Recursively evolve nested attrs classes

Fixes: #634

changelog.d/759.change.rst

@@ -0,0 +1 @@
+Let ``evolve()`` work with nested ``attrs`` classes. #634

docs/examples.rst

@@ -618,6 +618,27 @@ In Clojure that function is called `assoc <https://clojuredocs.org/clojure.core/
  >>> i1 == i2
  False
 
+This functions also works for nested ``attrs`` classes.
+You just pass a (possibly nested) dict with changes for an attribute:
+
+.. doctest::
+
+ >>> @attr.s(frozen=True)
+ ... class Child(object):
+ ... x = attr.ib()
+ ... y = attr.ib()
+ >>> @attr.s(frozen=True)
+ ... class Parent(object):
+ ... child = attr.ib()
+ >>> i1 = Parent(Child(1, 2))
+ >>> i1
+ Parent(child=Child(x=1, y=2))
+ >>> i2 = attr.evolve(i1, child={"y": 3})
+ >>> i2
+ Parent(child=Child(x=1, y=3))
+ >>> i1 == i2, i1.child == i2.child
+ (False, False)
+
 
 Other Goodies
 -------------

src/attr/_funcs.py

@@ -319,7 +319,8 @@ def evolve(inst, **changes):
  Create a new instance, based on *inst* with *changes* applied.
 
  :param inst: Instance of a class with ``attrs`` attributes.
- :param changes: Keyword changes in the new copy.
+ :param changes: Keyword changes in the new copy. Nested attrs classes ca
+ be updated by passing (nested) dicts of values.
 
  :return: A copy of inst with *changes* incorporated.
 
@@ -337,8 +338,13 @@ def evolve(inst, **changes):
  continue
  attr_name = a.name # To deal with private attributes.
  init_name = attr_name if attr_name[0] != "_" else attr_name[1:]
+ value = getattr(inst, attr_name)
  if init_name not in changes:
- changes[init_name] = getattr(inst, attr_name)
+ # Add original value to changes
+ changes[init_name] = value
+ elif has(value):
+ # Evolve nested attrs classes
+ changes[init_name] = evolve(value, **changes[init_name])
 
  return cls(**changes)
 

tests/test_funcs.py

@@ -597,3 +597,27 @@ class C(object):
  b = attr.ib(init=False, default=0)
 
  assert evolve(C(1), a=2).a == 2
+
+ def test_recursive(self):
+ """
+ evolve() recursively evolves nested attrs classes when a dict is
+ passed for an attribute.
+ """
+
+ @attr.s
+ class N2(object):
+ e = attr.ib(type=int)
+
+ @attr.s
+ class N1(object):
+ c = attr.ib(type=N2)
+ d = attr.ib(type=int)
+
+ @attr.s
+ class C(object):
+ a = attr.ib(type=N1)
+ b = attr.ib(type=int)
+
+ c1 = C(N1(N2(1), 2), 3)
+ c2 = evolve(c1, a={"c": {"e": 23}})
+ assert c2 == C(N1(N2(23), 2), 3)