Evaluating a formula given by user for multiple values

Question

I am writing a python script that needs to take an equation from a user in the form of something like this

z=x^2+3x+9 +y^3 or z =cos(pi/2+x) + 2sin(y)

and evaluate the function at runtime over many values for x and y. How would I go about using the input given by a user as a function? Meaning I would like to be able to do something like this:

input = input("please input 3 variable function.")
function = evaluate_function(input)
for x and y:
        result = evaluate function
        return result

Is something like this possible? I have looked around and the closed thing I have found to what I want to do seems to be this (How to process user supplied formulas?), but it is only talking about evaluating for a single value of x and z not iterating over many values. Any help would be greatly appreciated.

Update: As suggested below I found this (http://lybniz2.sourceforge.net/safeeval.html) about using eval() which seems to be pretty much what I want to do

Ever heard of [`eval`](https://docs.python.org/2/library/functions.html?highlight=eval#eval)? — ivan_pozdeev, Oct 17 '14 at 04:45
Yes but I read that is not a safe way to do this if you can not trust the input of your users — guribe94, Oct 17 '14 at 04:48

score 0 · Answer 1 · answered May 19 '19 at 13:02

0

Interpreting math formulas is sympy's domain. parse_expr is its safe parsing function, and its global_dict and local_dict arguments govern what predefined symbols are available.

answered May 19 '19 at 13:02

ivan_pozdeev

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score 0 · Answer 2 · answered May 19 '19 at 20:58

Yes, parsing and substitution should work:

from sympy.parsing.sympy_parser import parse_expr
eq = parse_expr(raw_input('enter an equation of x, y and/or z'))
for v in ((1,2,3),(1,2,4)):
  res = eq.subs(dict(zip((x,y,z),v)))
  dat = (tuple(v) + (eq, res))
  print('at x=%s, y=%s, z=%, %s = %s' % dat)

Evaluating a formula given by user for multiple values

2 Answers2