'''API''' is a shortcut for "Application Programming Interface".

Loosely defined, API describes everything an application programmer needs to know about
piece of code to know how to use it. The idea is that the programmer knows all [[Signature]]s
of classes and methods; their parameters, parameter types (or behaviour) as well as return values.

A programmer should always write against interface (API), '''not''' against implementation.
This is especially important in Python. If you know a class has method subfrob(), you'd be better off with

{{{
if hasattr(obj, 'subfrob'):
    do_something(obj)
else:
    well_do_something_else
}}}

instead of using `isinstance(obj, SomeSpecificClass)`. Effectively, this is applying idiom
"if it walks like a duck, quacks like a duck..." it's a duck alright. Controlling behaviour 
depending on class instance used is more like programming to implementation, whereas programming against "signature" is more like programming against interface.

An example of API can be found at [[Voicent Simple Telephone Call API]]. The published class is used to encapsulate the implementation details.