Class within class, is it possible? (Python)

Question

I can include a class within another class (Father and daughter rsrs)? Reason for the question? Simple, in Django I saw something like this

class Form(...):
  class Meta:
    (code)

Okay, I looked it up... I found this here, right here at SOPT

But since I’m a beginner in Python, the question:

That is correct?

Answer 1

It’s possible, but it has limited usefulness: it only serves when you want to use the inner class as a "namespace", and you’ll never want to create an instance of it.

Only correct. For example, when you want to leave some parameters for the class, but does not want to leave these parameters direct as class attributes - to avoid confusion of names.

That is, it’s exactly what Django does in his "models": it has a lot of parameters that he needs to write down in a model class - but if everything went right under the class, no model field would be able to have the same name as none of these attributes.

For example:

In [137]:  
     ...: class MinhaClasse: 
     ...:     a = 0 
     ...:     class Interna: 
     ...:        a = 5 
     ...:                                                                                                                                              

In [138]: obj = MinhaClasse                                                                                                                            

In [139]: obj.a                                                                                                                                        
Out[139]: 0

In [140]: obj.Interna.a                                                                                                                                
Out[140]: 5

However, you should not do this if you intend to instantiate the class Interna or if you want each instance of MinhaClasse have a class instance Interna. That is: for use normal class classes should not be used within classes.

If you want for example a class Veiculo has an instance of Motor within, the correct one is to create the two separate classes, and at the initialization of Veiculo create the instance of Motor to that vehicle:

In [141]: class Motor: 
     ...:     pass 
     ...:                                                                                                                                              

In [142]: class Veiculo: 
     ...:     def __init__(self): 
     ...:         self.motor = Motor() 
     ...:                                                                                                                                              

In [143]: meu_carro = Veiculo()                                                                                                                        

In [144]: meu_carro.motor                                                                                                                              
Out[144]: <__main__.Motor at 0x7f1dc57ab400>

I put this example here, because I’ve seen people think that by defining one class within another, by "magic" when you create an outside class instance, Python would create an inside class instance - that doesn’t happen.