Python for JavaScript Developers: Learn Python Using What You Already Know

JavaScript vs Python, side by side

Each pair below does the same job. Start with the code you know, then see how Python expresses it.

Variables: let, const and undefined

let count = 0;
const limit = 10;
let missing;            // undefined

count = 0
LIMIT = 10              # constant by convention only
missing = None

Python has no let or const: assignment creates the name, and an uppercase name is only a convention. There is no undefined: reading an unassigned name raises NameError, a missing key raises KeyError, and None is an explicit empty value.

Objects vs dictionaries

const user = { name: "Ada", age: 36 };
user.email = "ada@example.com";
delete user.age;
console.log("name" in user);

user = {"name": "Ada", "age": 36}
user["email"] = "ada@example.com"
del user["age"]
print("name" in user)

Dictionaries use [] with string keys, never dot access, and user.get("age") returns None instead of raising KeyError. A dictionary holds data only: for data with behavior, define a class.

map/filter vs comprehensions

const doubled = nums
  .filter(n => n > 0)
  .map(n => n * 2);

doubled = [n * 2 for n in nums if n > 0]

square = lambda n: n * n

A comprehension replaces most filter and map chains. Python also has lambda, but it is limited to a single expression, so use def for anything longer; there is no arrow-function syntax. Python's built-in versions return lazy iterators, not lists.

async/await

async function loadAll(urls) {
  return Promise.all(urls.map(fetchText));
}

import asyncio

async def load_all(urls):
    return await asyncio.gather(*(fetch_text(url) for url in urls))

async and await look alike, but a Python coroutine does not start until it is awaited or scheduled, while a JavaScript Promise starts running immediately. Python gives you no running event loop by default: start one with asyncio.run(...).