Dynamically add fields to dataclass objects

You could use make_dataclass to create X on the fly:

X = make_dataclass('X', [('i', int), ('s', str)])
x = X(i=42, s="text")

asdict(x)
# {'i': 42, 's': 'text'}

Or as a derived class:

@dataclass
class X:
    i: int

x = X(i=42)
x.__class__ = make_dataclass('Y', fields=[('s', str)], bases=(X,))
x.s="text"

asdict(x)
# {'i': 42, 's': 'text'}