Do not check parent binder for AssistedBuilder (#196)

Only check the current binder for bindings when resolving
`AssistedBuilder` instances, since any injected argument to the built
class should use the active injector when creating the `AssistedBuilder`
and not one of its parents.

Fixes #186.

injector/__init__.py

@@ -632,8 +632,9 @@ def _get_binding(self, key: type, *, only_this_binder: bool = False) -> Tuple[Bi
 
     def get_binding(self, interface: type) -> Tuple[Binding, 'Binder']:
         is_scope = isinstance(interface, type) and issubclass(interface, Scope)
+        is_assisted_builder = _is_specialization(interface, AssistedBuilder)
         try:
-            return self._get_binding(interface, only_this_binder=is_scope)
+            return self._get_binding(interface, only_this_binder=is_scope or is_assisted_builder)
         except (KeyError, UnsatisfiedRequirement):
             if is_scope:
                 scope = interface

injector_test.py

@@ -805,6 +805,19 @@ def __init__(self, builder: AssistedBuilder[NeedsAssistance]):
     assert (b1._injector, b2._injector) == (i1, i2)
 
 
+def test_assisted_builder_injection_is_safe_to_use_with_child_injectors():
+    class X:
+        @inject
+        def __init__(self, builder: AssistedBuilder[NeedsAssistance]):
+            self.builder = builder
+
+    i1 = Injector()
+    i2 = i1.create_child_injector()
+    b1 = i1.get(X).builder
+    b2 = i2.get(X).builder
+    assert (b1._injector, b2._injector) == (i1, i2)
+
+
 class TestThreadSafety:
     def setup(self):
         self.event = threading.Event()