Import Hook

When you import fragments.loader, it installs a custom MetaPathFinder on sys.meta_path. From that point on, every .py file that Python imports is intercepted. If the file contains <>, the source is transpiled before it reaches the interpreter. Files without <> pass through unchanged.

from fragments import loader  # installs the hook — must come first

from fastapi import FastAPI   # any fragment-containing module imported after
from routes import router     # this line is now transpiled automatically

The hook operates on source text, so nothing is written to disk — your .py files are never modified.