Fix custom scheme config type narrowing

[k5342]

Summary

• assert that the lazily loaded custom scheme configuration is initialized
• allow mypy to narrow the global configuration from None to ConfigParser

Background

mypy 2.0 enabled --local-partial-types by default. This option changes type inference when a module-level variable is initialized in another scope.¹

PFIO initializes _config to None at module scope and assigns a ConfigParser from _load_config(). The mypy migration notes use the same pattern—a module variable initialized to None and assigned through global in another function—and specify that the inferred type remains optional.²

A call to _load_config() therefore does not narrow _config in the caller. The assertions make the existing runtime invariant explicit to the type checker.

Testing

• python3 -m py_compile pfio/v2/config.py
• git diff --check

Footnotes

1. mypy, “Enable --local-partial-types by Default,” Mypy Release Notes: https://mypy.readthedocs.io/en/stable/changelog.html#enable-local-partial-types-by-default ↩

2. mypy, “Improved Compatibility for Local Partial Types,” Mypy Release Notes: https://mypy.readthedocs.io/en/stable/changelog.html#improved-compatibility-for-local-partial-types ↩