (Tim)]

limodou wrote:

> On 2/8/06, adam johnson <adam.sven.johnson at gmail.com> wrote:
>> Thanks for you answer.
>> 
>> I was under the impression that you could tack methods onto an
>> object at any time, your example almost works with old style classes
>> and would with a function instead of a method. 

In fact it will work if you change the A.hello to self.hello ...

    class A:

        def __init__(self):
            self.__call__ = self.hello

        def hello (self):
            print 'Hello, world!'

    a = A()
    a()

    Hello, world!

>> So now all I need to know is why now with new style classes the
>> special functions need to be defined on the class instead of
>> attached to the instance at any time. 

http://users.rcn.com/python/download/Descriptor.htm

> don't need do like above!
> 
>  >>> class A:
>  ...     def __init__(self):
>  ...             A.__call__ = A.hello
>  ...     def hello(self):
>  ...             print "Hello, world!"
> 
>  >>> a = A()
>  >>> a()
>  Hello, world!
> 
> You should review the bound method and unbound method.

This has nothing to do with it - in the above example, you are setting
the *class* attribute of a user-defined class (which allows setting
additional attributes) - which therefore makes instances callable.

Compare the equivalent for the current module:

    import sys

    def hello (self):
        print 'Hello, world!'

    m = sys.modules[__name__]
    mc = type(m)
    mc.__call__ = hello

    Traceback (most recent call last):
      File "D:\Python\modules\testclasscall.py", line 7, in ?
        mc.__call__ = hello
    TypeError: can't set attributes of built-in/extension type 'module'

Tim Delaney