python - what does `super()` in `__new__`

Question

Note: Part of a flyweight implementation with Python

import weakref

class CarModel:
    _models = weakref.WeakValueDictionary()

    def __new__(cls, model_name, *args, **kwargs):
        model = cls._models.get(model_name)
        if not model:
            model = super().__new__(cls)
            cls._models[model_name] = model    
        return model

    def __init__(self, model_name, air=False):
        if not hasattr(self, "initted"):
          self.model_name = model_name
          self.air = air
          self.initted=True

Question 1> what does super() mean? Does it mean the parent class of CarModel?

Question 2> I also have difficulties to understand the how the function __new__ works? Specifically, the following line.

model = super().__new__(cls)

Description for __new__:

The constructor function is called __new__ as opposed to __init__, and accepts exactly one argument, the class that is being constructed (it is called before the object is constructed, so there is no self argument). It also has to return the newly created object.

Answer 1

To begin with super() in itself is simply shorthand for super(A, B), where A is the class wherein the code occurs, and B is the first argument to the function in which the code occurs; so in your particular case, super().__new__(cls) expands to super(CarModel, cls).__new__(cls).

In turn, super(T, O) returns a "super object". To understand what a super object does, you need to understand how attribute references on instances and classes work in Python.

Assuming no __getattr__ or __getattribute__ methods are involved, referencing attribute A on an object O (that is, evaluating O.A or getattr(O, "A")) proceeds through the following steps:

1. If "A" is defined in O's instance dict (O.__dict__), then the value on that dict is returned directly, precisely as it is.
2. Otherwise, each of the classes in O's method resolution order are checked in turn, looking for "A" in each of their dicts. If found, call the value D.
3. If D, in turn, does not define a __get__, then it is returned as it is. If it does, however, then D is referred to as a "descriptor", and its __get__ method is called with O as the first argument, and type(O) as the second argument.

An attribute reference on a class works about the same, substituting the class being reference for the instance, with the following differences:

• Step 1 doesn't apply.
• The __get__ method is called with None as the first argument, and the class being referenced as the second.

Python uses descriptors to implement such things as instance methods, class methods, static methods, and properties.

A super object created with super(T, O), then, is a (built-in) object with a __getattribute__ method which is called on every attribute reference on it, and looks up the attribute in the dicts of the only classes following T in O's MRO. The value it then finds, it calls __get__ on as usual.

The process is a bit complex, so as an example, here's how it would work on your specific case. Since CarModel is defined as it is, its MRO is [CarModel, object].

1. super().__new__(cls) expands to super(CarModel, cls).__new__(cls), as described above.
2. super(CarModel, cls) is evaluated to yield a super object S.
3. Python fetches the attribute "__new__" on S (the equivalent of calling getattr(S, "__new__") in Python code).
4. Since S was created on the CarModel class, it considers the classes following CarModel in the MRO of CarModel, and finds "__new__" in the dict of the object class itself. Its value, a static method, has a __get__ method, which is called with the arguments None and cls. Since __new__ is a static method, its __get__ method simply returns the function as it is, unmodified. Therefore, super(CarModel, cls).__new__ is precisely the same as object.__new__.
5. The function obtained in the last step (that is, object.__new__) is called with the cls argument, where cls is probably CarModel, finally a new instance of the CarModel class.

I hope that was at all understandable.

(For the sake of completeness, it should be mentioned that the actual __new__ function on the object class is not actually a static method, but a special built-in function that simply has no __get__ method at all, but since the __get__ method on static methods just return the function they were defined with, the effect is the same.)