pattern: Fix and improve config overriding in from_configfile method

Fix error when a configuration value loaded from a config file is also
given as keyword parameter to the from_configfile method.

Override configuration loaded from config file only if the provided
value is not None.

swh/lister/pattern.py

@@ -251,13 +251,15 @@ class Lister(Generic[StateType, PageType]):
     @classmethod
     def from_configfile(cls, **kwargs: Any):
         """Instantiate a lister from the configuration loaded from the
-        SWH_CONFIG_FILENAME envvar, with potential extra keyword arguments.
+        SWH_CONFIG_FILENAME envvar, with potential extra keyword arguments
+        if their value is not None.
 
         Args:
             kwargs: kwargs passed to the lister instantiation
         """
-        config = load_from_envvar()
-        return cls.from_config(**config, **kwargs)
+        config = dict(load_from_envvar())
+        config.update({k: v for k, v in kwargs.items() if v is not None})
+        return cls.from_config(**config)
 
 
 class StatelessLister(Lister[None, PageType], Generic[PageType]):

swh/lister/tests/test_pattern.py

@@ -39,6 +39,29 @@ def test_instantiation(swh_scheduler):
         lister.run()
 
 
+def test_instantiation_from_configfile(swh_scheduler, mocker):
+    mock_load_from_envvar = mocker.patch("swh.lister.pattern.load_from_envvar")
+    mock_get_scheduler = mocker.patch("swh.lister.pattern.get_scheduler")
+    mock_load_from_envvar.return_value = {
+        "scheduler": {},
+        "url": "foo",
+        "instance": "bar",
+    }
+    mock_get_scheduler.return_value = swh_scheduler
+
+    lister = InstantiableLister.from_configfile()
+    assert lister.url == "foo"
+    assert lister.instance == "bar"
+
+    lister = InstantiableLister.from_configfile(url="bar", instance="foo")
+    assert lister.url == "bar"
+    assert lister.instance == "foo"
+
+    lister = InstantiableLister.from_configfile(url=None, instance="foo")
+    assert lister.url == "foo"
+    assert lister.instance == "foo"
+
+
 if TYPE_CHECKING:
     _Base = pattern.Lister[Any, PageType]
 else: