Calling dataclasses.replace(instance, **changes) returns a new object of the same type.

From my understanding there is, however, no method to update in place fields of a dataclass from another one.

I propose to add dataclasses.update(instance_to_update, other_instance, **changes).

This would for instance allow one to change all fields of current object in a sturdy way.
In my case, I currently call obj.__dict__.update(other_obj.__dict__) to perform the operation, but I know it has always to be done pretty carefully.

If this is accepted, I'm willing to post the change.

What would the interaction be between other_instance and changes? Why is this API different from .replace()?

I'm don't see the point of this proposal.  The fields of dataclasses are already mutable, so they can be updated just like any other object (no need for a special mechanism):

a = InventoryItem(name='Widget', unit_price=37.25, quantity_on_hand=10)

a.quantity_on_hand -= 1           # Sold one
a.unit_price *= 0.90              # Ten percent off sale

Also, it's not hard to directly take data from one instance to another:

a.name = master_catalog.name
a.unit_price = master_catalog.unit_price

I agree that I don't see the point, unless there's something I'm missing with other_instance. Or maybe the proposal is for it to also work with frozen dataclasses? I'm definitely -1 on that.

So unless there's something I'm missing where normal attribute assignment wouldn't work, I'm leaning toward rejecting this. What is it about dataclasses that would need this feature, where regular classes don't? Just do what you'd do with regular classes.

I understand your points, I'll give an example with a simplified version of my problem:


import dataclasses
import othermodule # is an external dependency

@dataclass
class City:
    name: str
    position: othermodule.Point # Point is a dataclass

    def update_position(self):
        obj = anymodule.get_position(name=self.name)
        
        # The classic solution would be to do
        self.position.x = obj.x
        self.position.y = obj.y

        # what if othermodule adds z (altitude) to Point?
        # we could imagine:
        dataclasses.update(self.position, obj)

        # I'm currently doing:
        self.position.__dict__.update(obj.__dict__)

Maybe I simply handle the issue the wrong way, so my dataclass proposal is out of scope.

I cannot do self.position = obj because I have references on position elsewhere.

I'm going to reject this. There's nothing special about dataclasses that would require a feature like this.