fix: prevent ErrorTree index access from mutating iteration

[BlocksecPHD]

Summary

Fixes #1328

The Bug: Accessing an index in that has no error incorrectly adds that index to the tree's iteration. This violates the documented behavior that should iterate over "indices in the instance with errors".

Root Cause: is a , and used which auto-creates entries for missing keys.

Fix:

1. In , explicitly create child trees and add to instead of relying on mutation
2. In , check if index exists in first, and return an empty without mutation if not

Before the fix:

tree = ErrorTree([ValidationError(path=['foo'], instance={'foo': 'bar', 'baz': 'qux'})])
print(list(tree))  # ['foo']
child = tree['baz']  # Access valid but error-free index
print(list(tree))  # BUG: ['foo', 'baz'] - mutation happened!

After the fix:

tree = ErrorTree([ValidationError(path=['foo'], instance={'foo': 'bar', 'baz': 'qux'})])
print(list(tree))  # ['foo']
child = tree['baz']  # Access valid but error-free index
print(list(tree))  # ['foo'] - no mutation!

Tests

Added 3 new tests to verify the fix:

• test_index_access_does_not_mutate_tree - basic mutation fix test
• test_nested_index_access_does_not_mutate_tree - nested path mutation test
• test_index_access_on_empty_tree_returns_empty_tree - empty tree edge case

All 7291 existing tests continue to pass.

[read-the-docs-community]

Documentation build overview

📚 python-jsonschema | 🛠️ Build #32020459 | 📁 Comparing 2be21cc against latest (ad0a1b3)

🔍 Preview build

Show files changed (1 files in total): 📝 1 modified | ➕ 0 added | ➖ 0 deleted

File	Status
_modules/jsonschema/exceptions/index.html	📝 modified