fix: use default_factory for mutable field defaults in ServerCallContext

[greysonlalonde]

summary

• replace default={} and default=UnauthenticatedUser() with default_factory=dict and default_factory=UnauthenticatedUser in ServerCallContext to prevent shared mutable default instances across all instances.

[gemini-code-assist]

Summary of Changes

Hello, 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 common pitfall in Python regarding mutable default arguments. By switching to default_factory for specific fields within the ServerCallContext class, it guarantees that new, distinct objects are created for each instance, thereby enhancing the reliability and predictability of the application's state management.

Highlights

• Mutable Default Arguments: Replaced direct assignment of mutable default objects ({} and UnauthenticatedUser()) with default_factory for state and user fields in ServerCallContext.
• Prevented Shared Instances: The change ensures that each instance of ServerCallContext receives its own unique state dictionary and User object, preventing unintended sharing and modification across instances.

Changelog

• src/a2a/server/context.py
  • Updated state field to use default_factory=dict instead of default={}.
  • Updated user field to use default_factory=UnauthenticatedUser instead of default=UnauthenticatedUser().

Activity

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[gemini-code-assist]

Code Review

This change correctly resolves an issue with mutable default arguments for the state and user fields in ServerCallContext. By using default_factory, you ensure that each model instance receives a new, unique object, preventing unintended side effects from shared state. This is a solid fix that aligns with best practices. To further improve the robustness of the code and prevent future regressions, I recommend adding a unit test that asserts two separate instances of ServerCallContext have distinct state and user objects.