Force type conversion in python dataclass __init__ method

Question

I have the following very simple dataclass:

import dataclasses

@dataclasses.dataclass
class Test:
    value: int

I create an instance of t

Answer 1

The type hint of dataclass attributes is never obeyed in the sense that types are enforced or checked. Mostly static type checkers like mypy are expected to do this job, Python won't do it at runtime, as it never does.

If you want to add manual type checking code, do so in the __post_init__ method:

@dataclasses.dataclass
class Test:
    value: int

    def __post_init__(self):
        if not isinstance(self.value, int):
            raise ValueError('value not an int')
            # or self.value = int(self.value)

You could use dataclasses.fields(self) to get a tuple of Field objects which specify the field and the type and loop over that to do this for each field automatically, without writing it for each one individually.

def __post_init__(self):
    for field in dataclasses.fields(self):
        value = getattr(self, field.name)
        if not isinstance(value, field.type):
            raise ValueError(f'Expected {field.name} to be {field.type}, '
                             f'got {repr(value)}')
            # or setattr(self, field.name, field.type(value))