Lazily import tomlkit in arguments_manager

Author: Pierre-Sassoulas

doc/whatsnew/fragments/2866.performance

@@ -1,6 +1,6 @@
-Lazily import ``dill`` in ``pylint.lint.parallel`` and ``isort`` in
-``pylint.checkers.imports`` so they are only loaded when actually needed.
-``dill`` (~13 ms) is now deferred until parallel checking runs, and
-``isort`` (~60 ms) until import-ordering messages are checked.
+Lazily import ``isort``, ``dill``, ``multiprocessing``/``concurrent.futures``,
+and ``tomlkit`` so they are only loaded when actually needed.
+This reduces startup time by ~25% (e.g. ``--version``: 91 => 67 ms,
+``--help``: 176 => 133 ms, single-file lint: 272 => 226 ms).
 
 Closes #2866

pylint/config/arguments_manager.py

@@ -14,8 +14,6 @@
 from collections.abc import Sequence
 from typing import TYPE_CHECKING, Any, TextIO
 
-import tomlkit
-
 from pylint import utils
 from pylint.config.argument import (
     _Argument,
@@ -302,6 +300,8 @@ def _generate_config_file(self, *, minimal: bool = False) -> str:
         """Write a configuration file according to the current configuration into
         stdout.
         """
+        import tomlkit  # pylint: disable=import-outside-toplevel
+
         toml_doc = tomlkit.document()
         tool_table = tomlkit.table(is_super_table=True)
         toml_doc.add(tomlkit.key("tool"), tool_table)