[The Python version described in this article is 3.x, more specifically - the 3.3 alpha release of CPython.]The concept of a callable is fundamental in Python. When thinking about what can be "called", the immediately obvious answer is functions. Whether it’s user defined functions (written by you), or builtin functions (most probably implemented in C inside the CPython interpreter), functions were meant to be called, right?Well, there are also methods, but they’re not very interesting because they’re just special functions that are bound to objects. What else can be call...