Python 3.7 dataclass: Raise error when assigning a value to undefined attribute

Question

I want to limit the usage of a dataclass to the users of my code and am wondering how can I raise an error in the following context:

from dataclasses import dataclass

@dataclass
class Foo:
    attr1: str

foo = Foo("1")
foo.attr2 = "3" #I want this line to raise an exception

Currently the last line succeeds and do not change the underlying object. I want the last line to throw an error.

Answer 1

You can add a __slots__ attribute to a dataclass like any other class. Attempting to create new attributes of an instance will then fail with AttributeError:

@dataclass
class Foo:
    __slots__ = ("attr1",)
    attr1: str

foo = Foo("1")
foo.attr2 = "3"
# AttributeError: 'Foo' object has no attribute 'attr2'