Callables

Python's definition of callable object includes the obvious function definitions created with the def statement.

It also includes, informally, any class with a __call__() method. We can see several examples of this in Python 3 Object Oriented Programming, Dusty Phillips, Packt Publishing. For it to be more formal, we should make every callable class definition a proper subclass of collections.abc.Callable.

When we look at any Python function, we see the following behavior:

>>> abs(3)
3
>>> isinstance(abs, collections.abc.Callable)
True

The built-in abs() function is a proper instance of collections.abc.Callable. This is also true for the functions we define. The following is an example:

>>> def test(n):
...     return n*n
...
>>> isinstance(test, collections.abc.Callable)
True

Every function reports itself as Callable. This simplifies the inspection of an argument value and helps write meaningful debugging messages.

We'll take a look at callables in...