Restore NullContext as a distinct class with virtual subclass semantics#218
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Previously `NullContext` was a direct alias of `ContextBase`. This preserved substitutability (any context could be passed where a `NullContext` was expected) but degraded introspection: `.context_type.__name__` reported "ContextBase" with the abstract base's docstring, which was confusing for users of models that declare `context: NullContext`. Reintroduce `NullContext` as a distinct subclass of `ContextBase` whose metaclass lies in `__instancecheck__` / `__subclasscheck__` so that any `ContextBase` instance/subclass virtually satisfies `NullContext`. The lie is gated to `NullContext` itself, so user-defined subclasses follow normal class-hierarchy rules. Pydantic's v2 instance fast-path in `model_validate` respects the lie, so original context instances flow through unchanged. Net effect: zero behavior change at runtime; introspection now reports `NullContext` with its own docstring. Signed-off-by: Pascal Tomecek <pascal.tomecek@cubistsystematic.com>
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #218 +/- ##
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Coverage 95.37% 95.37%
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Files 142 142
Lines 11428 11437 +9
Branches 623 625 +2
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Problem
NullContextis currently aliased toContextBase(commit fc64400, PR #166). This preserved substitutability (any context can be passed whereNullContextis expected) but degraded introspection: for a model declaringcontext: NullContext,.context_type.__name__reports"ContextBase"with the abstract base's docstring, which is confusing for users.Approach
Reintroduce
NullContextas a distinct subclass ofContextBasewith a metaclass that lies in__instancecheck__/__subclasscheck__: anyContextBaseinstance or subclass virtually satisfiesNullContext. The lie is gated toNullContextitself, so user-defined subclasses ofNullContextfollow normal class-hierarchy rules.Pydantic v2's instance fast-path in
model_validaterespects the lie, so original context instances flow through unchanged — same as the alias today.Behavior
m.context_type.__name__ContextBaseNullContext✅ContextBase'sNullContext's ✅isinstance(date_ctx, NullContext)m(date_ctx)onNullContextmodelMyNull(NullContext)subclass behaves as plain subclasstype_discriminatorContextBaseNullContextTests
All 841 tests pass on the local suite. No existing tests required modification.
Cache invalidation note
tokenize(NullContext())differs fromtokenize(ContextBase())(the discriminator value changes). Any persisted cache keyed under the old alias semantics will miss once on first re-evaluation. Not a correctness issue, just a one-time invalidation worth a release note.