C-like forward declaration in python class

Question

How would I properly do the following in python?

class AnyType:
    def __init__(self, Type:Union[PrimitiveType, ComplexType]):
        self.Type = Type

class PrimitiveType(AnyType):
    def __init__(self, Type):
        super().__init__(Type)

class NestedType(AnyType):
    def __init__(self, Type):
        super().__init__(Type)

NameError: name 'PrimitiveType' is not defined

I know in C there is a forward declaration but how do I prevent the following circular reference in python?

A class usually should not depend on the existence of any particular subclasses. What's your use case for this? — chepner
– chepner, Commented Nov 13, 2022 at 2:37
@chepner a bit more detail is here: stackoverflow.com/questions/74417988/… — David542
– David542, Commented Nov 13, 2022 at 2:38
That doesn't answer my question. Why would a type require a subtype in order to instantiate it? — chepner
– chepner, Commented Nov 13, 2022 at 2:45
@chepner it probably wouldn't or shouldn't, to be honest this is my first time trying to design a sort of type hierarchy like this so sort of playing around with it a bit... — David542
– David542, Commented Nov 13, 2022 at 9:39

kosciej16 · Accepted Answer · 2022-11-13 02:32:22Z

2

There is a PEP describing such feature. If you use python3.7+ you can add from __future__ import annotations at the beginning and it should work. In other case, using string fixes the problem

class AnyType:
    def __init__(self, Type:Union["PrimitiveType", "ComplexType"]):
        self.Type = Type

answered Nov 13, 2022 at 2:32

kosciej16

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