How to enforce dataclass fields' types? [duplicate]

Question

In this code:

import dataclasses

@dataclasses.dataclass
class MyClass:
    value: str

obj = MyClass(value=1)

the dataclass MyClass is instantiated with a value that does not obey the value type.

Is there a simple way (using a decorator, an argument in the dataclass decorator or library) of enforcing the fields' types so that the last line in my example raises a ValueError or something like that? Is there a major downside of enforcing types in this way?

Answer 1

You can declare a custom __post_init__ method (see python's doc) and put all checks there to force type checking. This method can be declare in parent's class to reduce changes amount.

import dataclasses


@dataclasses.dataclass()
class Parent:
    def __post_init__(self):
        for (name, field_type) in self.__annotations__.items():
            if not isinstance(self.__dict__[name], field_type):
                current_type = type(self.__dict__[name])
                raise TypeError(f"The field `{name}` was assigned by `{current_type}` instead of `{field_type}`")

        print("Check is passed successfully")


@dataclasses.dataclass()
class MyClass(Parent):
    value: str


obj1 = MyClass(value="1")
obj2 = MyClass(value=1)

The results:

Check is passed successfully
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 3, in __init__
  File "<stdin>", line 7, in __post_init__
TypeError: The field `value` was assigned by `<class 'int'>` instead of `<class 'str'>`