parsergen-backend-python.md

Parser Generator Backend: Python

This backend outputs a parser as Python source code. See parser generator backends for what every generated parser does and scanner generator backend: Python for the scanner side.

It targets Python 3.10 and imports nothing beyond the standard library.

--backend-python-scanner-module sets the module the token constants are imported from, which defaults to scanner. A leading dot makes the import relative, which is what a scanner and a parser sitting in the same package need.

Parser.parse takes a TokenSource and returns a ParseResult holding the tree and the errors, and can be called again with another scanner. The tree is None when the parse could not be finished. Errors are returned rather than raised, so reporting many of them costs nothing. Lexemes are bytes sliced out of the source.

Example

examples/calculator/python/ is a calculator built on this backend. Its parser was generated with:

golr parser \
  --frontend golr \
  --frontend-file-path calculator.golr \
  --backend python \
  --backend-python-scanner-module .scanner \
  --backend-file-path parser/parser.py