Recursively evolve nested attrs classes

Fixes: #634

src/attr/_funcs.py

@@ -337,8 +337,13 @@ 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:
+            e = attr.ib(type=int)
+
+        @attr.s
+        class N1:
+            c = attr.ib(type=N2)
+            d = attr.ib(type=int)
+
+        @attr.s
+        class C:
+            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)