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Using eval() To Transform Symbolic Expressions (Python recipe) by Moe Aboulkheir
ActiveState Code (http://code.activestate.com/recipes/491276/)

This was written as an underhanded solution to an problem in an online programming challenge (http://www.spoj.pl) that required converting simple arithmetic expressions from infix to postfix form. I thought the solution was pretty neat, and I suspect the general technique may be useful for other problems so i'm including it here.

Python, 24 lines
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import string

def make_transformer(op):
    def transform(self, other):
        return rpn_transformer(self.expr + other.expr + (op,))
    return transform

class rpn_transformer:
    __slots__ = ('expr',)

    def __init__(self, expr):
        self.expr = expr

    __mul__ = make_transformer('*')
    __div__ = make_transformer('/')
    __add__ = make_transformer('+')
    __sub__ = make_transformer('-')

names = dict((c, rpn_transformer((c,))) for c in string.ascii_lowercase)
to_postfix = lambda s: ' '.join(eval(s, names).expr)

# example

assert to_postfix('(a+b)/(c+d+e*f)-h') == 'a b + c d + e f * + / h -'
Tags: programs