[Python] How to face name mangling with two non-nested public classes with private attributes

I have two public classes in Python. They are not nested. They have some private attributes and the second class has an object whose type is the first class. I need to access to the private attributes of the object I created inside the second class.

I'm not an expert in name mangling but I have read this example.

I have understood what the problem is, but I don't know how to solve it in my case, since I have one private attribute which is a instance of a public class with some private attributes that need to be accessed from other public class.

Let's clarify the exact problem with an example:

class First:

def __init__(self) -> None:
self.__size: ...

def __len__(self) -> int:
return self.__size


class Second:

def __init__(self, first: First) -> None:
self.__first = first
self.__size = __first.__size

def __len__(self) -> int:
return self.__size


When I run my tests I obtain some errors:

self.__size: = first._Second__first
^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'First' object has no attribute '_Second__first'

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