Magic methods and definition order

Question

I'm looking at the source code for a trie implementation

On lines 80-85:

def keys(self, prefix=[]):
    return self.__keys__(prefix)

def __keys__(self, prefix=[], seen=[]):
    result = []
    etc.

1. What is def __keys__? Is that a magic object that is self-created? If so, is this poor code? Or does __keys__ exist as a standard Python magic method? I can't find it anywhere in the Python documentation, though.

2. Why is it legal for the function to call self.__keys__ before def __keys__ is even instantiated? Wouldn't def __keys__ have to go before def keys (since keys calls __keys__)?

Answer 1

The compilation of a class in Python is done way before the class is instantiated.

Whenever class type is created, the body of the class block is compiled and executed. Then, all the functions are transformed either into bound handles (normal functions) or into classmethod/staticmethod objects. Then, when a new instance is created, content of the type's __dict__ is copied over to the instance (and bound handles are transformed into methods).

Therefore, at the moment of calling instance.keys(), the instance already has both keys and __keys__ methods.

Also, there is no __keys__ method in any data mode, as far as I know.