fix(when): ensure then_enter_with accepts None (#137)

Fixes #135

Author: mcous

decoy/call_handler.py

@@ -4,7 +4,7 @@
 from .spy_log import SpyLog
 from .context_managers import ContextWrapper
 from .spy_events import SpyCall, SpyEvent
-from .stub_store import StubStore
+from .stub_store import MISSING, StubStore
 
 
 class CallHandlerResult(NamedTuple):
@@ -43,7 +43,7 @@ def handle(self, call: SpyEvent) -> Optional[CallHandlerResult]:
             else:
                 return_value = behavior.action()
 
-        elif behavior.context_value:
+        elif behavior.context_value is not MISSING:
             return_value = ContextWrapper(behavior.context_value)
 
         else:

decoy/stub_store.py

@@ -1,14 +1,26 @@
 """Stub creation and storage."""
-from typing import Any, Callable, List, NamedTuple, Optional
+from typing import Any, Callable, List, NamedTuple, Optional, Union
 
 from .spy_events import SpyEvent, WhenRehearsal, match_event
 
 
+class _MISSING:
+    pass
+
+
+MISSING = _MISSING()
+"""Value not specified sentinel.
+
+Used when `None` could be a valid value,
+so `Optional` would be inappropriate.
+"""
+
+
 class StubBehavior(NamedTuple):
     """A recorded stub behavior."""
 
     return_value: Optional[Any] = None
-    context_value: Optional[Any] = None
+    context_value: Union[_MISSING, Any] = MISSING
     error: Optional[Exception] = None
     action: Optional[Callable[..., Any]] = None
     once: bool = False

tests/test_call_handler.py

@@ -145,9 +145,11 @@ def test_handle_call_with_context_enter(
     stub_store: StubStore,
     subject: CallHandler,
 ) -> None:
-    """It return a Stub's configured context value."""
+    """It should return a Stub's configured context value."""
     spy_call = SpyEvent(
-        spy_id=42, spy_name="spy_name", payload=SpyCall(args=(), kwargs={})
+        spy_id=42,
+        spy_name="spy_name",
+        payload=SpyCall(args=(), kwargs={}),
     )
     behavior = StubBehavior(context_value="hello world")
 
@@ -157,3 +159,25 @@ def test_handle_call_with_context_enter(
         assert result == "hello world"
 
     decoy.verify(spy_log.push(spy_call))
+
+
+def test_handle_call_with_context_enter_none(
+    decoy: Decoy,
+    spy_log: SpyLog,
+    stub_store: StubStore,
+    subject: CallHandler,
+) -> None:
+    """It should allow a configured context value to be None."""
+    spy_call = SpyEvent(
+        spy_id=42,
+        spy_name="spy_name",
+        payload=SpyCall(args=(), kwargs={}),
+    )
+    behavior = StubBehavior(context_value=None)
+
+    decoy.when(stub_store.get_by_call(spy_call)).then_return(behavior)
+
+    with subject.handle(spy_call).value as result:  # type: ignore[union-attr]
+        assert result is None
+
+    decoy.verify(spy_log.push(spy_call))