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.

Also add some plugin support test cases, an example plugin as a
reference, and doc section about how to develop rules' plugins.

Signed-off-by: Satoru SATOH <satoru.satoh@gmail.com>
Co-authored-by: Adrien Vergé

tests/yamllint_plugin_example/README.rst

@@ -0,0 +1,61 @@
+yamllint plugin example
+=======================
+
+This is a yamllint plugin example as a reference, contains the following rules.
+
+- ``forbid-comments`` to forbid comments
+- ``random-failure`` to fail randomly
+
+To enable thes rules in yamllint, you must add them to your `yamllint config
+file <https://yamllint.readthedocs.io/en/stable/configuration.html>`_:
+
+.. code-block:: yaml
+
+ extends: default
+
+ rules:
+   forbid-comments: enable
+   random-failure: enable
+
+How to develop rule plugins
+---------------------------
+
+yamllint rule plugins must satisfy the followings.
+
+#. It must be a Python package installable using pip and distributed under
+   GPLv3+ same as yamllint.
+
+   How to make a Python package is beyond the scope of this README file. Please
+   refer to the official guide (`Python Packaging User Guide
+   <https://packaging.python.org/>`_ ) and related documents.
+
+#. It must contains the entry point configuration in ``setup.cfg`` or something
+   similar packaging configuration files, to make it installed and working as a
+   yamllint plugin like below. (``<plugin_name>`` is that plugin name and
+   ``<plugin_src_dir>`` is a dir where the rule modules exist.)
+   ::
+
+     [options.entry_points]
+     yamllint.plugins.rules =
+          <plugin_name> = <plugin_src_dir>
+
+#. It must contain custom yamllint rule modules:
+
+   - Each rule module must define a couple of global variables, ``ID`` and
+     ``TYPE``. ``ID`` must not conflicts with other rules' IDs.
+   - Each rule module must define a function named 'check' to test input data
+     complies with the rule.
+   - Each rule module may have other global variables.
+     - ``CONF`` to define its configuration parameters and those types.
+     - ``DEFAULT`` to provide default values for each configuration parameters.
+
+#. It must define a global variable ``RULES`` to provide an iterable object, a
+   tuple or a list for example, of tuples of rule ID and rule modules to
+   yamllint like this.
+   ::
+
+     RULES = (
+         # (rule module ID, rule module)
+         (a_custom_rule_module.ID, a_custom_rule_module),
+         (other_custom_rule_module.ID, other_custom_rule_module),
+     )

tests/yamllint_plugin_example/rules/__init__.py

@@ -0,0 +1,30 @@
+# -*- 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 (
+    forbid_comments, no_forty_two, random_failure
+)
+
+
+RULES = (
+    (forbid_comments.ID, forbid_comments),
+    (no_forty_two.ID, no_forty_two),
+    (random_failure.ID, random_failure)
+)

tests/yamllint_plugin_example/rules/forbid_comments.py

@@ -0,0 +1,61 @@
+#
+# 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 forbid comments.
+
+.. rubric:: Options
+
+* Use ``forbid`` to control comments. Set to ``true`` to forbid comments
+  completely.
+
+.. rubric:: Examples
+
+#. With ``forbid-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:
+  forbid-comments:
+    forbid: False
+
+"""
+from yamllint.linter import LintProblem
+
+
+ID = 'forbid-comments'
+TYPE = 'comment'
+CONF = {'forbid': bool}
+DEFAULT = {'forbid': False}
+
+
+def check(conf, comment):
+    if conf['forbid']:
+        yield LintProblem(comment.line_no, comment.column_no,
+                          'forbidden comment')

tests/yamllint_plugin_example/rules/no_forty_two.py

@@ -0,0 +1,49 @@
+#
+# 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 forbid 42 in any values.
+
+.. rubric:: Examples
+
+#. With ``no-forty-two: {}``
+
+   the following code snippet would **PASS**:
+   ::
+
+    the_answer: 1
+
+   the following code snippet would **FAIL**:
+   ::
+
+    the_answer: 42
+"""
+import yaml
+
+from yamllint.linter import LintProblem
+
+
+ID = 'no-forty-two'
+TYPE = 'token'
+
+
+def check(conf, token, prev, next, nextnext, context):
+    if (isinstance(token, yaml.ScalarToken) and
+            isinstance(prev, yaml.ValueToken) and
+            token.value == '42'):
+        yield LintProblem(token.start_mark.line + 1,
+                          token.start_mark.column + 1,
+                          '42 is forbidden value')

tests/yamllint_plugin_example/rules/random_failure.py

@@ -0,0 +1,29 @@
+# -*- coding: utf-8 -*-
+# Copyright (C) 2020 Adrien Vergé
+#
+# 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 random
+
+from yamllint.linter import LintProblem
+
+ID = 'random-failure'
+TYPE = 'token'
+
+
+def check(conf, token, prev, next, nextnext, context):
+    if random.random() > 0.9:
+        yield LintProblem(token.start_mark.line + 1,
+                          token.start_mark.column + 1,
+                          'random failure')

tests/yamllint_plugin_example/setup.cfg

@@ -0,0 +1,11 @@
+[metadata]
+name = yamllint_plugin_example
+version = 1.0.0
+
+[options]
+packages = find:
+install_requires = yamllint
+
+[options.entry_points]
+yamllint.plugins.rules =
+    example = rules

tests/yamllint_plugin_example/setup.py

@@ -0,0 +1,2 @@
+import setuptools
+setuptools.setup()