enhancement: add lint rules plugin support

Add plugin support using setuptools (pkg_resources) plugin mechanism to
yamllint to allow users to add their own custom lint rule plugins,
together with an example plugin implementation and test cases.

Signed-off-by: Satoru SATOH <satoru.satoh@gmail.com>

tests/plugins/example/__init__.py

@@ -0,0 +1,26 @@
+# -*- 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/>.
+
+"""yamllint plugin entry point
+"""
+from __future__ import absolute_import
+
+from . import override_comments
+
+
+RULES_MAP = {
+    override_comments.ID: override_comments
+}

tests/plugins/example/override_comments.py

@@ -0,0 +1,63 @@
+#
+# 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/>.
+#
+"""
+Use this rule to override some comments' rules.
+
+.. rubric:: Options
+
+* Use ``forbid`` to control comments. Set to ``true`` to forbid comments
+  completely.
+
+.. rubric:: Examples
+
+#. With ``override-comments: {forbid: true}``
+
+   the following code snippet would **PASS**:
+   ::
+
+    foo: 1
+
+   the following code snippet would **FAIL**:
+   ::
+
+    # baz
+    foo: 1
+
+.. rubric:: Default values (when enabled)
+
+.. code-block:: yaml
+
+rules:
+  override-comments:
+    forbid: False
+
+"""
+from yamllint.linter import LintProblem
+
+
+ID = 'override-comments'
+TYPE = 'comment'
+CONF = {'forbid': bool}
+DEFAULT = {'forbid': False}
+
+
+def check(conf, comment):
+    """Check if comments are found.
+    """
+    if conf['forbid']:
+        yield LintProblem(comment.line_no, comment.column_no,
+                          'forbidden comment')

tests/test_plugins.py

@@ -0,0 +1,80 @@
+# -*- 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
+import warnings
+
+try:
+    from unittest import mock
+except ImportError:  # for python 2.7
+    mock = False
+
+from tests.plugins import example
+
+import yamllint.plugins
+
+
+class FakeEntryPoint(object):
+    """Fake object to mimic pkg_resources.EntryPoint.
+    """
+    RULES_MAP = example.RULES_MAP
+
+    def load(self):
+        """Fake method to return self.
+        """
+        return self
+
+
+class BrokenEntryPoint(FakeEntryPoint):
+    """Fake object to mimic load failure of pkg_resources.EntryPoint.
+    """
+    def load(self):
+        raise ImportError("This entry point should fail always!")
+
+
+class PluginFunctionsTestCase(unittest.TestCase):
+
+    def test_validate_rule_module(self):
+        fun = yamllint.plugins.validate_rule_module
+        rule_mod = example.override_comments
+
+        self.assertFalse(fun(object()))
+        self.assertTrue(fun(rule_mod))
+
+    @unittest.skipIf(not mock, "unittest.mock is not available")
+    def test_validate_rule_module_using_mock(self):
+        fun = yamllint.plugins.validate_rule_module
+        rule_mod = example.override_comments
+
+        with mock.patch.object(rule_mod, "ID", False):
+            self.assertFalse(fun(rule_mod))
+
+        with mock.patch.object(rule_mod, "TYPE", False):
+            self.assertFalse(fun(rule_mod))
+
+        with mock.patch.object(rule_mod, "check", True):
+            self.assertFalse(fun(rule_mod))
+
+    def test_load_plugin_rules_itr(self):
+        fun = yamllint.plugins.load_plugin_rules_itr
+
+        self.assertEqual(list(fun([])), [])
+        self.assertEqual(sorted(fun([FakeEntryPoint(),
+                                     FakeEntryPoint()])),
+                         sorted(FakeEntryPoint.RULES_MAP.items()))
+
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore")
+            self.assertEqual(list(fun([BrokenEntryPoint()])), [])

yamllint/plugins.py

@@ -0,0 +1,61 @@
+# -*- 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/>.
+"""
+Plugin module utilizing setuptools (pkg_resources) to allow users to add their
+own custom lint rules.
+"""
+import warnings
+
+import pkg_resources
+
+
+PACKAGE_GROUP = "yamllint.plugins.rules"
+
+
+def validate_rule_module(rule_mod):
+    """Test if given rule module is valid.
+    """
+    return (getattr(rule_mod, "ID", False) and
+            getattr(rule_mod, "TYPE", False)
+            ) and callable(getattr(rule_mod, "check", False))
+
+
+def load_plugin_rules_itr(entry_points=None, group=PACKAGE_GROUP):
+    """Load custom lint rule plugins."""
+    if not entry_points:
+        entry_points = pkg_resources.iter_entry_points(group)
+
+    rule_ids = set()
+    for entry in entry_points:
+        try:
+            rules = entry.load()
+            for rule_id, rule_mod in rules.RULES_MAP.items():
+                if rule_id in rule_ids or not validate_rule_module(rule_mod):
+                    continue
+
+                yield (rule_id, rule_mod)
+                rule_ids.add(rule_id)
+
+        # pkg_resources.EntryPoint.resolve may throw ImportError.
+        except (AttributeError, ImportError):
+            warnings.warn("Could not load the plugin: {}".format(entry),
+                          RuntimeWarning)
+
+
+def get_plugin_rules_map():
+    """Get a mappings of plugin rule's IDs and rules."""
+    return dict((rule_id, rule_mod)
+                for rule_id, rule_mod in load_plugin_rules_itr())