Test the command class inheritance for each command.

tests/unit/test_commands.py

@@ -1,8 +1,22 @@
 import pytest
 
+from pip._internal.cli.req_command import (
+    IndexGroupCommand,
+    RequirementCommand,
+    SessionCommandMixin,
+)
 from pip._internal.commands import commands_dict, create_command
 
 
+def check_commands(pred, expected):
+    """
+    Check the commands satisfying a predicate.
+    """
+    commands = [create_command(name) for name in sorted(commands_dict)]
+    actual = [command.name for command in commands if pred(command)]
+    assert actual == expected, 'actual: {}'.format(actual)
+
+
 def test_commands_dict__order():
     """
     Check the ordering of commands_dict.
@@ -20,3 +34,43 @@ def test_create_command(name):
     command = create_command(name)
     assert command.name == name
     assert command.summary == commands_dict[name].summary
+
+
+def test_session_commands():
+    """
+    Test which commands inherit from SessionCommandMixin.
+    """
+    def is_session_command(command):
+        return isinstance(command, SessionCommandMixin)
+
+    expected = ['download', 'install', 'list', 'search', 'uninstall', 'wheel']
+    check_commands(is_session_command, expected)
+
+
+def test_index_group_commands():
+    """
+    Test the commands inheriting from IndexGroupCommand.
+    """
+    expected = ['download', 'install', 'list', 'wheel']
+
+    def is_index_group_command(command):
+        return isinstance(command, IndexGroupCommand)
+
+    check_commands(is_index_group_command, expected)
+
+    # Also check that the commands inheriting from IndexGroupCommand are
+    # exactly the commands with the --no-index option.
+    def has_option_no_index(command):
+        return command.parser.has_option('--no-index')
+
+    check_commands(has_option_no_index, expected)
+
+
+def test_requirement_commands():
+    """
+    Test which commands inherit from RequirementCommand.
+    """
+    def is_requirement_command(command):
+        return isinstance(command, RequirementCommand)
+
+    check_commands(is_requirement_command, ['download', 'install', 'wheel'])