module with __call__ defined is not callable?

[Scott David Daniels]

Steven D'Aprano wrote:
> On Wed, 08 Feb 2006 13:58:13 +1100, Delaney, Timothy (Tim) wrote:
> 
>> adam johnson wrote:
>>
>>> Hi All.
>>> I was wondering why defining a __call__ attribute for a module
>>> doesn't make it actually callable. 
>> For the same reason that the following doesn't work
> [snip example]
>> The __call__ attribute must be defined on the class (or type) - not on
>> the instance. A module is an instance of <type 'module'>.
> 
> That's not a _reason_, it is just a (re-)statement of fact. We know that
> defining a __call__ method on a module doesn't make it callable. Why not?
> The answer isn't "because defining a __call__ method on a module or an
> instance doesn't make it callable", that's just avoiding the question.

You missed it.  Steven D'Aprano was telling you why, and all you heard
was the no.  He stated a more general principal which controls why
modules in particular are not callable.  It is not a design decision
about modules; it is a design decision about classes and instances.

     class SomeClass(object):
          def __call__(self): return 'Text'

     class AnotherClass(object):
          def __repr__(self): return 'Missive'

     name = SomeClass()()  # this works
     name = AnotherClass()()  # this doesn't
     obj = AnotherClass()  # build an instance
     def fun(*args): return 'more text' # *args so nearly any call works
     obj.__call__ = fun    # tack a function onto an instance
     obj()  # note this doesn't call the function.

Now, if you think the last _should_ do the call, then let's step
back to classes.

     class SomeClass(object):
          def __call__(self): return 'Text'

Now the SomeClass object (which is a subclass of object) has an
attribute named "__call__".  Should that define how the expression
     SomeClass()
is evaluated?  Should that return the string 'Text' or create a
new instance of SomeClass?

--Scott David Daniels
scott.daniels at acm.org