Question

Which Python tool can you recommend to parse programming languages? It should allow for a readable representation of the language grammar inside the source, and it should be able to scale to complicated languages (something with a grammar as complex as e.g. Python itself).

When I search, I mostly find pyparsing, which I will be evaluating, but of course I’m interested in other alternatives.

Edit: Bonus points if it comes with good error reporting and source code locations attached to syntax tree elements.

Answer 1

I really like pyPEG. Its error reporting isn’t very friendly, but it can add source code locations to the AST.

pyPEG doesn’t have a separate lexer, which would make parsing Python itself hard (I think CPython recognises indent and dedent in the lexer), but I’ve used pyPEG to build a parser for subset of C# with surprisingly little work.

An example adapted from fdik.org/pyPEG/: A simple language like this:

function fak(n) {
    if (n==0) { // 0! is 1 by definition
        return 1;
    } else {
        return n * fak(n - 1);
    };
}

A pyPEG parser for that language:

def comment():          return [re.compile(r"//.*"),
                                re.compile("/\*.*?\*/", re.S)]
def literal():          return re.compile(r'\d*\.\d*|\d+|".*?"')
def symbol():           return re.compile(r"\w+")
def operator():         return re.compile(r"\+|\-|\*|\/|\=\=")
def operation():        return symbol, operator, [literal, functioncall]
def expression():       return [literal, operation, functioncall]
def expressionlist():   return expression, -1, (",", expression)
def returnstatement():  return keyword("return"), expression
def ifstatement():      return (keyword("if"), "(", expression, ")", block,
                                keyword("else"), block)
def statement():        return [ifstatement, returnstatement], ";"
def block():            return "{", -2, statement, "}"
def parameterlist():    return "(", symbol, -1, (",", symbol), ")"
def functioncall():     return symbol, "(", expressionlist, ")"
def function():         return keyword("function"), symbol, parameterlist, block
def simpleLanguage():   return function