Question

what is the python version of saying this.. this is in java

public static enum Operations {Add, Subtract, Multiply, Divide, None};

I am converting an entire program to python, i just cant figure out this part

heres my whole class

    import java.util.*;

public class Expression
{
    public static enum Operations {Add, Subtract, Multiply, Divide, None};
    int a;
    int b;
    Expression.Operations op;
    public Expression()
    {
        a = 0;
        b = 0;
        op = Expression.Operations.None;
    }
    public Expression(int value1, int value2, Expression.Operations operation)
    {
        a = value1;
        b = value2;
        op = operation;
    }

    public boolean parseString(String expressionString, Map<Character, Integer> vars)
    {
        Scanner scanner = new Scanner(expressionString);

        //Attempt to read the first value.
        if (scanner.hasNextInt())
            a = scanner.nextInt();
        else if (scanner.hasNext())
        {
            String var = scanner.next();
            //Ensure that the variable identifier is a single alphabetical character in length.
            if (!var.matches("[A-Z]{1}"))
            {
                return false;
            }
            if (vars.containsKey(var.charAt(0)))
                a = vars.get(var.charAt(0));
            else
            {
                System.err.println("ERROR: Uninitialized variable.");
                return false;
            }
        }
        else return false;

        //If more tokens exist, attempt to read the operator.
        if (scanner.hasNext())
        {
            String operator = scanner.next();
            if (operator.equals("+"))
                op = Expression.Operations.Add;
            else if (operator.equals("-"))
                op = Expression.Operations.Subtract;
            else if (operator.equals("*"))
                op = Expression.Operations.Multiply;
            else if (operator.equals("/"))
                op = Expression.Operations.Divide;
            else
                return false;

            //Attempt to read the second value.
            if (scanner.hasNextInt())
                b = scanner.nextInt();
            else if (scanner.hasNext())
            {
                String var = scanner.next();
                //Ensure that the variable identifier is a single alphabetical character in length.
                if (!var.matches("[A-Z]{1}"))
                {
                    return false;
                }
                b = vars.get(var.charAt(0));
            }
            else return false;
        }

        return true;
    }
    public int evaluate()
    {
        int value = 0;
        if (op == Expression.Operations.Add)
            value = a + b;
        if (op == Expression.Operations.Subtract)
            value = a - b;
        if (op == Expression.Operations.Multiply)
            value = a * b;
        if (op == Expression.Operations.Divide)
            value = a / b;
        if (op == Expression.Operations.None)
            value = a;
        return value;
    }
}

Answer 1

In Python any attribute or method is considered public unless you put underscores at the start of its name. Here is the relevant section in the Python 2.7 tutorial.

Python doesn’t have a way of exactly duplicating the function of static, but any attributes you define on a class will be visible in instances in the same way as static variables are. Just attribute = value inside your class definition and you’re fine.

You can’t make values constant in Python, but the convention is to use UPPERCASE_IDENTIFIERS to indicate that intention.

Enums do not exist. In Python ordinary strings constants are usually used for this purpose. Just pass "add" "subtract", "multiply", "divide" or None to your function.

For example, in your parser

if (operator.equals("+"))
    op = Expression.Operations.Add;

would become

if operator == "+":
    op = "add"

and in your evaluator

if (op == Expression.Operations.Add)
    value = a + b;

would become

if op == "add"
    value = a + b