How To Calculate An Equation In A String, Python

I have a variable that is function = '(2*1)+3'. How would I get it out of string form and calculate the answer? I tried using float(), int(float()) but I'm not sure if that's for n

Solution 1:

I've written this a couple times, and every time it seems that I lose the code...

A very simple (and "safe") calculator can be created using ast:

import ast
import operator

_OP_MAP = {
    ast.Add: operator.add,
    ast.Sub: operator.sub,
    ast.Mult: operator.mul,
    ast.Div: operator.div,
    ast.Invert: operator.neg,
}


classCalc(ast.NodeVisitor):

    defvisit_BinOp(self, node):
        left = self.visit(node.left)
        right = self.visit(node.right)
        return _OP_MAP[type(node.op)](left, right)

    defvisit_Num(self, node):
        return node.n

    defvisit_Expr(self, node):
        return self.visit(node.value)

    @classmethoddefevaluate(cls, expression):
        tree = ast.parse(expression)
        calc = cls()
        return calc.visit(tree.body[0])


print Calc.evaluate('1 + 3 * (2 + 7)')

This calculator supports numbers, addition, subtraction, division, multiplication and negation (e.g. -6) and parenthesised groups. Order of operations are the same as Python which should be relatively intuitive... It can (almost trivially) be extended to support just about any unary or binary operator that python supports by adding the ast node type and corresponding operator/function to the _OP_MAP above.

Solution 2:

You may use eval

>>>function = '(2*1)+3'>>>eval(function)
5

As @mgilson said,

Only do this if you completely trust the source of the string.