med ∩ ml

Validate Python dataclass types

Table of contents

In Python you can create dataclasses with the wrong type. The type checker should show an error, but nothing prevents creating the object. This small function validates that each attribute is of the correct type by using the __annotations__ attribute of the dataclass. You can also create a base class to inherit from, but it won’t work if you override the __post_init__ method in the child classes.

import dataclasses

def val(t):
    Validate the object `t` types based on the
    __annotations__ dictionary.
    for k,v in t.__annotations__.items():
        assert isinstance(getattr(t, k), v)

#Base class
@dataclasses.dataclass(frozen=True, slots=True)
class Base:
    def __post_init__(self):

# Inherit from base class
@dataclasses.dataclass(frozen=True, slots=True)
class User(Base):
    name: str
    age: int

Now when initializing the object, it will throw a runtime exception if any of the attributes is not of the correct type.

# ok
u = User(name="ric", age=12)

# error
u = User(name=123, age=12)


This is just a quick snipppet. I recommend using pydantic if you need a robust data validation library.