Creating and Implementing an interface using Python?

Question

I have two (2) questions: Firstly, how do I create the FlyBehavior interface using Python? Secondly, how do I implement the FlyBehavior interface in the FlyWithWings class, using Python (see below)? I'm learning Design Patterns by Head First and I want to rewrite the following Java classes using Python

public abstract class Duck {

    // Reference variables for the 
    // behavior interface types
    FlyBehavior flyBehavior;
    QuackBehavior quackBehavior;

    public Duck() {
    }

    // Delegate to the behavior class
    public void performFly(){
        flyBehavior.fly();
    }

    // Delegate to the behavior class
    public void performQuack(){
        quackBehavior.quack();
    }
}

Here is the interface that all flying behavior classes implement

public interface FlyBehavior {
    public void fly();
}

Here is the flying behavior implementation for ducks that do fly

public class FlyWithWings implements FlyBehavior {
    public void fly(){
    System.out.println("I'm flying");
    }
}

Here is what I have so far using Python. Below is my Python abstract Duck class

import abc

class Duck:
    __metaclass__=abc.ABCMeta


    FlyBehavior FlyBehavior;
    QuackBehavior QuackBehavior;

    @abc.abstractmethod
    def __init__():
        return

    @abc.abstractmethod
    def performFly():
        return

    @abc.abstractmethod
    def performQuack():
        return

Here is where I'm stuck trying to create the interface, and trying to implement the interface.

@immibis Abstract Base Class, I read about this at this link: docs.python.org/2/library/abc.html — Sparky
– Sparky, Commented Apr 30, 2015 at 1:40

Stack Exchange Broke The Law · Accepted Answer · 2017-02-21 21:20:56Z

6

As Alex Taylor pointed out, Python is a duck-typed language - you don't need to specify the types of things, you just use them.

However, I think his translation of the Java code is wrong. You do not need to use abc here - just use a normal class.

class Duck(object):
    # Like in Java, you don't need to write a __init__ if it's empty

    # You don't need to declare fields either - just use them.

    def performFly(self):
        self.flyBehaviour.fly()

    def performQuack(self):
        self.quackBehaviour.quack()

class FlyWithWings(object):
    def fly(self):
        print "I'm flying"

# Example:
d = Duck()
d.flyBehaviour = FlyWithWings()
d.performFly() # prints "I'm flying"

edited Feb 21, 2017 at 21:20

answered Apr 30, 2015 at 1:47

Stack Exchange Broke The Law

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1 Comment

twk Over a year ago

Shouldn't the last line be: 'd.performFly()' ?

Alex Taylor · Accepted Answer · 2015-04-30 01:39:29Z

6

Python is a duck typed language. You don't need interfaces - you pass in an object and if it supports the method you want it works. If it doesn't have the method it blows up. It doesn't have the compile-time checking that Java has. If you need to check, you do it yourself at run-time. So it should just be:

import abc

class Duck:
    __metaclass__=abc.ABCMeta


    FlyBehavior FlyBehavior;
    QuackBehavior QuackBehavior;

    @abc.abstractmethod
    def __init__():
        return

    @abc.abstractmethod
    def performFly():
        flyBehavior.fly()

    @abc.abstractmethod
    def performQuack():
        quackBehavior.quack()

As a broader point, not all design patterns are applicable to all languages. See Are Design Patterns Missing Language Features.

answered Apr 30, 2015 at 1:39

Alex Taylor

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1 Comment

Stack Exchange Broke The Law Over a year ago

Good point about design patterns not applying to all languages.

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