How to take a user defined function as user input in python?

Question

I am trying to write a program to define user-defined function with return expression as input from user. Suppose a function should return x+y.In python we can do it as---

def f(x,y):
 return x+y

But,i want something like

a=input("enter the function:")
def f(x,y):
 return a
print f(2,3)

If i enter x+y as input of a,it will give 5.How can it be done in Python(without Sympy or raw_function along with eval())?

Answer 1

This was actually easy.Suppose u want to evaluate the function at x=2

f1=str(input("Enter the test function:"))
def f(x):
 return eval(f1)
print f(2)

That's it!

Answer 2

The Pyparsing lib parses mathematical expressions.

You can use it like this for example :

from pyparsing import NumericStringParser
nsp = NumericStringParser()
print(nsp.eval('2+3')) # 5

You have to replace variables by numeric values.

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In case your really don't want to use external eval library, an application of RPN could be a solution.