fix(types): check min_length for parts in Artifact

[punkyoon]

Description

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Fixes #670 🦕

Summary

This PR adds validation to the Artifact model in a2a/types.py to ensure that the parts list is never empty.

Motivation

According to the A2A Protocol Specification (Section 4.17), an Artifact's parts list "Must contain at least one part."

Previously, it was possible to initialize an Artifact with an empty list []. This discrepancy allowed invalid artifacts to be created (e.g., by strands-agents) which could cause issues for downstream consumers.

This change also aligns the Python SDK with the Java SDK, which already strictly enforces this rule (see a2a-java implementation).

Changes

• Updated Artifact class in a2a/types.py.
• Used Pydantic's Field(min_length=1) to enforce the constraint at the model level.

Verification

Attempting to create an Artifact with empty parts now raises a ValidationError:

from a2a.types import Artifact
import pytest
from pydantic import ValidationError

# This now raises ValidationError
with pytest.raises(ValidationError):
    Artifact(
        artifact_id="test-id",
        parts=[], 
        metadata={}
    )

[gemini-code-assist]

Summary of Changes

Hello @punkyoon, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical data integrity issue by ensuring that the parts list in the Artifact model is never empty. This change prevents the creation of invalid artifacts that do not conform to the A2A Protocol Specification, thereby enhancing the robustness of the system and aligning the Python SDK with the strict validation already present in the Java SDK.

Highlights

• Artifact Model Validation: Enforced that the parts list within the Artifact model must contain at least one element, aligning with the A2A Protocol Specification (Section 4.17).
• Pydantic min_length Constraint: Utilized Pydantic's Field(min_length=1) to implement the validation constraint directly in the Artifact model definition in a2a/types.py.

Changelog

• src/a2a/types.py
  • Added a min_length=1 constraint to the parts field of the Artifact class, ensuring that an Artifact object always contains at least one Part.

Activity

• The author has confirmed adherence to the CONTRIBUTING guide and conventional commit standards.
• Linter and tests are confirmed to pass, and documentation updates were considered.
• This pull request addresses and closes issue [Bug]: Enforce non-empty parts in Artifact model to match A2A Specification #670.

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[gemini-code-assist]

Code Review

This pull request aims to enforce that an Artifact must contain at least one part, which aligns with the A2A Protocol Specification. The implementation using Pydantic's Field(min_length=1) is functionally correct. However, there's a significant maintenance concern as the change is made to an auto-generated file (src/a2a/types.py). My review comment details why this change should be made in the source JSON schema instead to ensure it's not lost upon regeneration.

[gemini-code-assist]

While this change correctly enforces the min_length constraint, this file is marked as auto-generated by datamodel-codegen at the top. Manual edits to generated files are fragile and likely to be overwritten the next time the code generation script is run. To make this change permanent, the constraint should be added to the source JSON schema (likely as "minItems": 1 for the parts array within the Artifact definition) and then this Python model file should be regenerated.

[punkyoon]

I missed that types.py is automatically generated. I have opened an issue in the main repository to ensure the constraint is included during model generation: a2aproject/A2A#1456