Add standalone .prompt file handler, opt-in folder loading, and lookup for Python Genkit "issue-3282"

[merongithub]

Summary

Introduce a standalone .prompt file handler for Python Genkit with:

• Directory scanning (sync/async), partial registration, name/variant parsing
• Optional folder preloading via Registry/Genkit without auto-registration
• Lookup of loaded prompts by name/variant/ns
• Tests validating loader behavior and metadata rendering

Scope of Changes

• New utilities under genkit/dotprompt/:
  • file_loader.py: load .prompt files/dirs, register partials, compute definition keys, async metadata rendering; supports current engine API naming.
  • __init__.py: export loader/metadata functions.
  • types.py: prompt identity and loaded prompt types.
• Registry integration (read-only, opt-in):
  • Registry.load_prompt_folder(dir, ns) caches loaded prompts in-memory.
  • Registry.list_loaded_prompts(), Registry.get_loaded_prompt(key), Registry.lookup_loaded_prompt(name, variant, ns).
• Genkit API helpers:
  • GenkitRegistry.load_prompt_dir, aload_prompt_dir, load_prompt_file, aload_prompt_file (no auto-registration).
• Tests:
  • file_loader_test.py: directory scan, partials, variant parsing, subdir prefixing, metadata rendering, definition-key and lookup.
• Cleanups:
  • Removed unused genkit/dotprompt/exceptions.py.
  • Adjusted comments; ensured real engine API support.

Behavior and Compatibility

• No breaking changes; existing prompts and execution paths are unchanged.
• .prompt loading is opt-in (invoked by API or Registry.load_prompt_folder).
• Tests pass locally: 6/6.

How to Test

• Using uv (recommended):
  • From py/:
    • uv sync
    • uv run pytest packages/genkit/tests/genkit/dotprompt/file_loader_test.py -q
• Or using venv:
  • Create/activate venv
  • Install pinned dotpromptz and dotprompt-handlebars per py/pyproject.toml (uv.lock has the commit)
  • pytest packages/genkit/tests/genkit/dotprompt/file_loader_test.py -q

Follow-ups (not included here)

• Optional: wire ai.prompt('name', { variant }) to use loaded prompts behind a flag.
• Caching optimizations (mtime/hash).
• Additional integration tests using real engine for end-to-end prompt execution.

Checklist

• Code compiles and lints cleanly
• Unit tests added/updated and passing (6/6)
• No breaking changes to public APIs
• Real Dotprompt engine (dotpromptz) supported (camelCase/snake_case compatibility)
• Read-only, opt-in integration only (no auto-registration)
• Removed unused files (genkit/dotprompt/exceptions.py)
• Comments updated to reflect current design
• Documentation update (optional) on new loader APIs and Registry helpers
• Release notes entry (optional)

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