config: Report if 'rules' is not a dict

In the current version, the error message states `TypeError: 'NoneType' object
is not iterable` when the 'rules' in the config file is not a dict (gh-274.) It
would be helpful to improve the error message.

Additionally, it is for consistency, as other elements (`ignore`, `locale`,
etc.) have similar error checks.

Author: koyuki7w

tests/test_config.py

@@ -197,6 +197,10 @@ class Rule:
                           {'multiple': ['item4']})
 
     def test_invalid_rule(self):
+        with self.assertRaisesRegex(
+                config.YamlLintConfigError,
+                'invalid config: rules should be a dict'):
+            config.YamlLintConfig('rules:\n')
         with self.assertRaisesRegex(
                 config.YamlLintConfigError,
                 'invalid config: rule "colons": should be either '

yamllint/config.py

@@ -82,6 +82,8 @@ def parse(self, raw_content):
             raise YamlLintConfigError('invalid config: not a dict')
 
         self.rules = conf.get('rules', {})
+        if not isinstance(self.rules, dict):
+            raise YamlLintConfigError('invalid config: rules should be a dict')
         for rule in self.rules:
             if self.rules[rule] == 'enable':
                 self.rules[rule] = {}