enhancement: enable rules' plugin support

Enable rules' plugin support and add its test cases.

tests/test_rules.py

@@ -0,0 +1,64 @@
+# -*- coding: utf-8 -*-
+# Copyright (C) 2020 Satoru SATOH
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+import unittest
+
+try:
+    from unittest import mock
+except ImportError:  # for python 2.7
+    mock = False
+
+from tests.plugins import example
+
+import yamllint.rules
+
+
+RULE_NEVER_EXISTS = "rule_never_exists"
+PLUGIN_RULES = example.RULES_MAP
+
+
+class TestCase(unittest.TestCase):
+    """Test cases for yamllint.rules.__init__.*.
+    """
+    def test_get_default_rule(self):
+        self.assertEqual(yamllint.rules.get(yamllint.rules.braces.ID),
+                         yamllint.rules.braces)
+
+    def test_get_rule_does_not_exist(self):
+        with self.assertRaises(ValueError):
+            yamllint.rules.get(RULE_NEVER_EXISTS)
+
+
+@unittest.skipIf(not mock, "unittest.mock is not available")
+class TestCaseUsingMock(unittest.TestCase):
+    """Test cases for yamllint.rules.__init__.* using mock.
+    """
+    def test_get_default_rule_with_plugins(self):
+        with mock.patch.dict(yamllint.rules._EXTERNAL_RULES, PLUGIN_RULES):
+            self.assertEqual(yamllint.rules.get(yamllint.rules.braces.ID),
+                             yamllint.rules.braces)
+
+    def test_get_plugin_rules(self):
+        plugin_rule_id = example.override_comments.ID
+        plugin_rule_mod = example.override_comments
+
+        with mock.patch.dict(yamllint.rules._EXTERNAL_RULES, PLUGIN_RULES):
+            self.assertEqual(yamllint.rules.get(plugin_rule_id),
+                             plugin_rule_mod)
+
+    def test_get_rule_does_not_exist_with_plugins(self):
+        with mock.patch.dict(yamllint.rules._EXTERNAL_RULES, PLUGIN_RULES):
+            with self.assertRaises(ValueError):
+                yamllint.rules.get(RULE_NEVER_EXISTS)

yamllint/rules/__init__.py

@@ -14,6 +14,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import yamllint.plugins
 from yamllint.rules import (
     braces,
     brackets,
@@ -62,9 +63,14 @@ _RULES = {
     truthy.ID: truthy,
 }
 
+_EXTERNAL_RULES = yamllint.plugins.get_plugin_rules_map()
 
-def get(id):
-    if id not in _RULES:
-        raise ValueError('no such rule: "%s"' % id)
 
-    return _RULES[id]
+def get(rule_id):
+    if rule_id in _RULES:
+        return _RULES[rule_id]
+
+    if rule_id in _EXTERNAL_RULES:
+        return _EXTERNAL_RULES[rule_id]
+
+    raise ValueError('no such rule: "%s"' % rule_id)