attr.evolve no longer supports setting an attr object as attribute

[progval]

Hi,

I don't know if this is an intentional side-effect of #759, but attr.evolve no longer works in the non-recursive recursive case if the argument is an attr class.

Here is a snippet to reproduce it:

import attr

@attr.s
class Cls1:
    param1 = attr.ib()


@attr.s
class Cls2:
    param2 = attr.ib()


obj2a = Cls2(param2="a")
obj2b = Cls2(param2="b")

obj1a = Cls1(param1=obj2a)
obj1b = attr.evolve(obj1a, param1=obj2b)

print(obj1b)

Before fe6eb31, it printed this: Cls1(param1=Cls2(param2='b'))

But after this commit, it raises this error:

Traceback (most recent call last):
  File "test.py", line 17, in <module>
    obj1b = attr.evolve(obj1a, param1=obj2b)
  File "/home/dev/.local/lib/python3.7/site-packages/attrs-21.2.0.dev0-py3.7.egg/attr/_funcs.py", line 347, in evolve
    changes[init_name] = evolve(value, **changes[init_name])
TypeError: evolve() argument after ** must be a mapping, not Cls2

Thanks!

[staaam]

same here. Issue is in attr._funcs.evolve at:

    for a in attrs:
        ...
        elif has(value):
            # Evolve nested attrs classes
            changes[init_name] = evolve(value, **changes[init_name])

value - is old value (param1 from example above)
changes[init_name] - is new value (obj2b from above)
but for recursive evolve it should be dict
probably the best way to make it backward compatible will be to check
elif has(value) and isinstance(changes[init_name], Mapping):

[hynek]

Yeah, we'll have to revert #759 and push out 21.2.

I'm gonna wait a bit with the release to see if there's other breakages that should be fixed.

[hynek]

21.2 with the fix is on PyPI.