fix(spy): allow spy creation when type hints cannot be resolved (#10)

Fixes #8

Author: mcous

decoy/spy.py

@@ -77,12 +77,20 @@ def __getattr__(self, name: str) -> Any:
         child_spec = None
 
         if isclass(self._spec):
-            hints = get_type_hints(self._spec)  # type: ignore[arg-type]
-            child_spec = getattr(
-                self._spec,
-                name,
-                hints.get(name),
-            )
+            try:
+                # NOTE(mc, 2021-01-05): `get_type_hints` may fail at runtime,
+                # e.g. if a type is subscriptable according to mypy but not
+                # according to Python, `get_type_hints` will raise.
+                # Rather than fail to create a spy with an inscrutable error,
+                # gracefully fallback to a specification-less spy.
+                hints = get_type_hints(self._spec)  # type: ignore[arg-type]
+                child_spec = getattr(
+                    self._spec,
+                    name,
+                    hints.get(name),
+                )
+            except Exception:
+                pass
 
         if isinstance(child_spec, property):
             hints = get_type_hints(child_spec.fget)

tests/test_spy.py

@@ -248,6 +248,27 @@ class _SomeClass:
     ]
 
 
+@pytest.mark.filterwarnings("ignore:'NoneType' object is not subscriptable")
+async def test_create_nested_spy_using_non_runtime_type_hints() -> None:
+    """It should gracefully degrade if type hints cannot be resolved."""
+
+    class _SomeClass:
+        _property: "None[str]"
+
+    calls = []
+    spy = create_spy(spec=_SomeClass, handle_call=lambda c: calls.append(c))
+    spy._property.do_something(7, eight=8, nine=9)
+
+    assert calls == [
+        SpyCall(
+            spy_id=id(spy._property.do_something),
+            spy_name="_SomeClass._property.do_something",
+            args=(7,),
+            kwargs={"eight": 8, "nine": 9},
+        ),
+    ]
+
+
 async def test_spy_returns_handler_value() -> None:
     """The spy should return the value from its call handler when called."""
     call_count = 0