Python: Callable behavior on new and old-style classes.

gistfile1.pytb

In [6]: callable?
Type:       builtin_function_or_method
String Form:<built-in function callable>
Namespace:  Python builtin
Docstring:
callable(object) -> bool

Return whether the object is callable (i.e., some kind of function).
Note that classes are callable, as are instances with a __call__() method.

In [7]: class Foo(object):
   ...:     pass
   ...: 

In [8]: callable(Foo)
Out[8]: True

In [9]: hasattr(Foo, '__call__')
Out[9]: True

In [10]: foo = Foo()

In [11]: callable(foo)
Out[11]: False

In [12]: hasattr(foo, '__call__')
Out[12]: False

In [13]: %timeit hasattr(foo, '__call__')
1000000 loops, best of 3: 579 ns per loop

In [14]: %timeit callable(foo)
10000000 loops, best of 3: 57.3 ns per loop

In [15]: class OldFoo:
   ....:     pass
   ....: 

In [16]: callable(OldFoo)
Out[16]: True

In [17]: hasattr(OldFoo, '__call__')
Out[17]: False