Add LazyType trait for deferred class imports

[Carreau]

Summary

Type resolves a string klass — that is, imports it — as soon as the owning HasTraits object is created. For a default like "ipykernel.debugger.Debugger" that means paying for the debugpy import on every kernel startup, even though most sessions never debug.

LazyType subclasses Type and waits until the class is actually needed: reading or writing the trait, or reading klass / default_value off the trait itself.

class A(HasTraits):
    encoder = LazyType("json.JSONEncoder")

a = A()      # json not imported
a.encoder    # now it is

Type itself is untouched.

Key changes

• New LazyType(Type) — overrides instance_init to skip the eager resolution, and makes klass / default_value properties that import on first read and store the result in place.
• TraitType._resolve_lazily — tells MetaHasTraits.setup_class not to read default_value while building _static_immutable_initial_values. Without it the import fires at class-creation time and laziness is dead. None of that block's cases ever matched a Type subclass, so skipping it changes nothing else.

Design notes

Why properties rather than intercepting validate() / default(). The smaller approach — resolve inside those two methods, leave the attributes as raw strings — is not equivalent. It makes issubclass(x, trait.klass) raise TypeError, hands @observe a string as change.old, and leaves the class permanently unresolved when a @default returns None under allow_none. The properties close all three.

Strings, but not enforced. Strings are what make this useful and the type hints say so, but a class object is accepted and stored as is — it is already imported, so there is nothing left to defer and such a trait simply behaves like Type.

Consequences worth knowing

• A bad class name surfaces at first use rather than at construction.
• Generating help resolves the class, so --help-all costs the same as it does for Type.

Testing

12 tests covering deferral through class definition and instantiation, resolution on read and on assignment, caching, allow_none with a None default staying lazy, observers receiving a resolved change.old, @default-returns-None not stranding the trait, forcing resolution via _resolve_classes, accepting an already-imported class, and that Type still resolves eagerly.

Full suite green, mypy traitlets clean, ruff and formatting unchanged from main.

[codecov]

Codecov Report

❌ Patch coverage is 94.35028% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.20%. Comparing base (1dae189) to head (172457d).

Files with missing lines	Patch %	Lines
traitlets/traitlets.py	77.77%	7 Missing and 3 partials ⚠️

Components	Coverage Δ
traitlets	85.45% <77.77%> (-0.05%)	⬇️
tests	99.09% <100.00%> (+0.02%)	⬆️

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[Carreau]

@claude

Remove this test, this is not a requirement this is just something we are fine happening, we don't want to constrain future evolution

[Carreau]

@claude delete this comment