> I think the best thing to do is write another decorator that adds this method. I've often thought that having a dataclasses_tools third-party module would be a good idea.

I'd be happy with a separate decorator in the standard library for adding these methods. Not so sure about a third-party module, the added value is probably not high enough to justify an extra dependency (assuming one is aware it exists in the first place).

> or assume the member type matches the type defined in the class. 

This doesn't seem an unreasonable assumption to me. If I'm using a dataclass, I probably care enough about its member types to bother declaring them and I wouldn't mind if a particular method expects that the members actually match the types. This behaviour would be clearly documented. 

Alternatively, if we go with a separate decorator, whether this assumption holds could be a parameter, something like:

    def add_asdict(cls, name='asdict', strict=True)