Plugin sibling imports — `load_sibling`

When a backend plugin spans multiple Python files, use context["load_sibling"] rather than bare import statements. This is the slopsmith#33 contract.

The problem

The plugin loader inserts each plugin's directory onto sys.path so from extractor import X works. But Python caches imports by module name in sys.modules. Two plugins that each ship a top-level extractor.py (or any other generic name — util.py, client.py, parser.py, config.py, …) collide: whichever loads first wins, and the other plugin's from extractor import X either gets the wrong module or fails with cannot import name 'X' from 'extractor'.

The fix

context["load_sibling"](name) loads the sibling under a namespaced module name plugin_<id>.<name>, so each plugin gets its own copy. The <id> portion is bijectively encoded so reverse-DNS-style ids like com.example.foo work without colliding: _ → _5f_, . → _2e_.

def setup(app, context):
    extractor = context["load_sibling"]("extractor")
    ArchiveReader = extractor.ArchiveReader
    # …

Notes

name is a bare module name — no .py suffix, no slashes, no .. The helper raises ValueError for path traversal / format issues and ImportError for missing files.
Both single-file siblings (extractor.py) and package-form siblings (extractor/__init__.py) work. Package form wins when both exist (matches CPython's import-resolution precedence).
Relative imports between siblings work — from .shared import X in a top-level helper, from ..shared import X from inside a sibling package. The synthetic parent package plugin_<id> carries the plugin directory in its __path__.
from . import sibling (attribute-style) also resolves: loaded children are exposed as attributes on the parent package.
Repeat calls return the cached module. Concurrent first-time calls are serialized via per-module locks so no caller observes a half-initialized module.
Don't mix load_sibling and bare import for the same module — they'd execute the file twice and split module-level state.

Migration

Bare import sibling from routes.py still works during the transition period, but the loader prints a startup warning when it detects two plugins shipping a same-named top-level module — covering both .py files and package directories. Migrate to load_sibling to silence the warning and immunize your plugin from future ecosystem collisions.

Verification

The collision-detection logic is encoded in tests/test_plugins.py. Run:

pytest tests/test_plugins.py -v

If you're authoring a plugin with a top-level helper, add a test that imports your plugin under a reset_plugin_state fixture to confirm clean import behaviour. See testing-plugins.md.

PLUGIN_AUTHORING.md — guide index
plugin-manifest.md — declaring routes.py
plugin-logging.md — context["log"] (also exposed via context)
testing-plugins.md — reset_plugin_state fixture

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Plugin sibling imports — load_sibling