numeric expression from string?

[Giovanni Bajo]

Brian Blais wrote:

> I have a string input from the user, and want to parse it to a
> number, and would like to know how to do it.  I would like to be able
> to accept arithmetic operations, like:
>
> '5+5'
> '(4+3)*2'
> '5e3/10**3'
>
> I thought of using eval, which will work, but could lead to bad
> security problems (not that it's a big  deal in my app, but still...)


eval() is the preferred way unless you have serious security reasons:

>>> def calc(s):
...   try:
...       return float(eval(s, dict(__builtins__=None)))
...   except Exception, e:
...       raise ValueError, "error during expression evaluation: %s" % e
...
>>> calc("3**4")
81.0
>>> calc("58+34*4")
194.0
>>> calc("58+34*4+a")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 5, in calc
ValueError: error during expression evaluation: name 'a' is not defined
>>> calc("object.__class__")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 5, in calc
ValueError: error during expression evaluation: name 'object' is not defined
>>> calc("3.__class__")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 5, in calc
ValueError: error during expression evaluation: unexpected EOF while parsing
(line 1)
>>> calc("type(3).__class__")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 5, in calc
ValueError: error during expression evaluation: name 'type' is not defined


Of course, one can still bring your system to its knees when
"1000**1000000000000000"...
-- 
Giovanni Bajo