chore: add typing to cloudinit.distros.parsers hostname and resolv_conf

[Ahmedaltu]

GH-5445

Add type annotations to fix check_untyped_defs mypy errors in:

• cloudinit/distros/parsers/hostname.py
• cloudinit/distros/parsers/resolv_conf.py

Changes:

• Added List, Optional, Tuple imports from typing
• Annotated all method signatures with parameter and return types
• Typed _contents as Optional[List[Tuple[str, List[str]]]]
• Added assert self._contents is not None guards after parse() calls
• Fixed __str__ in hostname.py — renamed contents variable to avoid
  shadowing StringIO object with str reassignment
• Removed cloudinit.distros.parsers.hostname and
  cloudinit.distros.parsers.resolv_conf from disabled modules in
  pyproject.toml

Verified with:
mypy cloudinit/distros/parsers/hostname.py cloudinit/distros/parsers/resolv_conf.py --ignore-missing-imports
Success: no issues found in 2 source files

[holmanb]

Please don't use assert for type narrowing.

[holmanb]

Please don't use assertions in runtime code for type narrowing.

[holmanb]

same

[holmanb]

same

[Ahmedaltu]

Thanks @holmanb. Replaced assert with a _parsed: bool = False flag (same pattern used in cloudinit/net/network_state.py). _contents is now typed as List[Tuple[str, List[str]]] directly without Optional, so no type narrowing is needed. mypy passes cleanly.

[holmanb]

Please don't make unnecessary variable renames.

[Ahmedaltu]

Done — reverted the variable rename. Kept contents as the StringIO object and used rendered only for the final string to avoid the mypy variable shadowing error while preserving the original naming.

[holmanb]

This new variable seems unnecessary and error-prone. It should be possible to simply check the truthy value of self._contents rather than manually using a boolean to track the same thing.

[Ahmedaltu]

Hi @holmanb, I considered using if not self._contents but there's a subtle issue — when the input text is empty or contains only comments, _parse() returns an empty list []. Since an empty list is falsy, using if not self._contents would cause parse() to re-run on every call for empty files, defeating the lazy-parse caching.

The _parsed flag correctly distinguishes between "not yet parsed" and "parsed but empty result". This is the same reason network_state.py uses a dedicated _parsed boolean rather than checking the contents directly.

Happy to use a different approach if you have a preferred solution for this case.

[github-actions]

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