Implement MockChecker

Note on message ids:

When using an already existing message-id, pylint will raise an error.
The message-ids of custom checkers appear to be starting at 9999
counting backwards, so I just took the next available id.

For reference: I looked at
`scripts/dev/pylint_checkers/qute_pylint/config.py::ConfigChecker`,
whose id is 9998.

scripts/dev/pylint_checkers/qute_pylint/mock.py

@@ -0,0 +1,81 @@
+# vim: ft=python fileencoding=utf-8 sts=4 sw=4 et:
+
+# Copyright 2014-2021 Florian Bruhin (The Compiler) <mail@qutebrowser.org>
+#
+# This file is part of qutebrowser.
+#
+# qutebrowser 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.
+#
+# qutebrowser 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 qutebrowser.  If not, see <https://www.gnu.org/licenses/>.
+
+"""Custom astroid checker for mock calls."""
+
+from qutebrowser.utils import utils
+
+import astroid
+from pylint import interfaces, checkers
+
+
+class MockChecker(checkers.BaseChecker):
+
+    """Custom astroid checker for mock calls."""
+
+    __implements__ = interfaces.IAstroidChecker
+    name = 'mock'
+    msgs = {
+        'E9997': ('Mock/MagicMock requires the spec argument',  # flake8: disable=S001
+                  'mock-requires-spec',
+                  None),
+    }
+    priority = -1
+    printed_warning = False
+
+    def _is_mock_call(self, node):
+        """Check if node is a mock call.
+
+        Args:
+            node: the currently visited node
+
+        Returns:
+            True if `node` is a mock call
+        """
+        if isinstance(node.func, astroid.nodes.node_classes.Attribute):
+            # namespaced imports, e.g.
+            # >>> import unittest.mock
+            # >>> unittest.mock.Mock()
+            return node.func.attrname in ('Mock', 'MagicMock')
+        elif isinstance(node.func, astroid.nodes.node_classes.Name):
+            # imports without namespace, e.g.
+            # >>> from unittest.mock import Mock
+            # >>> Mock()
+            return node.func.name in ('Mock', 'MagicMock')
+
+        raise utils.Unreachable()
+
+    def visit_call(self, node: astroid.nodes.Call):
+        """Emit a message for mock calls without a spec argument.
+
+        Args:
+            node: the currently visited node
+        """
+        if not self._is_mock_call(node):
+            return
+
+        if any(kw.arg == 'spec' for kw in node.keywords):
+            return
+
+        self.add_message('mock-requires-spec', node=node)
+
+
+def register(linter):
+    """Register this checker."""
+    linter.register_checker(MockChecker(linter))

tests/unit/scripts/test_pylint_checkers.py

@@ -0,0 +1,80 @@
+# vim: ft=python fileencoding=utf-8 sts=4 sw=4 et:
+
+# Copyright 2015-2021 Florian Bruhin (The Compiler) <mail@qutebrowser.org>
+#
+# This file is part of qutebrowser.
+#
+# qutebrowser 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.
+#
+# qutebrowser 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 qutebrowser.  If not, see <https://www.gnu.org/licenses/>.
+
+"""Tests for custom pylint checkers."""
+
+
+import astroid
+import pylint.testutils
+import pytest
+
+from scripts.dev.pylint_checkers.qute_pylint.mock import MockChecker
+
+
+class TestMockChecker(pylint.testutils.CheckerTestCase):
+    CHECKER_CLASS = MockChecker
+
+    @pytest.mark.parametrize(
+        'call',
+        [
+            'Mock()',
+            'MagicMock()',
+        ]
+    )
+    @pytest.mark.parametrize(
+        'module_path',
+        [
+            '',
+            'mock.'
+            'unittest.mock.'
+        ]
+    )
+    def test_find_mocks_with_missing_spec(self, module_path, call):
+        call_node = astroid.extract_node(f'{module_path}{call}')
+
+        with self.assertAddsMessages(
+            pylint_testutils.MessageTest(
+                msg_id='mock-requires-spec',
+                node=call_node,
+            ),
+        ):
+            self.checker.visit_call(call_node)
+
+    @pytest.mark.parametrize(
+        'class_',
+        [
+            'Mock(spec=int)',
+            'MagicMock(spec=int)',
+            'mock.Mock(spec=int)',
+            'mock.MagicMock(spec=int)',
+            'unittest.mock.Mock(spec=int)',
+            'unittest.mock.MagicMock(spec=int)',
+        ]
+    )
+    def test_ignores_mocks_with_spec(self, class_):
+        call_node = astroid.extract_node(class_)
+
+        with self.assertNoMessages():
+            self.checker.visit_call(call_node)
+
+    def test_ignores_non_mocks(self):
+        call_node = astroid.extract_node('open()')
+
+        with self.assertNoMessages():
+            self.checker.visit_call(call_node)