Eliminate evolve() boilerplate with evolvers/copy

[energizah]

When I want to evolve() an object, I sometimes have to do a bit of boilerplate copy()ing:

from typing import List

import attr


@attr.s
class Car:
    model: str = attr.ib()
    occupants: List = attr.ib()


car1 = Car("Toyota", ["Alice", "Bob"])
car2 = attr.evolve(car1, model="Honda", occupants=car1.occupants.copy())

To eliminate this boilerplate, one option would be:

@attr.s
class Car:
    model: str = attr.ib()
    occupants: List = attr.ib(evolver=lambda x: x.copy())

Or similarly:

@attr.s
class Car:
    model: str = attr.ib()
    occupants: List = attr.ib()

    @occupants
    def evolver(self):
        return self.occupants.copy()

Or using copy.copy() via the __copy__ method:

@attr.s
class Car:
    model: str = attr.ib()
    occupants: List = attr.ib(evolve_copy=True)
    

The latter idea suggests the option of putting the copy functionality into @attr.s, which could add a __copy__ method to the class, and like the other methods it could be enabled/disabled for each attribute with attr.ib(copy=True/False), and evolve() could use copy.copy() by default.

@attr.s(copy=True)
class Car:
    model: str = attr.ib()
    occupants: List = attr.ib()
    components: pyrsistent.PSet = attr.ib(copy=False)

---

cc: @altendky

[hynek]

Hm, you basically want a deep copy for evolve? That sounds too complex to be in scope. Is there anything stopping you to implement it yourself on top of evolve?

[energizah]

I can do most of it from outside, using the metadata field. The semantics are clearer for evolver= than for copy=. For example:

import copy
from typing import List

import attr


def fancy_evolve(obj, **kwargs):

    evolve_fields = {}
    for field_name, class_attrib in attr.fields_dict(type(obj)).items():
        if "evolver" not in class_attrib.metadata:
            continue

        evolver = class_attrib.metadata["evolver"]
        evolve_fields[field_name] = evolver(getattr(obj, field_name))

    evolve_fields.update(kwargs)
    return attr.evolve(obj, **evolve_fields)


# User code below.
##################

@attr.dataclass
class Driver:
    name: str
    age: int


@attr.s
class Vehicle:
    model: str = attr.ib()
    driver: Driver = attr.ib(metadata={"evolver": attr.evolve})
    parcel_ids: List = attr.ib(metadata={"evolver": copy.copy})


vehicle = Vehicle(model="Prius", driver=Driver("Alice", age=21), parcel_ids=[1, 4, 8])
new_vehicle = fancy_evolve(vehicle)

assert new_vehicle == vehicle
assert new_vehicle.driver == vehicle.driver
assert new_vehicle.parcel_ids == vehicle.parcel_ids
assert new_vehicle.driver is not vehicle.driver
assert new_vehicle.parcel_ids is not vehicle.parcel_ids