[Rajatendu1]

Problem

A nested UserDict or ChainMap is treated differently from a nested dict during recursive merges. A small override replaces the entire mapping and loses unrelated settings. With defaults_deep, the existing mapping stays but missing defaults are not added. Customizers also never reach the nested entries.

For example, merging {"settings": UserDict({"host": "localhost", "timeout": 30})} with {"settings": {"timeout": 60}} currently drops host.

Change

Recognize any Mapping source paired with a MutableMapping destination as a recursive mapping merge. Keep the destination mapping object, the existing literal-key lookup, source deep-copying, list behavior and scalar replacement rules. Add regression coverage for mixed dict/UserDict/ChainMap pairs, deep defaults, customizers, mappings nested in lists, ChainMap parent layers and source isolation. No dependencies or public signatures change.

Validation

Base: develop at b6bd031de47134b8e3c3977fbe65bf13cc51a205.

• Added 34 cases: 29 fail on the unchanged source and 5 compatibility controls already pass. All 34 pass with the change.
• Full configured unit/doctest suite on Linux CPython 3.10.12, 3.11.17, 3.12.15, 3.13.17 and 3.14.9: baseline 3,324 passed, patched 3,358 passed, 100% library statement coverage in every run.
• Full inv ci before and after on Python 3.10: source/wheel builds, Sphinx with warnings as errors, Ruff format/lint, mypy, generated chaining-stub freshness and the configured tests pass.
• git diff --check passes.

The documented test command excludes tests/pytest_mypy_testing; source mypy and generated-stub checks are included. An initial broader exploratory collection included that optional directory and failed 334 assertions; those are not claimed as passing. Windows, macOS and hosted CI have not been run locally.

Patch prepared and tests executed by Instinct, an AI assistant, at my request.

[coveralls]

coverage: 100.0%. remained the same — Rajatendu1:fix-nested-mapping-merge into dgilland:develop