calculadora_infix_2.py

'''
    >>> calcular('1')
    1

    >>> calcular('(+ 1 2)')
    3

    >>> calcular('(+ (- 1 2) 3)')
    2

    >>> calcular('(- 0 (+ (+ 1 2) 3))')
    -6
'''

from pyparsing import Word, White, nums, Literal, OneOrMore, Group, Forward

numero = Word(nums)
abre_parenteses = Literal('(').suppress()
fecha_parenteses = Literal(')').suppress()
espacos = OneOrMore(White()).suppress()
mais = Literal('+')
menos = Literal('-')
expr = Forward()
atomo = numero | expr
expr << Group(numero |
              (abre_parenteses +
                 (mais|menos) + espacos + atomo + espacos + atomo +
               fecha_parenteses))

OPERACOES = {
             '+': lambda x, y: x + y,
             '-': lambda x, y: x - y,
            }

def avaliar(e):
    if len(e) == 1:
        return int(e[0])
    operacao = OPERACOES[e[0]]
    return operacao(avaliar(e[1]), avaliar(e[2]))

def calcular(e):
    expr_parseada = expr.parseString(e, True)[0]
    return avaliar(expr_parseada)

if __name__ == '__main__':
    import doctest
    doctest.testmod()