Given one of the motivations of @dataclass is to reduce boilerplate code then, in the context of @dataclass,

x: list = [] should be equal to x: list = field(default_factory=lambda: [])

The example in PEP 557 is not reasonable. It should be:

class D:
    def __init__(self, x=[]):
        self.x = x

That x = None works (without specifying a default factory, and is different from plain "x")  makes the whole "factory" argument even more bizarre. Why would a None Type work, but a List Type not?

I think either the behavior of this should be different or the docs should at address this more clearly.