Enums in Python: How to enforce in method arguments

Question

I want to use enums in python like in code below (java). I am a greenhorn in Python. I have the following code in Java and want to replicate the functionality in Python:

class Direction {
  public enum Direction {LEFT, RIGHT, UP, DOWN}

  public void navigate(Direction direction)
    switch(direction){
        case Direction.LEFT:
            System.out.print("left");
            break;
        case Direction.RIGHT:
            System.out.print("right");
            break;
        case Direction.UP:
            System.out.print("up");
            break;
        case Direction.DOWN:
            System.out.print("down");
            break;
  }
}

How can I enforce users to only provide an enum to a Python method?

Possible duplicate of [How can I represent an 'Enum' in Python?](http://stackoverflow.com/questions/36932/how-can-i-represent-an-enum-in-python) — Alastair McCormack, Feb 15 '16 at 10:58
I saw that one, but it doesn't explain (at least to me), how to pass enum element as parameter and force user to use enum elements instead of typing raw strings or different values. — user2678074, Feb 15 '16 at 12:21
I've updated the question with that as the thrust of your question. Ideally you would've provided Python code so we could see what you were trying to achieve — Alastair McCormack, Feb 15 '16 at 12:41

Alastair McCormack · Accepted Answer · 2019-07-01T10:21:00.083

28

Python is dynamic and duck typed - variables can change type and you can't force types on methods.

You can, however, check for types in the body of a method using isinstance().

~~isinstance() will allow users to subclass your enum for future extensibility.~~ - See comments

E.g.

# Python 2.x: pip install enum34
from enum import Enum

class Direction(Enum):
    LEFT = "left"
    RIGHT = "right"
    UP = "up"
    DOWN = "down"

def move(direction):

    # Type checking
    if not isinstance(direction, Direction):
        raise TypeError('direction must be an instance of Direction Enum')

    print direction.value

>>> move(Direction.LEFT)
left
>>> move("right")
TypeError: direction must be an instance of Direction Enum

edited Jul 01 '19 at 10:21

answered Feb 15 '16 at 12:34

Alastair McCormack

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Actually, `Enum`s with members cannot be subclassed. – Ethan Furman Jul 01 '19 at 00:56
@EthanFurman oh wow. I just tested extending an Enum. Sure enough, you can't - thanks for the heads-up! – Alastair McCormack Jul 01 '19 at 10:19

alexis · Answer 2 · 2016-02-15T18:11:17.167

The "pythonic" thing to do is to follow the principle of duck-typing: Try to accept the value you are passed without making too big of a fuss. In this case, instead of enforcing the type I would simply check for equality with each enum value, and raise an error for anything that cannot be handled:

def navigate(direction):
    """Turn toward `direction` (an enum of type `Direction`)"""

    if direction == Direction.left:
         print("Left")
    elif direction == Direction.right:
         (etc., etc.)
    else:
         # Hmm, `direction` does not compare equal to any enum value:
         raise ValueError("Invalid direction "+ str(direction))

I got this and now I'm so thankful for all the coding gods for languages with object types. Thank you all for help though. — user2678074, Mar 10 '16 at 14:59

Enums in Python: How to enforce in method arguments

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