Performance: decrease runtime overhead for constructing HasTraits (up to 20x faster) (#777)

* perf: find default generators at class contruction time

instead of at runtime, which gives a runtime overhead.

* perf: find descriptors at class construction time

* perf: only add notifiers when they exist

* perf: avoid cross_validation_lock context manager

this has a runtime overhead (about 20% faster without it)

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* perf: get app traits during class construction instead of runtime

* typing

* perf: avoid dynamic default code path for static immutable defaults

* typing

* perf: avoid unnecessary dict presence checks

* more trivial immutable static initial values

* perf: better scaling due to skipping instance_init when possible.

* perf: specialized version of hold_trait_notifications in ctor

* perf: skip setup if no kwargs are given

* fix: validate return value should be used

* flake8: unused variable

* perf: opt out instance_init for bool and enum

* perf: all instances that are None are immutable and static

* perf: remove usage of named attribute accesses on bunches

* trigger ci

* trigger ci

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Nate Rush <naterush1997@gmail.com>

Author: 3 people

traitlets/tests/test_traitlets.py

@@ -123,6 +123,9 @@ class A(HasTraitsStub):
         a.tt = 10  # type:ignore
         self.assertEqual(a.tt, -1)
 
+        a = A(tt=11)
+        self.assertEqual(a.tt, -1)
+
     def test_default_validate(self):
         class MyIntTT(TraitType):
             def validate(self, obj, value):
@@ -2035,6 +2038,23 @@ def check_valid(self, proposal):
         with self.assertRaises(TraitError):
             u.even = 3  # Trait Error
 
+    def test_validate_used(self):
+        """Verify that the validate value is being used"""
+
+        class FixedValue(HasTraits):
+            value = Int(0)
+
+            @validate("value")
+            def _value_validate(self, proposal):
+                return -1
+
+        u = FixedValue(value=2)
+        assert u.value == -1
+
+        u = FixedValue()
+        u.value = 3
+        assert u.value == -1
+
 
 class TestLink(TestCase):
     def test_connect_same(self):