summary: (was: callback + __call__ + TypeError: unbound method; on non method)

[Alex Martelli]

<posted & mailed>

Volker Apelt wrote:

> ## Thank you, Alex Martelli for your help on this problem.

You're welcome, but:
        ...
>   class replacement for staticmethod and add a second
>   group of () to the call.

The "second group of ()" is needed only because:

> def call_me():
>     return time.time

CALLING call_me returns (doesn't CALL) the function object
time.time.  Therefore:

> class Callback_good:
>     _default_Func = staticmethod(call_me) #class wide default callback

now Callback_good._default_Func is a callable returning a
callable when called, therefore:

>     def __init__(self):
>         self._m_func = Callback_good._default_Func
>         
>     def __call__(self):
>         # uncomment one of these
>         #return self._m_func()() # for python < 2.2
>         #return self._m_func()   # for ptyton >= 2.2

Nope -- with your current definitions you need ()() in any
version of Python.


I have another preference, anyway:

class Callback_better:
    _m_func = staticmethod(call_me())  # record the final-callable
    def __call__(self): return self._m_func()

No need for __init__: self._m_func defaults to Callback_better._m_func
unless specifically set as an instance attribute of self.

This works either in 2.2, or 2.1 with the staticmethod class I
proposed (comes from the Python Cookbook, by the way -- both the
online version, and the enhanced one we edited into an O'Reilly
printed book:-0).


Alex