[backend] Set default category if none is provided

The default category is now set to the first category listed in the backend's
`CATEGORIES` list. This change is necessary to accommodate the removal of
backend-specific `fetch` methods, which previously handled category assignment.

Updated the tests to validate the default and specific category assignments,
and removed the obsolete `test_fetch_archive_needs_category`.

Signed-off-by: Venu Vardhan Reddy Tekula <vt2182@nyu.edu>

perceval/backend.py

@@ -688,9 +688,9 @@ def parse(self, *args):
         """
         parsed_args = self.parser.parse_args(args)
 
-        # Category was not set, remove it
+        # Ensure category is set
         if parsed_args.category is None:
-            delattr(parsed_args, 'category')
+            parsed_args.category = self._backend.CATEGORIES[0]
 
         if self._from_date:
             parsed_args.from_date = str_to_datetime(parsed_args.from_date)

tests/test_backend.py

@@ -1093,26 +1093,19 @@ def test_incompatible_fetch_archive_and_no_archive(self):
         with self.assertRaises(AttributeError):
             _ = parser.parse(*args)
 
-    def test_fetch_archive_needs_category(self):
-        """Test if fetch-archive needs a category"""
-
-        args = ['--fetch-archive']
-        parser = BackendCommandArgumentParser(MockedBackendCommand.BACKEND,
-                                              archive=True)
-
-        with self.assertRaises(AttributeError):
-            _ = parser.parse(*args)
-
-    def test_remove_empty_category(self):
-        """Test whether category argument is removed when no value is given"""
+    def test_default_category(self):
+        """Test whether a default category is set if none is provided"""
 
+        # No category is provided
         args = []
         parser = BackendCommandArgumentParser(MockedBackendCommand.BACKEND,
                                               archive=True)
         parsed_args = parser.parse(*args)
 
-        with self.assertRaises(AttributeError):
-            _ = parsed_args.category
+        self.assertEqual(parsed_args.category, MockedBackendCommand.BACKEND.DEFAULT_CATEGORY)
+
+    def test_specific_category(self):
+        """Test whether a specific category is set when provided"""
 
         # An empty string is parsed
         args = ['--category', '']