Equivalents in Python and JavaScript. Part 4

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(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(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):

    title='Hello, World!', 
    content='This is our first 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) {

    'title': 'Hello, World!', 
    'content': 'This is our first post.',
    'is_published': true
    '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__(),
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
# 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;
// 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 = ''
    def slug(self):
        return self._slug
    def slug(self, value):
        self._slug = value
post = new Post(1, 'Hello, World!')
post.slug = 'hello-world'

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';

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!

Get the Ultimate Cheat Sheet of Equivalents in Python and JavaScript

Use it for good!

Cover photo by Andre Benz


Equivalents in Python and JavaScript. Part 3

This is the 3rd part of 4-article series about analogies in Python and JavaScript. In the previous parts we covered a large part of the traditional Python 2.7 and JavaScript based on the ECMAScript 5 standard. This time we will start looking into Python 3.6 and JavaScript based on the ECMAScript 6 standard. ECMAScript 6 standard is pretty new and supported only the newest versions of browsers. For older browsers you will need Babel to compile your next-generation JavaScript code to the cross-browser-compatible equivalents. It opens the door to so many interesting things to explore. We will start from string interpolation, unpacking lists, lambda functions, iterations without indexes, generators, and sets!

Variables in strings

The old and inefficient way to build strings with values from variables is this concatenation:

name = 'World'
value = 'Hello, ' + name + '!\nWelcome!'

This can get very sparse and difficult to read. Also it is very easy to miss whitespaces in the sentence around variables.

Since Python version 3.6 and JavaScript based on the ECMAScript 6 standard, you can use so called string interpolation. These are string templates which are filled in with values from variables.

In Python they are also called f-string, because their notation starts with letter "f":

name = 'World'
value = f"""Hello, {name}!

price = 14.9
value = f'Price: {price:.2f} €'  # 'Price: 14.90 €'

In JavaScript string templates start and end with backticks:

name = 'World';
value = `Hello, ${name}!

price = 14.9;
value = `Price ${price.toFixed(2)} €`;  // 'Price: 14.90 €'

Note that string templates can be of a single line as well as of multiple lines. For f-strings in Python you can pass the format for variables, but you can't call methods of a variable unless they are properties and call getter methods.

Unpacking lists

Python and now JavaScript has an interesting feature to assign items of sequences into separate variables. For example, we can read the three values of a list into variables a, b, and c with the following syntax:

[a, b, c] = [1, 2, 3]

For tuples the parenthesis can be omitted. The following is a very popular way to swap values of two variables in Python:

a = 1
b = 2
a, b = b, a  # swap values

With the next generation JavaScript this can also be achieved:

a = 1;
b = 2;
[a, b] = [b, a];  // swap values

In Python 3.6 if we have an unknown number of items in a list or tuple, we can assign them to a tuple of several values while also unpacking the rest to a list:

first, second, *the_rest = [1, 2, 3, 4]
# first == 1
# second == 2
# the_rest == [3, 4]

This can also be done with JavaScript (ECMAScript 6):

[first, second, ...the_rest] = [1, 2, 3, 4];
// first === 1
// last === 2
// the_rest === [3, 4]

Lambda functions

Python and JavaScript have a very neat functionality to create functions in a single line. These functions are called lambdas. Lambdas are very simple functions that take one or more arguments and return some calculated value. Usually lambdas are used when you need to pass a function to another function as a callback or as a function to manipulate every separate elements in a sequence.

In Python, you would define a lambda using the lambda keyword, like this:

sum = lambda x, y: x + y
square = lambda x: x ** 2

In JavaScript lambdas use the => notation. If there are more than one arguments, they have to be in parenthesis:

sum = (x, y) => x + y;
square = x => Math.pow(x, 2);

Iteration without indexes

Many programming languages allow iterating through a sequence only by using indexes and incrementing their values. Then to get an item at some position, you would read it from an array, for example:

for (i=0; i<items.length; i++) {

This is not a nice syntax and is very technical - it doesn't read naturally. What we really want is just to grab each value from the list. And Python has a very neat possibility just to iterate through the elements:

for item in ['A', 'B', 'C']:

In the modern JavaScript this is also possible with the for..of operator:

for (let item of ['A', 'B', 'C']) {

You can also iterate through a string characters in Python:

for character in 'ABC':

And in the modern JavaScript:

for (let character of 'ABC') {


Python and modern JavaScript has a possibility to define special functions through which you can iterate. With each iteration they return the next generated value in a sequence. These functions are called generators. With generators you can get numbers in a range, lines from a file, data from different paginated API calls, fibonacci numbers, and any other dynamicly generated sequences.

Technically generators are like normal functions, but instead of returning a value, they yield a value. This value will be returned for one iteration. And this generation happens as long as the end of the function is reached.

