I have a snippet below which runs fine on python 3.7.0 but raises a ValueError exception on 3.7.1. I believe it's related to https://bugs.python.org/issue33805.

The error: c:\python\lib\dataclasses.py:1219: ValueError

The script:

from dataclasses import replace, dataclass, InitVar

@dataclass
class Test:
    a:int = 1
    b:InitVar[int] = None

    def __post_init__(self, b):
        if b is not None:
            self.a = b


if __name__ == '__main__':
    t = Test()
    t1 = Test(b=5)
    assert t1.a == 5

    t2 = replace(t1, **{})
    print(t2)