lwcalc.py

  import operator, re
  lookup = { 
      '-': operator.sub,
      '+': operator.add,
      '*': operator.mul,
      '/': operator.div,
      '^': operator.pow,
  } 
  emdas = [['\^'], ['\/\*'], ['\+\-']]
  def lwe(expr):
      for ops in reversed(emdas):
          expr = re.split("([{}])".format("\\".join(ops)), expr, 1)
          if len(expr) == 3:
              return lookup[expr[1]](lwe(expr[0]), lwe(expr[2]))
          expr = expr[0]
      return float(expr)
  print lwe("4.5*3+3*2^2")