Summary

Fix RecursionError when comparing BaseModel instances that contain circular self-references.

Changes

• Added try-except RecursionError handlers in BaseModel.__eq__ to gracefully handle circular references
• When RecursionError is caught during comparison, we return True (assuming equal structure)
• Added tests for self-referential and mutual-reference equality scenarios

Why This Approach

This implementation has zero performance overhead for normal (non-circular) comparisons:

• No additional tracking variables or ContextVar usage
• No set allocations or lookups on every comparison
• The try-except only triggers when RecursionError actually occurs

This addresses the performance concerns raised in PR #11581 which proposed using ContextVar tracking for every equality comparison.

Behavior

Example

from pydantic import BaseModel
from typing import Optional

class SelfRef(BaseModel):
    value: int = 0
    ref: Optional['SelfRef'] = None

a = SelfRef()
a.ref = a  # a refers to itself

a2 = SelfRef()
a2.ref = a2  # a2 refers to itself

# Before: RecursionError
# After: True (same structure)
a == a2

Testing

• Added test_self_referential_equality for direct self-reference scenarios
• Added test_mutual_reference_equality for mutual circular references
• All existing equality tests continue to pass

Fixes #10630

🤖 Generated with Claude Code