manager: ignore register(None) instead of raising TypeError

changelog/731.bugfix.rst

@@ -1 +1 @@
-:func:`PluginManager.register(None) <pluggy.PluginManager.register>` is now rejected early with a ``TypeError`` instead of having some weird behaviors.
+:func:`PluginManager.register(None) <pluggy.PluginManager.register>` is now ignored instead of having some weird behaviors.

src/pluggy/_manager.py

@@ -197,7 +197,10 @@ def register(self, plugin: _Plugin, name: str | None = None) -> str | None:
         If the plugin is already registered, raises a :exc:`ValueError`.
         """
         if plugin is None:
-            raise TypeError("plugin must not be None")
+            # Registering a `None` plugin causes problems (#731). However, some
+            # users are doing it, so can't error out (#749). Just ignore
+            # instead.
+            return None
 
         plugin_name = name or self.get_canonical_name(plugin)
 

testing/test_pluginmanager.py

@@ -29,10 +29,9 @@ def test_plugin_double_register(pm: PluginManager) -> None:
         pm.register(42, name="def")
 
 
-def test_register_rejects_none(pm: PluginManager) -> None:
-    """``None`` collides with the blocked-name sentinel and must be rejected."""
-    with pytest.raises(TypeError, match="plugin must not be None"):
-        pm.register(None)
+def test_register_ignores_none(pm: PluginManager) -> None:
+    """``None`` collides with the blocked-name sentinel and must be ignored."""
+    assert pm.register(None) is None
     assert pm.get_plugins() == set()
     assert not pm.is_registered(None)