Equivalents in Python and JavaScript. Part 4

#advanced #intermediate #javascript #python

In the last three parts of the series of articles about analogies in Python and JavaScript, we explored lots of interesting concepts like serializing to JSON, error handling, using regular expressions, string interpolation, generators, lambdas, and many more. This time we will delve into function arguments, creating classes, using class inheritance, and defining getters and setters of class properties.

Function arguments

Python is very flexible with argument handling for functions: you can set default values there, allow a flexible amount of positional or keyword arguments (*args and **kwargs). When you pass values to a function, you can define by name to which argument that value should be assigned. All that in a way is now possible in JavaScript too.

Default values for function arguments in Python can be defined like this:

from pprint import pprint

def report(post_id, reason='not-relevant'):
    pprint({'post_id': post_id, 'reason': reason})

report(42)
report(post_id=24, reason='spam')

In JavaScript that can be achieved similarly:

function report(post_id, reason='not-relevant') {
    console.log({post_id: post_id, reason: reason});
}

report(42);
report(post_id=24, reason='spam');

Positional arguments in Python can be accepted using the * operator like this:

from pprint import pprint

def add_tags(post_id, *tags):
    pprint({'post_id': post_id, 'tags': tags})

add_tags(42, 'python', 'javascript', 'django')

In JavaScript positional arguments can be accepted using the ... operator:

function add_tags(post_id, ...tags) {
    console.log({post_id: post_id, tags: tags});
}

add_tags(42, 'python', 'javascript', 'django');

Keyword arguments are often used in Python when you want to allow a flexible amount of options:

from pprint import pprint

def create_post(**options):
    pprint(options)

create_post(
    title='Hello, World!', 
    content='This is our first post.',
    is_published=True,
)
create_post(
    title='Hello again!',
    content='This is our second post.',
)

A common practice to pass multiple optional arguments to a JavaScript function is through a dictionary object, for example, options.

function create_post(options) {
    console.log(options);
}

create_post({
    'title': 'Hello, World!', 
    'content': 'This is our first post.',
    'is_published': true
});
create_post({
    'title': 'Hello again!', 
    'content': 'This is our second post.'
});

Classes and inheritance

Python is an object-oriented language. Since ECMAScript 6 standard support, it's also possible to write object-oriented code in JavaScript without hacks and weird prototype syntax.

In Python you would create a class with the constructor and a method to represent its instances textually like this:

class Post(object):
    def __init__(self, id, title):
        self.id = id
        self.title = title

    def __str__(self):
        return self.title

post = Post(42, 'Hello, World!')
isinstance(post, Post) == True
print(post)  # Hello, World!

In JavaScript to create a class with the constructor and a method to represent its instances textually, you would write:

class Post {
    constructor (id, title) {
        this.id = id;
        this.title = title;
    }
    toString() {
        return this.title;
    }
}

post = new Post(42, 'Hello, World!');
post instanceof Post === true;
console.log(post.toString());  // Hello, World!

Now we can create two classes Article and Link in Python that will extend the Post class. Here you can also see how we are using super to call methods from the base Post class.

class Article(Post):
    def __init__(self, id, title, content):
        super(Article, self).__init__(id, title)
        self.content = content

class Link(Post):
    def __init__(self, id, title, url):
        super(Link, self).__init__(id, title)
        self.url = url

    def __str__(self):
        return '{} ({})'.format(
            super(Link, self).__str__(),
            self.url,
        )

article = Article(1, 'Hello, World!', 'This is my first article.')
link = Link(2, 'DjangoTricks', 'https://djangotricks.blogspot.com')
isinstance(article, Post) == True
isinstance(link, Post) == True
print(link)
# DjangoTricks (https://djangotricks.blogspot.com)

In JavaScript the same is also doable by the following code:

class Article extends Post {
    constructor (id, title, content) {
        super(id, title);
        this.content = content;
    }
}

class Link extends Post {
    constructor (id, title, url) {
        super(id, title);
        this.url = url;
    }
    toString() {
        return super.toString() + ' (' + this.url + ')';
    }
}

article = new Article(1, 'Hello, World!', 'This is my first article.');
link = new Link(2, 'DjangoTricks', 'https://djangotricks.blogspot.com');
article instanceof Post === true;
link instanceof Post === true;
console.log(link.toString());
// DjangoTricks (https://djangotricks.blogspot.com)

Class properties: getters and setters

In object oriented programming, classes can have attributes, methods, and properties. Properties are a mixture of attributes and methods. You deal with them as attributes, but in the background they call special getter and setter methods to process data somehow before setting or returning to the caller.

The basic wireframe for getters and setters of the slug property in Python would be like this:

class Post(object):
    def __init__(self, id, title):
        self.id = id
        self.title = title
        self._slug = ''

    @property
    def slug(self):
        return self._slug

    @slug.setter
    def slug(self, value):
        self._slug = value

post = new Post(1, 'Hello, World!')
post.slug = 'hello-world'
print(post.slug)

In JavaScript getters and setters for the slug property can be defined as:

class Post {
    constructor (id, title) {
        this.id = id;
        this.title = title;
        this._slug = '';
    }

    set slug(value) {
        this._slug = value;
    }

    get slug() {
        return this._slug;
    }
}

post = new Post(1, 'Hello, World!');
post.slug = 'hello-world';
console.log(post.slug);

The Takeaways

In both languages, you can define default argument values for functions.
In both languages, you can pass a flexible amount of positional or keyword arguments for functions.
In both languages, object-oriented programming is possible.

As you might have noticed, I am offering a cheat sheet with the full list of equivalents in Python and JavaScript that you saw here described. At least for me, it is much more convenient to have some printed sheet of paper with valuable information next to my laptop, rather than switching among windows or tabs and scrolling to get the right piece of snippet. So I encourage you to get this cheat sheet and improve your programming!

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Equivalents in Python and JavaScript
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