Merging two dataclasses

Question

Let A be a dataclass that contains at least the attributes in the dataclass B.

In python, is there a simple way to merge an instance of B and an instance of A in the same way two dictionaries are merged with the | operator as in the example below?

from dataclasses import dataclass, asdict, replace

@dataclass
class A:
    x: int 
    y: int 

@dataclass
class B:
    x: int 

a=A(x=2,y=6)
b=B(x=4)

The current simplest solutions i found were:

1. To convert both instances to dictionaries, do the merge into a single dictionary, then convert back to dataclass as in the code below:

c_dict : dict = asdict(a) | asdict(b)
c_1=A(**c_dict)

2. To use the replace function and asdict conversion as in the code below:

c_2=replace(a,**asdict(b))

Is this the simplest way? For instance trying a|b fails as an 'unsupported operand'.

Answer 1

Since there is nothing built-in that will support the a | b behavior, you can implement on your own a mixin class with the __or__ method defined:

@dataclass
class Unionable:
    def __or__(self, other):
        return self.__class__(**asdict(self) | asdict(other))

so that:

@dataclass
class A(Unionable):
    x: int
    y: int

@dataclass
class B(Unionable):
    x: int

a = A(x=2, y=6)
b = B(x=4)
print(a | b)

outputs:

A(x=4, y=6)

Demo: https://ideone.com/rEDBzN