Core Python Programming (2nd Edition)

13.16. Advanced Features of New-Style Classes (Python 2.2+)
13.16.1. General Features of New-Style Classes
We have already discussed some of the features tied to new-style classes. With the unification of types
and classes, the most significant of these features is the ability to subclass Python data types. One side
effect of this is that all of the Python "casting" or conversion built-in functions are now factory functions.
When you call them, you are really instantiating an instance of the corresponding type.
The following built-in function, which have been around Python for a while, have been quietly (or
perhaps not) converted to factory functions:
● int(), long(), float(), complex()
● str(), unicode()
● list(), tuple()
● type()
In addition, several new ones have been added to round out the posse:
● basestring() [1]
● dict()
● bool()
[1] New in Python 2.3.
● set(), [2] frozenset() [2]
[2] New in Python 2.4.
● object()
● classmethod()
● staticmethod()
● super()
● property()
● file()
These class names and factory functions have flexible usage. In addition to creating new objects of
those types, they can be used as base classes when subclassing types, and they can now be used with
the isinstance() built-in function. Using isinstance() can help replace tired old idioms with one that
requires fewer functions calls resulting in cleaner code. For example, to test if an object is an integer,
we had to call type() twice or import the types module and use its attributes; but now we can just use
isinstance() and even gain in performance: