Speed up startup: defer heavy imports and cut redundant class setup work

tests/test_traitlets.py

@@ -937,6 +937,32 @@ class A(HasTraits):
         traits = a.traits(config_key=lambda v: True)
         self.assertEqual(traits, dict(i=A.i, f=A.f, j=A.j))
 
+    def test_traits_metadata_filter_caching(self):
+        # metadata-filtered class_traits()/traits() results are memoized per
+        # class; make sure the cache preserves the "fresh dict" contract and is
+        # invalidated when metadata is mutated after class creation.
+        class A(HasTraits):
+            i = Int().tag(config=True)
+            j = Int()
+
+        # returned dict is a fresh copy the caller may mutate freely
+        first = A.class_traits(config=True)
+        self.assertEqual(first, dict(i=A.i))
+        first["injected"] = "oops"
+        self.assertEqual(A.class_traits(config=True), dict(i=A.i))
+
+        # tagging a trait after the result was cached must be reflected
+        A.j.tag(config=True)
+        self.assertEqual(A.class_traits(config=True), dict(i=A.i, j=A.j))
+        self.assertEqual(A().traits(config=True), dict(i=A.i, j=A.j))
+
+        # a subclass has its own cache and does not pollute the parent's
+        class B(A):
+            k = Int().tag(config=True)
+
+        self.assertEqual(B.class_traits(config=True), dict(i=A.i, j=A.j, k=B.k))
+        self.assertEqual(A.class_traits(config=True), dict(i=A.i, j=A.j))
+
     def test_traits_metadata_deprecated(self):
         with expected_warnings([r"metadata should be set using the \.tag\(\) method"] * 2):
 

traitlets/config/application.py

@@ -5,17 +5,14 @@
 from __future__ import annotations
 
 import functools
-import json
 import logging
 import os
-import pprint
 import re
 import sys
 import typing as t
 from collections import OrderedDict, defaultdict
 from contextlib import suppress
 from copy import deepcopy
-from logging.config import dictConfig
 from textwrap import dedent
 
 from traitlets.config.configurable import Configurable, SingletonConfigurable
@@ -286,6 +283,8 @@ def _observe_logging_default(self, change: Bunch) -> None:
         self._configure_logging()
 
     def _configure_logging(self) -> None:
+        from logging.config import dictConfig
+
         config = self.get_default_logging_config()
         nested_update(config, self.logging_config or {})
         dictConfig(config)
@@ -483,6 +482,8 @@ def start_show_config(self) -> None:
                 cls_config.pop("show_config_json", None)
 
         if self.show_config_json:
+            import json
+
             json.dump(config, sys.stdout, indent=1, sort_keys=True, default=repr)
             # add trailing newline
             sys.stdout.write("\n")
@@ -498,6 +499,8 @@ def start_show_config(self) -> None:
             class_config = config[classname]
             if not class_config:
                 continue
+            import pprint
+
             print(classname)
             pformat_kwargs: StrDict = dict(indent=4, compact=True)  # noqa: C408
 
@@ -929,6 +932,8 @@ def _load_config_files(
                     if log:
                         log.debug("Loaded config file: %s", loader.full_filename)
                 if config:
+                    import json
+
                     for filename, earlier_config in zip(filenames, loaded, strict=True):
                         collisions = earlier_config.collisions(config)
                         if collisions and log:

traitlets/config/configurable.py

@@ -171,12 +171,15 @@ def _load_config(
     ) -> None:
         """load traits from a Config object"""
 
+        my_config = self._find_my_config(cfg)
+        if not my_config:
+            # Nothing in the config applies to this instance; avoid the cost of
+            # computing traits() and entering hold_trait_notifications() for the
+            # many leaf Configurables that carry no matching config.
+            return
+
         if traits is None:
             traits = self.traits(config=True)
-        if section_names is None:
-            section_names = self.section_names()
-
-        my_config = self._find_my_config(cfg)
 
         # hold trait notifications until after all config has been loaded
         with self.hold_trait_notifications():

traitlets/config/loader.py

@@ -7,7 +7,6 @@
 import argparse
 import copy
 import functools
-import json
 import os
 import re
 import sys
@@ -565,6 +564,8 @@ def load_config(self) -> Config:
         return self.config
 
     def _read_file_as_dict(self) -> dict[str, t.Any]:
+        import json
+
         with open(self.full_filename) as f:
             return t.cast("dict[str, t.Any]", json.load(f))
 
@@ -591,6 +592,8 @@ def __exit__(self, exc_type: object, exc_value: object, traceback: object) -> No
         configuration to disk.
         """
         self.config.version = 1
+        import json
+
         json_config = json.dumps(self.config, indent=2)
         with open(self.full_filename, "w") as f:
             f.write(json_config)