Evaluating mathematical expressions in Python

Question

I want to tokenize a given mathematical expression into a parse tree like this:

((3 + 4 - 1) * 5 + 6 * -7) / 2

                          \'/\'
                      


        

Answer 1

You can do this with the Python ast module.

https://docs.python.org/3.6/library/ast.html

theoperation is our mathematical operation we want to evaluate, we use the isinstance in order to know the type it is, if its a number, if its a binary operator(+,*,..). You can read at https://greentreesnakes.readthedocs.io/en/latest/tofrom.html , how the ast work

And in order to make the method work we sould use: evaluate(ast.parse(theoperation, mode='eval').body)

def evaluate(theoperation): 
    if (isinstance(theoperation, ast.Num)):
        return theoperation.n
    if (isinstance(theoperation, ast.BinOp)):
        leftope= evaluate(theoperation.left)
        rightope=evaluate(theoperation.right)   
        if (isinstance(theoperation.op, ast.Add)):
            return left+right
        elif (isinstance(theoperation.op, ast.Sub)):
            return left-right
        elif (isinstance(theoperation.op, ast.Mult)):
            return left*right
        elif (isinstance(theoperation.op, ast.Div)):
            return left/right
        elif (isinstance(theoperation.op, ast.Pow)):
            return left**right