User input a function

Question

I'm writing a program that involves calculating the maximum of a user defined function, however if the user enters a function in terms of a variable such as x, i.e.

input = x**2 + 2*x + 1

I (fairly expectedly) get an error as x is not defined.

Does anyone know of an effective way to allow a user to input a function in terms of a single variable such as x?

• KCH: Look here: stackoverflow.com/questions/594266/equation-parsing-in-python
• bpgergo: int(raw_input("please enter x")) docs.python.org/library/functions.html#raw_input

Answer 1

If you are not worried about security much, simplest solution is to eval the expression e.g.

def calculate(value, function):
 x = value
 return eval(function)
print calculate(2, "x**2 + 2*x + 1")
print calculate(2, "x**3 - x**2 + 1")

output:

9
5

You can make it more secure by passing an empty builtin dict but still I think some clever code will be able to access internals of your code.

• Jon Egeland: And then the user can input a value if you used the int(raw_input(...)) strategy @bpgergo suggested.
• Anurag Uniyal: @Jon yes there can be various ways to get x value and function string from user e.g. from gui or command-line or from an API
• Artsiom Rudzenka: The only thing i can add that maybe would be better to use docs.python.org/library/…
• Anurag Uniyal: @ArtsiomRudzenka thanks I may edit that, but I don't think it evaluates such expressions