Split name-to-backend conversion into its own function and associated…

… exception

assigner/backends/__init__.py

@@ -3,3 +3,17 @@
 from assigner.backends.mock import MockBackend
 
 pyflakes = [BackendBase, RepoError, GitlabBackend, MockBackend]
+
+backend_names = {
+    "gitlab": GitlabBackend,
+    "mock": MockBackend,
+    }
+
+class NoSuchBackend(Exception):
+    pass
+
+def from_name(name: str):
+    try:
+        return backend_names[name]
+    except KeyError:
+        raise NoSuchBackend("Cannot find backend with name {}".format(name))

assigner/backends/decorators.py

@@ -1,6 +1,6 @@
 import logging
 
-from assigner.backends import GitlabBackend, MockBackend
+from assigner.backends import from_name
 from assigner.config import requires_config
 
 
@@ -11,15 +11,8 @@ def requires_config_and_backend(func):
     """Provides a backend depending on configuration."""
     @requires_config
     def wrapper(config, cmdargs, *args, **kwargs):
-        try:
-            config.backend
-        except KeyError:
-            logger.info(
-                "The 'backend' field in config is not set; it will default to Gitlab."
-            )
-            return func(config, GitlabBackend, cmdargs, *args, **kwargs)
 
-        if config.backend["name"] == "gitlab":
-            return func(config, GitlabBackend, cmdargs, *args, **kwargs)
-        return func(config, MockBackend, cmdargs, *args, **kwargs)
+        backend = from_name(config.backend["name"])
+
+        return func(config, backend, cmdargs, *args, **kwargs)
     return wrapper

assigner/tests/commands/get_integration_test.py

@@ -14,7 +14,7 @@ def setUp(self):
             "assigner.commands.get.get_filtered_roster", autospec=True
         )
         self.mock_backend = self._create_patch(
-            "assigner.backends.decorators.GitlabBackend", autospec=True
+            "assigner.backends.GitlabBackend", autospec=True
         )
         self.mock_os = self._create_patch(
             "assigner.commands.get.os", autospec=True
@@ -27,7 +27,7 @@ def setUp(self):
             "path": "somepath/",
             "hw_name": "HW2",
             "section": "SP2017",
-            "student": "",
+            "student": MagicMock(),
             "branch": ["a", "b"],
             "force": MagicMock()
         })