Python Fundamentals for a JavaScript Developer

I'll guide you through Python fundamentals by comparing concepts with JavaScript. Let's start!

1. Hello World & Basic Syntax

JavaScript

console.log("Hello World");
let x = 5;

Python

print("Hello World")
x = 5  # No semicolon, no let/const

Key Differences:

• No semicolons in Python
• Indentation matters (replaces curly braces)
• Comments use # instead of //

2. Variables & Data Types

JavaScript

let name = "Alice";  // string
let age = 30;        // number
let isStudent = true; // boolean
let scores = [95, 87, 91]; // array
let person = {        // object
    name: "Bob",
    age: 25
};
let nothing = null;
let notDefined = undefined;

Python

name = "Alice"        # str
age = 30              # int (or float for decimals)
is_student = True     # bool (capital T/F)
scores = [95, 87, 91] # list (mutable)
person = {            # dict (dictionary)
    "name": "Bob",
    "age": 25
}
nothing = None        # Python's null/undefined

Key Differences:

• Python uses snake_case (not camelCase)
• True/False capitalized
• None instead of null/undefined
• Lists ≈ Arrays, Dicts ≈ Objects

3. Control Flow

JavaScript

// If-else
if (age >= 18) {
    console.log("Adult");
} else if (age >= 13) {
    console.log("Teen");
} else {
    console.log("Child");
}

// For loop
for (let i = 0; i < 5; i++) {
    console.log(i);
}

// While loop
let count = 0;
while (count < 5) {
    console.log(count);
    count++;
}

Python

# If-else (indentation instead of braces)
if age >= 18:
    print("Adult")
elif age >= 13:  # NOT else if
    print("Teen")
else:
    print("Child")

# For loop (more like for...of in JS)
for i in range(5):  # range(5) = [0, 1, 2, 3, 4]
    print(i)

# Iterate over list (like for...of)
for score in scores:
    print(score)

# While loop
count = 0
while count < 5:
    print(count)
    count += 1  # No ++ operator in Python

4. Functions

JavaScript

// Function declaration
function add(a, b) {
    return a + b;
}

// Arrow function
const multiply = (a, b) => a * b;

// Default parameters
function greet(name = "Guest") {
    return `Hello ${name}`;
}

Python

# Function definition (def instead of function)
def add(a, b):
    return a + b  # Indented body

# Lambda functions ≈ Arrow functions
multiply = lambda a, b: a * b

# Default parameters
def greet(name="Guest"):
    return f"Hello {name}"  # f-strings like template literals

# Multiple return values (tuples)
def get_coordinates():
    return 10, 20  # Returns a tuple (10, 20)

x, y = get_coordinates()  # Destructuring assignment

5. Data Structures Comparison

Arrays/Lists

// JavaScript Arrays
let arr = [1, 2, 3];
arr.push(4);          // [1, 2, 3, 4]
arr.pop();            // [1, 2, 3]
let sliced = arr.slice(0, 2);  // [1, 2]

# Python Lists
arr = [1, 2, 3]
arr.append(4)         # [1, 2, 3, 4]
arr.pop()             # [1, 2, 3] (removes last)
sliced = arr[0:2]     # [1, 2] (slicing syntax)
arr.insert(1, 99)     # [1, 99, 2, 3]

# List comprehension (unique to Python)
squares = [x**2 for x in range(5)]  # [0, 1, 4, 9, 16]

Objects/Dictionaries

// JavaScript Objects
let person = {
    name: "Alice",
    age: 30,
    greet() {
        return `Hello, I'm ${this.name}`;
    }
};
console.log(person.name);
console.log(person["age"]);

# Python Dictionaries
person = {
    "name": "Alice",
    "age": 30,
    "greet": lambda self: f"Hello, I'm {self['name']}"
}
print(person["name"])  # Access with brackets
print(person.get("age"))  # Safer access

# Methods don't naturally have 'this' context
# Usually you'd use classes for that (see below)

6. Classes & OOP

JavaScript (ES6+)

class Person {
    constructor(name, age) {
        this.name = name;
        this.age = age;
    }

    greet() {
        return `Hello, I'm ${this.name}`;
    }

    static species = "Human";
}

const alice = new Person("Alice", 30);

