fix(signatures): raise clear error for field names that shadow SignatureMeta properties

[STiFLeR7]

📝 Changes Description

Declaring a Signature field whose name collides with a SignatureMeta property (e.g. instructions, the metaclass property used to read/write the docstring) crashes signature class construction with an opaque internal error instead of a clear validation message:

class MySig(dspy.Signature):
    instructions: str = dspy.InputField()
    answer: str = dspy.OutputField()
# TypeError: 'NoneType' object is not subscriptable

Root cause: Pydantic detects the name collision itself (it emits its own UserWarning: Field name "instructions" ... shadows an attribute in parent "Signature") and, for that field only, produces a plain FieldInfo with json_schema_extra=None instead of preserving the InputField()/OutputField() instance from the class body. SignatureMeta already has validation for exactly this shape of problem — _validate_fields() raises a clear TypeError when a field's json_schema_extra doesn't carry the expected __dspy_field_type key — but it ran too late in __new__: when no docstring is given, _default_instructions(cls) runs first, which calls cls.input_fields → _get_fields_with_type(), which indexes json_schema_extra directly and crashes before _validate_fields() ever gets a chance to run.

Fix: move the existing _validate_fields() call to run immediately after Pydantic builds the class, before anything reads json_schema_extra. No new validation logic — just correct ordering of what was already there. After the fix:

TypeError: Field `instructions` in `MySig` must be declared with InputField or OutputField, but field `instructions` has `field.json_schema_extra=None`

Fixes #1710.

✅ Contributor Checklist

• Pre-Commit checks are passing (locally and remotely)
• Title of your PR / MR corresponds to the required format
• Commit message follows required format fix(signatures): ...

⚠️ Warnings

None — this only changes the order of two pre-existing steps in SignatureMeta.__new__; no new validation logic was added, so behavior for non-colliding field names (the common case) is unchanged.

Testing

• Reverted the fix locally and reproduced the exact reported TypeError: 'NoneType' object is not subscriptable before reapplying it.
• Added a regression test asserting the clear _validate_fields error is now raised instead.
• uv run pytest tests/signatures/ tests/primitives/ -q: 172 passed (6 deselected pre-existing Windows temp-file-permission failures, confirmed present on unmodified main too and unrelated to signature construction).
• uv run pytest tests/ -q (full suite, minus pre-existing environment failures — see companion PR fix(primitives): prevent named_parameters()/named_sub_modules() dict-key collisions #10241 for the same three confirmed-pre-existing exclusions): all passing.

[greptile-apps]

Greptile Summary

This change validates DSPy Signature fields immediately after Pydantic creates the class, preventing metadata access from obscuring malformed field declarations. The focused before-and-after check confirmed that normal Signature construction, field classification, generated instructions, and serialization still work, while an instructions field collision now raises the intended clear field-declaration error instead of an internal NoneType exception.

T-Rex validation blocked

The focused repository pytest command could not collect because the litellm package is missing from the environment. The independently executed harness exercised the modified class-construction paths successfully.

Confidence Score: 5/5

Safe to merge based on the exercised Signature construction behavior.

The before-and-after harness directly exercised the changed metaclass path: valid Signatures remained usable, and the property-shadowing failure now produces the intended validation error. No defects remain to report.

Files Needing Attention: No additional files need attention; the focused implementation path in dspy/signatures/signature.py and its regression scenario were exercised.

T-Rex Logs

What T-Rex did

• Ran the pre-PR focused signature validation script; it showed a valid Signature construction and that the collision produced a prior NoneType error.
• Ran the PR revision of the signature validation script; the process exited successfully, the Signature construction remained valid, and the collision raised a Field 'instructions' error that must be declared with InputField or OutputField.
• Attempted targeted pytest collection for signature tests, but collection was blocked by a missing environment dependency (litellm); the independent focused harness executed the changed code path successfully.
• Uploaded and organized supporting artifacts to enable review, including the focused signature validation script and the run outputs from pre-PR and PR executions.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "fix(signatures): raise clear error for f..." | Re-trigger Greptile}