Python dataclass gotcha

python_dataclass_gotcha.md

Small Python dataclass gotcha

Members not set via the constructor are ignored in equality comparisons. Consider other in the code example below:

from dataclasses import dataclass

@dataclass
class Data:
    value: int
    other = 1  # conceptually not part of a dataclass

a = Data(0)
b = Data(0)
assert a == b, "values are equal"
b.other = 100
assert a == b, "values are still equal!"

print("a:", a)
print("b:", a) # even in printouts "other" is ignored
print("a == b:", a == b)

As you can see both assertion checks pass:

a: Data(value=0)
b: Data(value=0)
a == b: True