Summary

Enables the RUF012 Ruff rule and adds required typing.ClassVar annotations to mutable class-level attributes across core modules, examples, and tests.

Motive

The codebase contained several mutable class attributes (dictionaries, lists, and defaultdict instances) that were not annotated. Without ClassVar, it is not explicitly clear to static analysis tools and developers whether these variables are intended to be shared across all instances or are actually intended to be instance-level defaults. Enabling RUF012 enforces the best practice of using ClassVar, preventing potential bugs where shared state is accidentally shadowed or misused.

Implementation

• Configuration: Removed RUF012 from the extend-ignore list in pyproject.toml.
• Core Annotations:

1. mesa/agent.py: Annotated _ids (the global agent ID tracker).
2. mesa/discrete_space/cell_agent.py: Annotated DIRECTION_MAP.
3. mesa/mesa_logging.py: Annotated FORMATS in the log formatter.

• Example Improvements:
  mesa/examples/advanced/pd_grid/model.py: Annotated activation_regimes and payoff.
• Test Consistency:
  tests/visualization/test_backends.py: Annotated dummy space/agent attributes used for backend validation.

Usage Examples

# Before
class PdGrid(mesa.Model):
    activation_regimes = ["Sequential", "Random", "Simultaneous"]

# After
class PdGrid(mesa.Model):
    activation_regimes: typing.ClassVar[list[str]] = ["Sequential", "Random", "Simultaneous"]

Additional Notes

• Verified that all checks pass by running ruff check . --select RUF012.
• This is a non-breaking change with no impact on simulation logic or performance.