If you place `# coding: utf-8` on the first line of a Python script, it'll try to interpret the raw bytes contained later in the file by passing them through a relevant codec[1]. Since the codec receives the raw source and transforms it before any interpretation happens, you can (here's the point I'm starting to guess) supply a codec that parses the code, transforms the AST, and dumps it back to the source for execution. It would be similar to how "parse transforms" work in Erlang, though there you're handed AST and not bytes.

[1] https://docs.python.org/3/library/codecs.html