python-ply

PLY is a straightforward lex/yacc implementation. Here is a list of its essential features:

It is implemented entirely in Python.
It uses LR-parsing which is reasonably efficient and well suited for larger grammars.
PLY provides most of the standard lex/yacc features including support for empty productions, precedence rules, error recovery, and support for ambiguous grammars.
PLY is straightforward to use and provides very extensive error checking.
PLY doesn't try to do anything more or less than provide the basic lex/yacc functionality. In other words, it's not a large parsing framework or a component of some larger system..