website/content/blog/python-dataclasses-derived-fields-validation.md

title	date	draft	tags	math	medium_enabled
Python Dataclasses: Derived Fields and Validation	2024-01-15T11:02:21-05:00	false	Python	false	false

Python dataclasses provide a simplified way of creating simple classes that hold data.

from dataclasses import dataclass

@dataclass
class Person:
    name: str
    birth_year: int

The above code is equivalent to:

class A:
    def __init__(name: str, birth_year: int):
        self.name = name
        self.birth_year = birth_year
        self.__post__init__()

Notice the call to __post__init__ at the end. We can override that method to do whatever we'd like. I have found two great use cases for this.

Use Case 1: Derived Fields

Straight from the Python documentation, this use case is for when we want to use some variables to create a new variable.

For example, to compute a new field age from a person's birth_year:

class Person:
    name: str
    birth_year: int
    age: int = field(init=False)
    
    def __post_init__(self):
        # Assuming the current year is 2024 and their birthday already passed
        self.age = 2024 - self.birth_year

Use Case 2: Validation

Another use case is to make sure that the user instantiates the fields of a data class in a way we expect.

class Person:
    name: str
    birth_year: int
    
    def __post__init__(self):
        assert self.birth_year > 0
        assert isinstance(self.name, str)

Nothing is stopping us from combining both of these use cases within the __post_init__ method!