Turning String into Numerical Equation

[Giovanni Bajo]

Michael Spencer wrote:

> * this means that, eval("sys.exit()") will likely stop your
> interpreter, and
> there are various other inputs with possibly harmful consequences.
>
> Concerns like these may send you back to your original idea of doing
> your own expression parsing.

I use something along these lines:

def safe_eval(expr, symbols={}):
    return eval(expr, dict(__builtins__=None, True=True, False=False), symbols)

import math
def calc(expr):
    return safe_eval(expr, vars(math))

>>> calc("2+3*(4+5)*(7-3)**2")
434
>>> calc("sin(pi/2)")
1.0
>>> calc("sys.exit()")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 2, in calc
  File "<stdin>", line 2, in safe_eval
  File "<string>", line 0, in ?
NameError: name 'sys' is not defined
>>> calc("0x1000 | 0x0100")
4352

-- 
Giovanni Bajo