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?

Look here: http://stackoverflow.com/questions/594266/equation-parsing-in-python — KCH, Dec 12 '11 at 21:48
int(raw_input("please enter x")) http://docs.python.org/library/functions.html#raw_input — bpgergo, Dec 12 '11 at 21:48

score 2 · Accepted Answer · answered Dec 12 '11 at 21:49

2

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.

answered Dec 12 '11 at 21:49

Anurag Uniyal

81,711
39
167
215

And then the user can input a value if you used the `int(raw_input(...))` strategy @bpgergo suggested. – Jon Egeland Dec 12 '11 at 21:52
1

@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 – Anurag Uniyal Dec 12 '11 at 21:54
The only thing i can add that maybe would be better to use http://docs.python.org/library/ast.html?highlight=eval#ast.literal_eval – Artsiom Rudzenka Dec 12 '11 at 21:56
@ArtsiomRudzenka thanks I may edit that, but I don't think it evaluates such expressions – Anurag Uniyal Dec 12 '11 at 22:31

User input a function

1 Answers1