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 concept here is composition. However, composition doesn’t necessarily mean nesting, right?

I have code that looks something like this:

class DownloadThread:     def foo(self):         pass  class DownloadManager():     def __init__(self):         dwld_threads = []     def create_new_thread():         dwld_threads.append(DownloadThread()) 

But now I’m wondering if there’s a situation where nesting would be better. Something like:

class DownloadManager():     class DownloadThread:         def foo(self):             pass     def __init__(self):         dwld_threads = []     def create_new_thread():         dwld_threads.append(DownloadManager.DownloadThread()) 

Answer 1

You might want to do this when the ‘inner’ class is a one-off, which will never be used outside the definition of the outer class. For example to use a metaclass, it’s sometimes handy to do

class Foo(object):     class __metaclass__(type):         ....  

instead of defining a metaclass separately, if you’re only using it once.

The only other time I’ve used nested classes like that, I used the outer class only as a namespace to group a bunch of closely related classes together:

class Group(object):     class cls1(object):        ...      class cls2(object):        ... 

Then from another module, you can import Group and refer to these as Group.cls1, Group.cls2 etc. However one might argue that you can accomplish exactly the same (perhaps in a less confusing way) by using a module.