gistfile1.sc

from pyparsing import *

def buildVar(s, l, t):

    return [["id", t[0][0]]]

def buildExpr(s, l, t):

    e = t[0]

    if e[1] == "+":
        return [["add", e[0], e[2]]]
    elif e[1] == "-":
        return [["sub", e[0], e[2]]]
    elif e[1] == "*":
        return [["mul", e[0], e[2]]]
    elif e[1] == "/":
        return [["div", e[0], e[2]]]
    else:
        raise Exception("Unknown expression:", t)

integer = Combine(Optional(oneOf("+ -")) + Word(nums)).setParseAction(buildVar)

mulOp = oneOf("* /")
addOp = oneOf("+ -")
lpar  = Literal("(").suppress()
rpar  = Literal(")").suppress()


def evalExpr(e):

    op = e[0]

    if op == "id":
        return int(e[1])
    elif op == "add":
        return evalExpr(e[1]) + evalExpr(e[2])
    elif op == "sub":
        return evalExpr(e[1]) - evalExpr(e[2])
    elif op == "mul":
        return evalExpr(e[1]) * evalExpr(e[2])
    elif op == "div":
        return evalExpr(e[1]) / evalExpr(e[2])
    else:
        raise Exception("Unknown expression:", e)


expr = operatorPrecedence(integer,
        [ (mulOp, 2, opAssoc.LEFT, buildExpr)
        , (addOp, 2, opAssoc.LEFT, buildExpr)
        ]
        )

program = expr + StringEnd()

try:
    input = "(4+3)*7+9"
    print "input =", input
    e = program.parseString(input)
    print "AST =", e
    print "eval =", evalExpr(e[0])

    print ""
    input = "(4+3)*a"   # expects parse err
    print "input =", input
    e = program.parseString(input)
    print "AST =", e
    print "eval =", evalExpr(e[0])

except ParseException, err:
    print err.line
    print " "*(err.column-1) + "^"
    print err