Is there a benefit to defining a class inside another class in Python?

Question

What I\'m talking about here are nested classes. Essentially, I have two classes that I\'m modeling. A DownloadManager class and a DownloadThread class. The obvious OOP conc

Answer 1

There is another usage for nested class, when one wants to construct inherited classes whose enhanced functionalities are encapsulated in a specific nested class.

See this example:

class foo:

  class bar:
    ...  # functionalities of a specific sub-feature of foo

  def __init__(self):
    self.a = self.bar()
    ...

  ...  # other features of foo


class foo2(foo):

  class bar(foo.bar):
    ... # enhanced functionalities for this specific feature

  def __init__(self):
    foo.__init__(self)

Note that in the constructor of foo, the line self.a = self.bar() will construct a foo.bar when the object being constructed is actually a foo object, and a foo2.bar object when the object being constructed is actually a foo2 object.

If the class bar was defined outside of class foo instead, as well as its inherited version (which would be called bar2 for example), then defining the new class foo2 would be much more painful, because the constuctor of foo2 would need to have its first line replaced by self.a = bar2(), which implies re-writing the whole constructor.