Expression evaluator

from parcon import *
from decimal import Decimal
import operator
expr = Forward()
number = (+Digit() + -(SignificantLiteral(".") + +Digit()))[flatten]["".join][Decimal]
term = number | "(" + expr + ")"
term = InfixExpr(term, [("*", operator.mul), ("/", operator.truediv)])
term = InfixExpr(term, [("+", operator.add), ("-", operator.sub)])
expr << term

This implements a simple expression evaluator, and shows how Parcon allows the evaluation logic to be specified as part of the parser. It uses Python's decimal module for arbitrary-precision arithmetic. This expression parser can then be used thus:

print expr.parseString("1+2") # prints 3
print expr.parseString("1+2+3") # prints 6
print expr.parseString("1+2+3+4") # prints 10
print expr.parseString("3*4") # prints 12
print expr.parseString("5+3*4") # prints 17
print expr.parseString("(5+3)*4") # prints 32
print expr.parseString("10/4") # prints 2.5