Errors

The native boundary converts failures into stable Python exception categories:

Exception	Meaning
ValueError	Unknown/removed system, invalid contest or rating configuration, malformed JSON, schema mismatch, incompatible checkpoint metadata
OSError	Checkpoint open, read, temporary write, sync, or atomic replacement failure
RuntimeError	An unexpected panic or internal Rust failure
TypeError	A Python argument has the wrong structural type, such as a bytes checkpoint path

Validation occurs before the rating calculation. The accepted contest shape is defined in Concepts, initial-parameter and system constraints in Rating systems, and persistence validation in Checkpoints. Integers outside the supported Rust range raise ValueError rather than a conversion-specific OverflowError.

from elo_mmr_py import Contest

try:
    Contest([('alice', 0, 0), ('alice', 1, 1)])
except ValueError as error:
    print(error)

An unexpected Rust panic is caught only as a final safety boundary. It is a bug; report it with a minimal contest/checkpoint reproducer and the package version. If it occurs during a Rater operation, that instance is poisoned and rejects subsequent access rather than exposing partially updated state.