How I learned Python as Javascript dev

Python and Javascript share many similarities. They are both interpreted languages, dynamically typed and support both object-oriented and functional programming.

But syntax has a lot of differences, starting with tab indentation that plays big a role in Python and missing curly brackets from C-family style.

So now let’s dive into it! 🏊🏼‍♂️

Important: If you have Copilot then turn it off 🛑! It will only do harm in the beginning!

Variables

In Python, declaring variable is very short. Just give it a name. No const, let, var etc..

age = 21

full_name = 'John Doe'

Naming

Python is using snake_case instead of the common camelCase from Javascript. I personally don’t care, both are fine. One saves chars but hurts readability. Common trade-off in software development. 🤷

Comments

Comments just # instead //.

# this is a comment in Python

Primitive data types

Almost the same, just boolean is uppercase 🥲.

salary = 50000 # int
price = 13.55 # float
name = 'John' # str
is_active = True # bool

Arithmetic operators

Numbers in Python work close to Javascript. Most operators are exactly the same: Addition, Subtraction, Modulo, Multiplication, Division.

But there is one extra // called Floor Division. Let’s have a look at the example to understand:

print(12.4 / 2) # 6.2
print(12.4 // 2) # 6.0

In floor division, the result is floored to the nearest smaller integer. It is also known as integer division.

Comparison operators

No difference here in syntax, but a big difference in type comparison.

print('10' == 10) # False

As we all know in magnificent Javascript 😅 it will be true and you must put triple equal sign === to make it false.

Logical operators

It took some time to get used to this. Python doesn’t have &&, || and !. Instead you use keywords and, or, not.

result = True and False
print(result) # False

Strings

There is a lot of cool stuff when working with strings in Python. One thing I really liked is string multiplication.

print('W' * 3) # WWW

In Javascript it will be just Nan. Also search in string is much simpler:

print('World' in 'Hello World') # True

Working with substrings is a pleasure. Just look at this beauty 😍!

text = 'Hello World'
print(text[0:5]) # 'Hello'

Array

Arrays syntax is exactly the same. No difference here.

my_list = [1, 2.5, "A string", True] # you can put different data types
my_list[0] # 1

One cool thing you can do with arrays in Python is concatenation with just + sign.

arr1 = [1, 2, 3]
arr2 = [4, 5, 6]
print(arr1 + arr2) # [1, 2, 3, 4, 5, 6]

Conditional statements

If-else block in Python looks like this. Mind the : in the end and indentation!

age = 18

if age >= 18:
    print("Adult")
else:
    print("Too young")

One thing that I find very ugly in Python is the ternary operator.

age = 18

print('Adult') if age >= 18 else print('Too young')

I know it reads more like human language but I just find it confusing because the condition is in the middle 😢.

// In js
const age = 18;
age >= 18 ? console.log('Adult') : console.log('Too young');

Functions

Simple syntax just use def instead of function and don’t forget no curly brackets no semicolons:

def add(a, b):
    return a + b

In Python there is also a lambda keyword for one line functions.

add = lambda a, b : a + b
add(1,2) # 3

In Python the number of arguments passed to the function should match the number of parameters defined. Otherwise it will be an error! 💥

Loops

In Javascript we have many ways to iterate (foreach, forof, forin, for, while). But there are only two loops in Python: for and while.

for i in range(10): # a sequence from 0 to 9
  print(i)

count = 0
while count < 10: # same with while
  print(count)
  count += 1

Note that we don’t even need round parenthesis ()!

Very handy builtin function is range. It defines a sequence. You can also define start and step.

for i in range(1, 10, 2): # a sequence from 1 to 10 with step 2
  print(i) # 1 3 5 7 9

Iterating over array is extremely easy and logical.

arr = [1, 2, 3]
for i in arr:
  print(i) # 1 2 3

None vs null

Javascript has a horrible mess with null and undefined values. Which are the same but not really the same. In Python you use None instead.

Type operators

Unfortunately Javascript has very sad situation when it comes to type checking 😶. Too much stuff goes wrong. For example, an array is an object and null is also an object.

function func() {}
const obj = {};

console.log(typeof 10); // number
console.log(typeof 'name'); // string
console.log(typeof [1, 2, 3]); // object!
console.log(typeof true); // boolean
console.log(typeof func); // function
console.log(typeof obj); // object
console.log(typeof null); // object!!
console.log(typeof undefined); // undefined

While in Python you don’t have this problem and everything is very clear.

def func():
    pass

obj = {}

print(type(10)) #int
print(type('name')) # str
print(type([1,2,3])) # list
print(type(True)) # bool
print(type(func)) # function
print(type(obj)) # dict
print(type(None)) # NoneType

Classes

Classes in Python work similarly as in Javascript. Defined by keyword class and class name. Constructor is called in Python __init__.

The first argument is self which is a replacement of this to access the current class instance. This is a bit weird for me because you must define it as an argument. 😕

class Person:
    def __init__(self, first_name, last_name):
        self.__first_name = first_name
        self.__last_name = last_name

    def get_full_name(self):
        return self.__first_name + ' ' + self.__last_name

person = Person('John', 'Doe')

print(person.get_full_name()) # John Doe

Double underscore in the beginning of a class property name is needed to make property private. In Python it will give you an error if you try to access private fields.

If you define a variable outside of constructor, it becomes a class variable and it’s shared across all class instances!

class Person:
    country = 'USA'  # class variable

    def __init__(self, name):
        self.name = name  # instance variable


person1 = Person('John Doe')
person2 = Person('Jane Doe')

Person.country = 'Canada' # change class variable

print(person1.name)  # John Doe
print(person1.country)  # Canada

print(person2.name)  # Jane Doe
print(person2.country)  # Canada

Builtin functions

Python has very rich standard library with many utility functions. I will show just some of them.

len() returns length of array or string.

len([1, 2, 3]) # 3
len("Hello") # 5

sum() returns sums of items of an iteratable

sum([10, 10, 10]) # 30

id() returns identity of an object

arr1 = [1, 2, 3]
arr2 = [1, 2 ,3]

arr1 == arr2 # True
id(arr1) == id(arr2) # False

When we compared two arrays with the same items the equality check returned true, because they are the same. But when we do identity check it returns false, because each array points to a different index in memory (not the same instance).

map() returns an iterator that applies function to every item of iterable.

def double(element):
  return element * 2

nums = [1, 2, 3, 4]
doubled = map(double, nums)

print(list(doubled)) # 2 4 6 8

That’s it

Thank you for reading, I hope it was interesting!

I plan to add more content about me learning Python. Stay tuned and see you soon! 🐍