Add a command to list enabled/disabled messages

When enabling/disabling several messages and groups in a config
file, it can be unclear which messages are actually enabled and
which are disabled. This new command produces the final resolved
lists of enabled/disabled messages, sorted by symbol but with the
ID provided for use with `--help-msg`.

This commit includes a test, documentation, and adding myself to
the contributors list, as this is my first contribution.

6 files changed, 77 insertions, 6 deletions

diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 4279fbaec..b46b2d0e8 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -326,3 +326,6 @@ contributors:
 * Robert Schweizer: contributor
 
 * Hugo van Kemenade: contributor
+
+* Trevor Bekolay: contributor
+    - Added --list-msgs-enabled command
diff --git a/ChangeLog b/ChangeLog
index 5f16e8f7e..432dd90be 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -9,7 +9,7 @@ Release date: TBA
 
 * Added a new check, ``consider-using-sys-exit``
 
-  This check is emitted when we detect that a quit() or exit() is invoked 
+  This check is emitted when we detect that a quit() or exit() is invoked
   instead of sys.exit(), which is the preferred way of exiting in program.
 
   Close #2925
@@ -263,6 +263,7 @@ must rename the associated identification.
 
 Closes #2395
 
+* Added ``--list-msgs-enabled`` command to list all enabled and disabled messages given the current RC file and command line arguments.
 
 What's New in Pylint 2.3.0?
 ===========================
diff --git a/doc/user_guide/run.rst b/doc/user_guide/run.rst
index ca7e3f16b..338a07064 100644
--- a/doc/user_guide/run.rst
+++ b/doc/user_guide/run.rst
@@ -119,6 +119,8 @@ Other useful global options include:
 --output-format=<format>   Select output format (text, json, custom).
 --msg-template=<template>  Modify text output message template.
 --list-msgs                Generate pylint's messages.
+--list-msgs-enabled        Display a list of what messages are enabled and
+                           disabled with the given configuration.
 --full-documentation       Generate pylint's full documentation, in reST
                              format.
 
diff --git a/doc/whatsnew/2.4.rst b/doc/whatsnew/2.4.rst
index 15fe03361..e5e5d86af 100644
--- a/doc/whatsnew/2.4.rst
+++ b/doc/whatsnew/2.4.rst
@@ -15,11 +15,11 @@ New checkers
 
 * Added a new check, ``consider-using-sys-exit``
 
-  This check is emitted when we detect that a quit() or exit() is invoked 
+  This check is emitted when we detect that a quit() or exit() is invoked
   instead of sys.exit(), which is the preferred way of exiting in program.
 
   Close #2925
-  
+
 * Added a new check, ``arguments-out-of-order``
 
   This check warns if you have arguments with names that match those in
@@ -125,7 +125,7 @@ New checkers
 Other Changes
 =============
 
-* Don't emit ``protected-access`` when a single underscore prefixed attribute is used 
+* Don't emit ``protected-access`` when a single underscore prefixed attribute is used
   inside a special method
 
   Close #1802
@@ -208,3 +208,10 @@ would mean that `no-value-for-parameter` would not be raised for::
 to control which logging string format style is valid.
 To allow this, a new `fstr` value is valid for the `logging-format-style`
 option.
+
+* ``--list-msgs-enabled`` command was added.
+
+When enabling/disabling several messages and groups in a config file,
+it can be unclear which messages are actually enabled and which are disabled.
+This new command produces the final resolved lists of enabled/disabled messages,
+sorted by symbol but with the ID provided for use with ``--help-msg``.
diff --git a/pylint/lint.py b/pylint/lint.py
index a43c4b95a..4d8e87d83 100644
--- a/pylint/lint.py
+++ b/pylint/lint.py
@@ -820,6 +820,25 @@ class PyLinter(
             self.global_set_option("disable", value)
         self._python3_porting_mode = True
 
+    def list_messages_enabled(self):
+        enabled = [
+            "  %s (%s)" % (message.symbol, message.msgid)
+            for message in self.msgs_store.messages
+            if self.is_message_enabled(message.msgid)
+        ]
+        disabled = [
+            "  %s (%s)" % (message.symbol, message.msgid)
+            for message in self.msgs_store.messages
+            if not self.is_message_enabled(message.msgid)
+        ]
+        print("Enabled messages:")
+        for msg in sorted(enabled):
+            print(msg)
+        print("\nDisabled messages:")
+        for msg in sorted(disabled):
+            print(msg)
+        print("")
+
     # block level option handling #############################################
     #
     # see func_block_disable_msg.py test case for expected behaviour
@@ -1514,6 +1533,18 @@ group are mutually exclusive.",
                     },
                 ),
                 (
+                    "list-msgs-enabled",
+                    {
+                        "action": "callback",
+                        "metavar": "<msg-id>",
+                        "callback": self.cb_list_messages_enabled,
+                        "group": "Commands",
+                        "level": 1,
+                        "help": "Display a list of what messages are enabled "
+                        "and disabled with the given configuration.",
+                    },
+                ),
+                (
                     "list-groups",
                     {
                         "action": "callback",
@@ -1750,6 +1781,11 @@ group are mutually exclusive.",
         self.linter.msgs_store.list_messages()
         sys.exit(0)
 
+    def cb_list_messages_enabled(self, option, optname, value, parser):
+        """optik callback for printing available messages"""
+        self.linter.list_messages_enabled()
+        sys.exit(0)
+
     def cb_list_groups(self, *args, **kwargs):
         """List all the check groups that pylint knows about
 
diff --git a/tests/unittest_lint.py b/tests/unittest_lint.py
index e27fa172b..4f942cd02 100644
--- a/tests/unittest_lint.py
+++ b/tests/unittest_lint.py
@@ -596,6 +596,28 @@ def test_full_documentation(linter):
         assert re.search(regexp, output)
 
 
+def test_list_msgs_enabled(init_linter, capsys):
+    linter = init_linter
+    linter.enable("W0101", scope="package")
+    linter.disable("W0102", scope="package")
+    linter.list_messages_enabled()
+
+    lines = capsys.readouterr().out.splitlines()
+
+    assert "Enabled messages:" in lines
+    assert "  unreachable (W0101)" in lines
+
+    assert "Disabled messages:" in lines
+    disabled_ix = lines.index("Disabled messages:")
+
+    # W0101 should be in the enabled section
+    assert lines.index("  unreachable (W0101)") < disabled_ix
+
+    assert "  dangerous-default-value (W0102)" in lines
+    # W0102 should be in the disabled section
+    assert lines.index("  dangerous-default-value (W0102)") > disabled_ix
+
+
 @pytest.fixture
 def pop_pylintrc():
     os.environ.pop("PYLINTRC", None)
@@ -776,7 +798,7 @@ def test_by_module_statement_value(init_linter):
     linter = init_linter
     linter.check(os.path.join(os.path.dirname(__file__), "data"))
 
-    for module, module_stats in linter.stats['by_module'].items():
+    for module, module_stats in linter.stats["by_module"].items():
 
         linter2 = init_linter
         if module == "data":
@@ -786,4 +808,4 @@ def test_by_module_statement_value(init_linter):
 
         # Check that the by_module "statement" is equal to the global "statement"
         # computed for that module
-        assert module_stats['statement'] == linter2.stats['statement']
+        assert module_stats["statement"] == linter2.stats["statement"]