No experience with them in python, but look also for PEG grammars. They are significantly simpler than the traditional tower of lexer + ambiguous limited looka

No experience with them in python, but look also for PEG grammars. They are significantly simpler than the traditional tower of lexer + ambiguous limited lookahead grammar. Plus the memoized Packrat algorithm allows throwing in functions with custom conditions. (Somewhat like parser combinators, they also support custom logic.)