How does one ignore extra arguments passed to a data class?

Question

I'd like to create a config dataclass in order to simplify whitelisting of and access to specific environment variables (typing os.environ['VAR_NAME'] is tedious relative to config.VAR_NAME). I therefore need to ignore unused environment variables in my dataclass's __init__ function, but I don't know how to extract the default __init__ in order to wrap it with, e.g., a function that also includes *_ as one of the arguments.

import os
from dataclasses import dataclass

@dataclass
class Config:
    VAR_NAME_1: str
    VAR_NAME_2: str

config = Config(**os.environ)

Running this gives me TypeError: __init__() got an unexpected keyword argument 'SOME_DEFAULT_ENV_VAR'.

Answer 1

Cleaning the argument list before passing it to the constructor is probably the best way to go about it. I'd advice against writing your own __init__ function though, since the dataclass' __init__ does a couple of other convenient things that you'll lose by overwriting it.

Also, since the argument-cleaning logic is very tightly bound to the behavior of the class and returns an instance, it might make sense to put it into a classmethod:

from dataclasses import dataclass
import inspect

@dataclass
class Config:
    var_1: str
    var_2: str

    @classmethod
    def from_dict(cls, env):      
        return cls(**{
            k: v for k, v in env.items() 
            if k in inspect.signature(cls).parameters
        })


# usage:
params = {'var_1': 'a', 'var_2': 'b', 'var_3': 'c'}
c = Config.from_dict(params)   # works without raising a TypeError 
print(c)
# prints: Config(var_1='a', var_2='b')