To illustrate that, the following Python code will create a generator countdown() which will return numbers from the given one back to 1, (like 10, 9, 8, ..., 1):

def countdown(counter):
    while counter > 0:
        yield counter
        counter -= 1
for counter in countdown(10):

Exactly the same can be achieved in modern JavaScript, but notice the asterisk at the function statement. It defines that it's a generator:

function* countdown(counter) {
    while (counter > 0) {
        yield counter;
for (let counter of countdown(10)) {


We already had a look at lists, tuples and arrays. But here is another type of data - sets. Sets are groups of elements that ensure that each element there exists only once. Set theory also specifies set operations like union, intersection, and difference, but we won't cover them here today.

This is how to create a set, add elements to it, check if a value exists, check the total amount of elements in a set, and iterate through its values, and remove a value using Python:

s = set(['A'])
s.add('B'); s.add('C')
'A' in s
len(s) == 3
for elem in s:

Here is how to achieve the same in modern JavaScript:

s = new Set(['A']);
s.has('A') === true;
s.size === 3;
for (let elem of s.values()) {

The Takeaways

  • String interpolation or literal templates allows you to have much cleaner and nicer code even with a possibility to have multiple lines of text.
  • You can iterate through elements in a sequence or group without using indexes.
  • Use generators when you have a sequence of nearly unlimited elements.
  • Use sets when you want to ensure fast check if data exists in a collection.
  • Use lambdas when you need short and clear single-line functions.

As you know from the previous parts, I am offering a cheat sheet with the whole list of equivalents in Python and JavaScript, both, time honored and future proof. To have something printed in front of your eyes is much more convenient than switching among windows or scrolling up and down until you find what you exactly were searching for. So I suggest you to get the cheat sheet and use it for good!

Get the Ultimate Cheat Sheet of Equivalents in Python and JavaScript

In the next and last part of the series, we will have a look at function arguments, classes, inheritance, and properties. Stay tuned!

Cover photo by Alex Knight


Equivalents in Python and JavaScript. Part 2

Last time we started a new series of articles about analogies in Python and JavaScript. We had a look at lists, arrays, dictionaries, objects, and strings, conditional assignments, and parsing integers. This time we will go through more interesting and more complex things like serializing dictionaries and lists to JSON, operations with regular expressions, as well as raising and catching errors.


When working with APIs it is very usual to serialize objects to JSON format and be able to parse JSON strings.

In Python it is done with the json module like this:

import json
json_data = json.dumps(dictionary, indent=4)
dictionary = json.loads(json_data)

Here we'll indent the nested elements in the JSON string by 4 spaces.

In JavaScript there is a JSON object that has methods to create and parse JSON strings:

json_data = JSON.stringify(dictionary, null, 4);
dictionary = JSON.parse(json_data);

Splitting strings by regular expressions

Regular expressions are multi-tool that once you master, you can accomplish lots of things.

In the last article, we saw how one can join lists of strings into a single string. But how can you split a long string into lists of strings? What if the delimiter can be not a single character as the comma, but a range of possible variations? This can be done with regular expressions and the split() method.

In Python, the split() method belongs to the regular expression pattern object. This is how you could split a text string into sentences by punctuation marks:

import re

# One or more characters of "!?." followed by whitespace
delimiter = re.compile(r'[!?\.]+\s*')

text = "Hello!!! What's new? Follow me."
sentences = delimiter.split(text)
# sentences == ['Hello', "What's new", 'Follow me', '']

In JavaScript the split() method belongs to the string:

// One or more characters of "!?." followed by whitespace
delimiter = /[!?\.]+\s*/;

text = "Hello!!! What's new? Follow me.";
sentences = text.split(delimiter)
// sentences === ["Hello", "What's new", "Follow me", ""]

Matching regular expression patterns in strings

Regular expressions are often used to validate data from the forms.

For example, to validate if the entered email address is correct, you would need to match it against a regular expression pattern. In Python that would look like this:

import re

# name, "@", and domain
pattern = re.compile(r'([\w.+\-]+)@([\w\-]+\.[\w\-.]+)')

match = pattern.match('hi@example.com')
# match.group(0) == 'hi@example.com'
# match.group(1) == 'hi'
# match.group(2) == 'example.com'

If the text matches the pattern, it returns a match object with the group() method to read the whole matched string, or separate captures of the pattern that were defined with the parenthesis. 0 means getting the whole string, 1 means getting the match in the first group, 2 means getting the match in the second group, and so on. If the text doesn't match the pattern, the None value will be returned.

