inner-class

Advanced inner classes for Python

An inner or nested class is a class declared entirely within the body of another class.

Theses classes can be used when you do not want to expose the class, and it is closely related to another class.

The four features

Python does have built-in support for inner classes.

Unfortunately this support is rather minimalistic and lack some useful features :

 class MyOuter:
    @inner
    class MyInner:
       def hello(self):
           print('Hello')

outer attribute

The inner class gain an 'outer' attribute that reference its outer class.

  >>> MyOuter.MyInner.outer 
  <class '__main__.MyOuter'>

inner derivation

When the outer class is derived, the inner class is also derived in order to point at the derived outer class.

 class MyChildOuter(MyOuter):
    pass

 >>> MyChildOuter.MyInner.outer
 <class '__main__.MyChildOuter'>

carried inheritance

When the inner class is redefined in an outer subclass, it will automatically derivate from the inner class of the outer superclass.

  class MyChildOuter(MyOuter):
     class MyInner:
       pass

  >>> MyChildOuter.MyInner().hello()
  Hello

This may seem a bit implicit but is useful for type conformance (code must continue to work when the parent class is replaced by a child class), particularly with mix-in classes.

inner instantiation

the outer attribute of an inner instance store it's outer instance

  outer = MyOuter()
  
  >>> outer
  <__main__.MyOuter object at 0x03BAA990>
  >>> outer.MyInner().outer
  <__main__.MyOuter object at 0x03BAA990>

This attribute is already available into inner __init__ constructor.

If the inner object is created directly from the outer outer class (and not an instance), it will still store the outer class.

Usage

See the samples directory.

implementation

This module provide 6 decorators :

@inner : support the 4 features

@class_inner : remove inner instantiation feature

@static_inner : only keep outer attribute feature

@raw_inner : a do nothing, useful if the language would change in the futur to make @inner the default behavior of inner class in python

@inner.property : access to target from outer instance automatically create an instance of the inner class

@inner.cached_property : the inner instance is only created once and then cached in the __dict__ of the outer instance

install

The module have been tested on Python 3.7

python setup.py test python setup.py install