dataclasses.dataclass wrapper brokes Fraction

[TiunovNN]

Bug report

Bug description:

from dataclasses import dataclass
from fractions import Fraction

print(Fraction('30/1'))
dataclass(Fraction)
print(Fraction('30/1'))

If the dataclass function is called with argument Fraction, it brokes __init__ for Fraction

output is:

30
Traceback (most recent call last):
  File "/Users/tiunovnn/git/video-compression-model/web/api/src/tasks/test.py", line 6, in <module>
    print(Fraction('30/1'))
          ~~~~~~~~^^^^^^^^
TypeError: Fraction.__init__() takes 1 positional argument but 2 were given

expected output is:

30
30

CPython versions tested on:

3.13

Operating systems tested on:

macOS

[JelleZijlstra]

Why would you expect this to work? The dataclass decorator shouldn't be called on arbitrary library classes.

[ericvsmith]

I agree with Jelle. Further, what would you expect using @dataclass to do? What are you trying to accomplish?

[TiunovNN]

What are you trying to accomplish?

Actually, i've came across this issue, when using marshmallow-dataclass

from dataclasses import dataclass
from fractions import Fraction

import marshmallow_dataclass


@dataclass
class CustomClass:
    value: Fraction

print(Fraction('30'))
marshmallow_dataclass.class_schema(CustomClass)
print(Fraction('30'))

But the question is why @dataclass mutates the original class rather than create the new one?

[ericvsmith]

But the question is why @DataClass mutates the original class rather than create the new one?

Ah, that’s a different question. It’s because it was the least surprising behavior (to me, at least). The idea is that it just some adds stuff to a class, so you don’t have to type it yourself. In any event, it’s not going to change at this point, since it would break code.