Python

class Person:
    species = "Human"  # Class attribute (static)

    def __init__(self, name, age):  # Constructor
        self.name = name  # Instance attribute
        self.age = age

    def greet(self):  # Methods always have self parameter
        return f"Hello, I'm {self.name}"

    @staticmethod
    def static_method():
        return "This is static"

alice = Person("Alice", 30)
print(alice.greet())  # No parentheses needed for self when calling

7. Modules & Imports

JavaScript (ES6 Modules)

// math.js
export const PI = 3.14159;
export function add(a, b) { return a + b; }

// main.js
import { PI, add } from './math.js';
import * as math from './math.js';

Python

# math.py
PI = 3.14159
def add(a, b):
    return a + b

# main.py
from math import PI, add
import math  # Then use math.PI, math.add
import math as m  # Alias

8. Error Handling

JavaScript

try {
    throw new Error("Something went wrong");
} catch (error) {
    console.error(error.message);
} finally {
    console.log("Cleanup");
}

Python

try:
    raise Exception("Something went wrong")
except Exception as e:  # 'as' instead of variable declaration
    print(f"Error: {e}")
finally:
    print("Cleanup")

9. Async Programming

JavaScript (Promises/Async-Await)

// Promise
fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(data => console.log(data));

// Async/await
async function getData() {
    const response = await fetch(url);
    return await response.json();
}

Python (Async/Await)

import asyncio
import aiohttp  # External library for HTTP

async def fetch_data(url):
    async with aiohttp.ClientSession() as session:
        async with session.get(url) as response:
            return await response.json()

# Run async function
asyncio.run(fetch_data('https://api.example.com/data'))

10. Common Patterns & Tips

1. Type Checking (Python is dynamically typed but has type hints)

def add(a: int, b: int) -> int:  # Type hints (optional)
    return a + b

2. String Formatting (multiple ways)

name = "Alice"
# f-strings (Python 3.6+, like template literals)
print(f"Hello {name}")

# .format() method
print("Hello {}".format(name))

# % formatting (older style)
print("Hello %s" % name)

3. Tuple vs List

# List - mutable
my_list = [1, 2, 3]
my_list[0] = 99  # OK

# Tuple - immutable
my_tuple = (1, 2, 3)
# my_tuple[0] = 99  # ERROR!

4. Sets (unique unordered collection)

my_set = {1, 2, 3, 3, 2}  # {1, 2, 3} (duplicates removed)
another_set = set([1, 2, 3, 4])  # Alternative creation

Quick Reference Table

JavaScript	Python	Notes
let x = 5;	x = 5	No declaration keywords
const arr = []	arr = []	No const, just assign
null / undefined	None	Single null value
=== strict equality	== and is	== value, is identity
array.length	len(list)	Function, not property
array.map()	List comprehensions	[x*2 for x in arr]
for (let i=0; i<n; i++)	for i in range(n)	Different pattern
function fn() {}	def fn():	def keyword
obj.property	dict["key"] or obj.attr	Depends on type
class MyClass {}	class MyClass:	Colon and indentation

Practice Exercise

Convert this JavaScript code to Python:

function filterEvenSquares(numbers) {
    return numbers
        .filter(n => n % 2 === 0)
        .map(n => n ** 2);
}

const result = filterEvenSquares([1, 2, 3, 4, 5]);
console.log(result); // [4, 16]

Python solution:

def filter_even_squares(numbers):
    return [n**2 for n in numbers if n % 2 == 0]

result = filter_even_squares([1, 2, 3, 4, 5])
print(result)  # [4, 16]

Next Steps

1. Install Python and a good IDE (VS Code with Python extension works well)
2. Practice by rewriting your JS projects in Python
3. Explore Python-specific features: decorators, generators, context managers
4. Learn popular libraries:
   • Web: Flask/Django (Express equivalents)
   • Data: NumPy, Pandas
   • AI/ML: TensorFlow, PyTorch

The main mindset shift: Python emphasizes readability and simplicity over cleverness. You'll write less code to accomplish the same tasks!