functools.singledispatch has poor static analysis semantics. This PR bans it via Ruff's flake8-tidy-imports.banned-api and replaces all existing usages with explicit isinstance-based if-branch dispatch.

Config

Added to pyproject.toml:

lint.flake8-tidy-imports.banned-api."functools.singledispatch".msg = "Use if-branch isinstance logic instead of singledispatch."
lint.flake8-tidy-imports.banned-api."functools.singledispatchmethod".msg = "Use if-branch isinstance logic instead of singledispatchmethod."

Refactoring pattern

All @singledispatch / @singledispatchmethod usages replaced:

# Before
@singledispatch
def _as_dict(value: INIDocument | Section) -> ...:
    raise NotImplementedError

@_as_dict.register(INIDocument)
def _(value: INIDocument) -> ...:
    return {k: _as_dict(v) for k, v in value.items()}

@_as_dict.register(Section)
def _(value: Section) -> ...:
    return {option.key: option.value for option in value.iter_options()}

# After
def _as_dict(value: INIDocument | Section) -> ...:
    if isinstance(value, INIDocument):
        return {k: _as_dict(v) for k, v in value.items()}
    elif isinstance(value, Section):
        return {option.key: option.value for option in value.iter_options()}
    else:
        assert_never(value)

Files changed

• _file/ini/io_.py — _as_dict
• _pipeweld/func.py — Adder.partition_component (method) and _get_instructions_for_insertion; complex branches extracted into _partition_series_component, _partition_parallel_component, _partition_depgroup_component
• _integrations/ci/bitbucket/pipeweld.py — get_pipeweld_object, _extract_step_from_item, _insert_parallel_step, _is_insertion_necessary; extracted _extract_step_from_parallel_item
• _integrations/ci/bitbucket/steps.py — _censor_step, get_steps_in_pipeline_item; extracted _censor_parallel_item_step, _censor_stage_item_step
• _integrations/pydantic/dump.py — fancy_model_dump; extracted _fancy_model_dump_list, _fancy_model_dump_dict, _fancy_model_dump_base_model

Where functions were large, helpers were extracted to stay within Ruff's branch/return-count limits (PLR0912, PLR0911). All dispatch exhaustiveness is enforced via assert_never.

• Fixes Forbid usage of functools.singledispatch #1360

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