Meta Classes

Note

Work in progress.

Preliminaries

Note

[Metaclasses] are deeper magic than 99% of users should ever worry about. If you wonder whether you need them, you don’t (the people who actually need th em

When will you need it:

  • When you develop a library or an API that will be used by people.
  • When you want to enforce a coding standard in a huge application.
  • They can be used to enhance classes with features like tracing, object persis tence, exception logging

If this is the case why bother understand it?

It demystifies the underlying working of python classes.

Note

Classes Are Instances of type

  • In Python 3.X, user-defined class objects are instances of the object named ty pe,which is itself a class.
  • In Python 2.X, new-style classes inherit from object, which is a subclass of t ype;classic classes are instances of type and are not created from a class.

Their differences are highlighted in the following code-snippet.

python3

>>> type([]), type(type([])
(<class 'list'>, <class 'type'>)

>>> type(list), type(type) # Same, but with type names
(<class 'type'>, <class 'type'>)

python2

>>> type(list), type(type) # Same, but with type names
(<type 'list'>, <type 'type'>)
>>> type(list), type(type)
(<type 'type'>, <type 'type'>)
  • Types are defined by classes that derive from type.
  • User-defined classes are instances of type classes
  • User-defined classes are types that generate instances of their own.