fix: lookup_default returns None instead of leaking internal sentinel

src/click/core.py

@@ -685,6 +685,21 @@ def ensure_object(self, object_type: type[V]) -> V:
             self.obj = rv = object_type()
         return rv
 
+    def _lookup_default(self, name: str, call: bool = True) -> t.Any:
+        """Internal version of :meth:`lookup_default` that returns
+        :data:`UNSET` when no value is found, for use by internal
+        methods that need to distinguish missing from ``None``.
+        """
+        if self.default_map is not None:
+            value = self.default_map.get(name, UNSET)
+
+            if call and callable(value):
+                return value()
+
+            return value
+
+        return UNSET
+
     @t.overload
     def lookup_default(
         self, name: str, call: t.Literal[True] = True
@@ -705,15 +720,12 @@ def lookup_default(self, name: str, call: bool = True) -> t.Any | None:
         .. versionchanged:: 8.0
             Added the ``call`` parameter.
         """
-        if self.default_map is not None:
-            value = self.default_map.get(name, UNSET)
-
-            if call and callable(value):
-                return value()
+        value = self._lookup_default(name, call=call)
 
-            return value
+        if value is UNSET:
+            return None
 
-        return UNSET
+        return value
 
     def fail(self, message: str) -> t.NoReturn:
         """Aborts the execution of the program with a specific error
@@ -2278,7 +2290,7 @@ def get_default(
         .. versionchanged:: 8.0
             Added the ``call`` parameter.
         """
-        value = ctx.lookup_default(self.name, call=False)  # type: ignore
+        value = ctx._lookup_default(self.name, call=False)
 
         if value is UNSET:
             value = self.default
@@ -2321,7 +2333,7 @@ def consume_value(
                 source = ParameterSource.ENVIRONMENT
 
         if value is UNSET:
-            default_map_value = ctx.lookup_default(self.name)  # type: ignore
+            default_map_value = ctx._lookup_default(self.name)
             if default_map_value is not UNSET:
                 value = default_map_value
                 source = ParameterSource.DEFAULT_MAP

tests/test_context.py

@@ -780,3 +780,20 @@ def test_propagate_opt_prefixes():
     ctx = click.Context(click.Command("test2"), parent=parent)
 
     assert ctx._opt_prefixes == {"-", "--", "!"}
+
+
+def test_lookup_default_returns_none_when_missing():
+    """lookup_default should return None, not an internal sentinel,
+    when the key is not in default_map or default_map is None."""
+    cmd = click.Command("test")
+    ctx = click.Context(cmd)
+
+    # No default_map at all
+    assert ctx.lookup_default("missing") is None
+
+    # default_map exists but key is missing
+    ctx.default_map = {"other": "value"}
+    assert ctx.lookup_default("missing") is None
+
+    # default_map has the key
+    assert ctx.lookup_default("other") == "value"