Is there an alternative to exec in python?

Question

So I'm making an integrator program with the simpson 1/3 method and I want the user to give me a function and interval of integration, and then return the result. I figure that I can use exec to make dynamic code, so I use it to create the function. This is my code:

from math import *
class CreaFormula:
 def __init__(self,formula):
 self.fun = "def f(x):\n return %s" % formula
class Integrador:
 def __init__(self,f):
 #integration interval 
 a = 0
 b = 1
 n = 600 
 h = (b-a)/n
 #method
 self.s = f(a) + f(b)
 for i in range(1,n):
 if i%2 == 0:
 self.s = self.s + 2*f(a+i*h)
 else:
 self.s = self.s + 4*f(a+i*h)
 self.s = self.s*(h/3)
 vr = -cos(1) + e
 self.er = abs((vr -self.s) /vr)
formula = input()
MiFo = CreaFormula(formula)
f1 = MiFo.fun
exec(f1)
MyInt = Integrador(f)
a = MyInt.s
b = MyInt.er
print(a)

So basically I want to put everything that is at the end of the code inside a class, so I can call it later by another code, but if I do that it shows an error that f is not defined, because it is created with exec. So is there a way to not use exec but still create a function from user's input?

Thanks in advance

• roganjosh: I don't understand the purpose of the string. Why don't you just bind an actual function to the attribute?
• Daniel Pryden: The problem with exec is that it allows a user to create a function that can do anything your program can do. If that's what you're trying to accomplish, then that's the right tool tool to use. That said, you should understand the security implications of this: what users are allowed to use this program? Is it OK that they can run any code they want on this machine?
• roganjosh: You might also want to look at sympy
• Alexander B.: I would try ast.parse('formula') and then try to process the tree as an expression. I would avoid using eval or exec unless it's 'just to test' code for yourself.

Answer 1

If the question is just how to construct a function object that can evaluate a user-supplied expression, you'll probably have an easier time using eval than exec:

def create_function_from_formula(formula):
 def user_function(x):
 return eval(formula, globals(), {'x': x})
 return user_function

Of course, even with eval, if someone provides a malicious formula, it can do anything, up to and including executing any other program included on the computer. So only do this if you trust the person providing the formula to essentially take over your computer. In particular, you should never do this if formula can come from a user who is not physically logged in to your computer already.