Dataclass nested within another dataclass does not update data correctly

Question

I generate two different instances of a python dataclass which includes a nested dataclass. When I update a value in the nested dataclass in one instance (but not in the other), the same data is placed in the nested dataclass in both instances. This is not what I expected.

from dataclasses import dataclass


@dataclass
class sub1:
    q: int = 10
    r: str = "qrst"


@dataclass
class A:
    a: int = 1
    s1: sub1 = sub1()


if __name__ == '__main__':
    a = A()
    aa = A()
    aa.a = 9
    aa.s1.r = "92"
    print("a:", repr(a))
    print("aa:", repr(aa))

''' Produces --
a: A(a=1, s1=sub1(q=10, r='92'))
aa: A(a=9, s1=sub1(q=10, r='92'))
'''

I expected the nested dataclass to be updated in only the specified instance (aa) and for the nested dataclass in the other instance (a) to remain unchanged.

What am I doing wrong, or is dataclass the wrong tool?

Answer 1

What you are currently doing is providing a default value for the field. As that value is a mutable object, changes to that object will be visible to all instances of your dataclass.

What you should do instead is provide a default factory that produces sub1 instances for each new A instance:

from dataclasses import field

@dataclass
class A:
    a: int = 1
    s1: sub1 = field(default_factory=sub1)

a = A()
aa = A()
aa.a = 9
aa.s1.r = "92"
print("a:", repr(a))  # a: A(a=1, s1=sub1(q=10, r='qrst'))
print("aa:", repr(aa))  # aa: A(a=9, s1=sub1(q=10, r='92'))