How do I programmatically create class methods in Python?

Question

Suppose I want to define a Python class whose instances have several members of similar form:

class Bar:
    def __init__(self):
        self.baz=3
        self.x=1
        self.name=4

I can instead create all members at once using explicit manipulation of __dict__ :

class Bar:
    def __init__(self):
        self.__dict__.update({name: len(name) for name in ("baz", "x", "name")})

However, if the associated members are class data members, instead of instance members, then I am aware of no analogous way to programmatically mutate Bar.__dict__ :

class Bar:
    #Fails with "AttributeError: 'mappingproxy' object has no attribute 'update'"
    Bar.__dict__.update({name: len(name) for name in ("baz", "x", "name")})

In Javascript, programmatically manipulating the properties of a constructor is possible, because constructors are just ordinary functions, which are just ordinary objects, and therefore just a mutable dictionary mapping properties to more objects. Is it possible to write analogous code in Python?

Answer 1

I guess you can use a metaclass, a base class with __init_subclass__ or use eval to create your class. Here's an example using a base class:

class Base:
    attrs_to_set: set[str] = set()

    def __init_subclass__(cls) -> None:
        for attr in cls.attrs_to_set:
            setattr(cls, attr, cls.get_attr_value(attr))

    @classmethod
    def get_attr_value(cls, attr_name: str):
        # default, you can change it depending on your use case i guess.
        return len(attr_name)


class Bar(Base):
    attrs_to_set = {"x", "baz", "name"}


print(Bar.x, Bar.name, Bar.baz)
# prints: 1 4 3