In JavaScript the match() method belongs to the string and it returns either a match object, or null. Pretty similar:

// name, "@", and domain
pattern = /([\w.+\-]+)@([\w\-]+\.[\w\-.]+)/;

match = 'hi@example.com'.match(pattern);
// match[0] === 'hi@example.com'
// match[1] === 'hi'
// match[2] === 'example.com'

The match object in JavaScript acts as an array. Its value at the zeroth position is the whole matched string. The other indexes correspond to the captures of the pattern defined with the parenthesis.

Moreover, sometimes you need to search if a specific value exists in a string and at which letter position it will be found. That can be done with the search() method.

In Python this method belongs to the regular expression pattern and it returns the match object. The match object has the start() method telling at which letter position the match starts:

text = 'Say hi at hi@example.com'
first_match = pattern.search(text)
if first_match:
    start = first_match.start()  # start == 10

In JavaScript the search() method belongs to the string and it returns just an integer telling at which letter position the match starts. If nothing is found, -1 is returned:

text = 'Say hi at hi@example.com';
first_match = text.search(pattern);
if (first_match > -1) {
    start = first_match;  // start === 10

Replacing patterns in strings using regular expressions

Replacing with regular expressions usually happen when cleaning up data, or adding additional features. For example, we could take some text and make all email addresses clickable.

Python developers would use the sub() method of the regular expression pattern:

html = pattern.sub(
    r'<a href="mailto:\g<0>">\g<0></a>',
    'Say hi at hi@example.com',
# html == 'Say hi at <a href="mailto:hi@example.com">hi@example.com</a>'

JavaScript developers would use the replace() method of the string:

html = 'Say hi at hi@example.com'.replace(
    '<a href="mailto:$&">$&</a>',
// html === 'Say hi at <a href="mailto:hi@example.com">hi@example.com</a>'

In Python the captures, also called as "backreferences", are accessible in the replacement string as \g<0>, \g<1>, \g<2>, etc. In JavaScript the same is accessible as $&, $1, $2, etc. Backreferences are usually used to wrap some strings or to switch places of different pieces of text.

It is also possible to replace a match with a function call. This can be used to do replacements within replacements or to count or collect some features of a text. For example, using replacements with function calls in JavaScript, I once wrote a fully functional HTML syntax highlighter.

Here let's change all email addresses in a text to UPPERCASE.

In Python, the replacement function receives the match object. We can use its group() method to do something with the matched text and return a text as a replacement:

text = pattern.sub(
    lambda match: match.group(0).upper(), 
    'Say hi at hi@example.com',
# text == 'Say hi at HI@EXAMPLE.COM'

In JavaScript the replacement function receives the whole match string, the first capture, the second capture, etc. We can do what we need with those values and then return some string as a replacement:

text = 'Say hi at hi@example.com'.replace(
    function(match, p1, p2) {
        return match.toUpperCase();
// text === 'Say hi at HI@EXAMPLE.COM'

Error handling

Contrary to Python, client-side JavaScript normally isn't used for saving or reading files or connecting to remote databases. So try..catch blocks are quite rare in JavaScript compared to try..except analogy in Python.

Anyway, error handling can be used with custom user errors implemented and raised in JavaScript libraries and caught in the main code.

The following example in Python shows how to define a custom exception class MyException, how to raise it in a function, and how to catch it and handle in a try..except..finally block:

class MyException(Exception):
    def __init__(self, message):
        self.message = message
    def __str__(self):
        return self.message
def proceed():
    raise MyException('Error happened!')

except MyException as err:
    print('Sorry! {}'.format(err))

The following example in JavaScript does exactly the same: here we define a MyException class, throw it in a function, and catch it and handle in the try..catch..finally block.

function MyException(message) {
   this.message = message;
   this.toString = function() {
       return this.message;

function proceed() {
    throw new MyException('Error happened!');

try {
} catch (err) {
    if (err instanceof MyException) {
        console.log('Sorry! ' + err);
} finally {

The MyException class in both languages has a parameter message and a method to represent itself as a string using the value of the message.

Of course, exceptions should be raised/thrown just in the case of errors. And you define what is an error in your module design.

The Takeaways

  • Serialization to JSON is quite straightforward in both, Python and JavaScript.
  • Regular expressions can be used as multi-tools when working with textual data.
  • You can do replacements with function calls in both languages.
  • For more sophisticated software design you can use custom error classes.

As I mentioned last time, you can grab a side-by-side comparison of Python and JavaScript that I compiled for you (and my future self). Side by side you will see features from traditional list, array, dictionary, object, and string handling to modern string interpolation, lambdas, generators, sets, classes, and everything else. Use it for good.

Get the Ultimate Cheat Sheet of Equivalents in Python and JavaScript

In the next part of the series, we will have a look at textual templates, list unpacking, lambda functions, iteration without indexes, generators, and sets. Stay tuned!

Cover photo by Benjamin Hung.