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: stackoverflow.com/questions/594266/equation-parsing-in-python — KCH
– KCH, Commented Dec 12, 2011 at 21:48
int(raw_input("please enter x")) docs.python.org/library/functions.html#raw_input — bpgergo
– bpgergo, Commented Dec 12, 2011 at 21:48

Anurag Uniyal · Accepted Answer · 2011-12-12 21:49:31Z

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, 2011 at 21:49

Anurag Uniyal

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4 Comments

Jon Egeland Over a year ago

And then the user can input a value if you used the int(raw_input(...)) strategy @bpgergo suggested.

Anurag Uniyal Over a year ago

@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 Over a year ago

The only thing i can add that maybe would be better to use docs.python.org/library/…

Anurag Uniyal Over a year ago

@ArtsiomRudzenka thanks I may edit that, but I don't think it evaluates such expressions

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