PYTHON · STEP 1 · INTRO

Python · Intro to the language

Python 3.15 (any 3.10+ works): variables, input, conditionals, loops, lists, dictionaries and functions — each shown first in the JavaScript you know, then in Python.

Step 1 of 5Django© Arthur Bernier Jrrewritten · verified16 code blocks579 words

What you will learn#

Creating Variables in a New Language
Conditionals in a New Language
Looping in a New Language
Collections in a New Language
Writing Functions in a New Language

Getting Started#

Install the current Python 3 from python.org/downloads (3.15 as of this writing; anything from 3.10 up runs everything on this page). Then open a terminal and start the interactive shell:

python3 --versionpython3python3 --version
python3

On Windows the command is usually py or python; on macOS and Linux it is python3. Type exit() (or Ctrl-D) to leave the REPL. To run a file: python3 hello.py.

If you'd rather not install anything yet, any browser-based Python runner works for today's exercises.

Creating Variables#

How we did it in Javascript

// We create constant variables with the const keywordconst myConstant = 5// We create re-assignable variables with the let keywordlet myVariable = "Hello World" // We print variables to the console using console.logconsole.log(myConstant)console.log(myVariable)// We create constant variables with the const keyword
const myConstant = 5
// We create re-assignable variables with the let keyword
let myVariable = "Hello World"

// We print variables to the console using console.log
console.log(myConstant)
console.log(myVariable)

How we do this in Python

# Constants don't exist in Python, but the convention is to name them in ALL_CAPSMY_CONSTANT = 5# Every variable is re-assignable; snake_case is the norm in Pythonmy_variable = "Hello World" # We print variables to the console with the print functionprint(MY_CONSTANT)print(my_variable)# Constants don't exist in Python, but the convention is to name them in ALL_CAPS
MY_CONSTANT = 5
# Every variable is re-assignable; snake_case is the norm in Python
my_variable = "Hello World"

# We print variables to the console with the print function
print(MY_CONSTANT)
print(my_variable)

Receiving Input#

In JavaScript in the browser, input comes from forms or prompt. In Node, reading from the terminal goes through the readline module; the promise-based version reads cleanly with await:

// Node: ask a question in the terminal and wait for the answerimport readline from "node:readline/promises"import { stdin as input, stdout as output } from "node:process" const rl = readline.createInterface({ input, output })const name = await rl.question("Who are you? ")console.log(`Hey there ${name}!`)rl.close()// Node: ask a question in the terminal and wait for the answer
import readline from "node:readline/promises"
import { stdin as input, stdout as output } from "node:process"

const rl = readline.createInterface({ input, output })
const name = await rl.question("Who are you? ")
console.log(`Hey there ${name}!`)
rl.close()

How would we do that in Python?

# input() prints the prompt, waits for a line, and returns it as a stringuser_input = input("Who are you? ") # f-strings interpolate variables into textprint(f"Hey there {user_input}")# input() prints the prompt, waits for a line, and returns it as a string
user_input = input("Who are you? ")

# f-strings interpolate variables into text
print(f"Hey there {user_input}")

input() always returns a string. If you want a number, convert it: age = int(input("Age? ")).

Conditionals#

If statements in Javascript

const num = 5 if (num > 3){    console.log("num is greater than 3")} else if (num > 1){    console.log("num is greater than 1")} else {    console.log("num is 1 or less")}const num = 5

if (num > 3){
    console.log("num is greater than 3")
} else if (num > 1){
    console.log("num is greater than 1")
} else {
    console.log("num is 1 or less")
}

If statements in Python

num = 5 # No parentheses or braces: the colon opens the block and the indentation *is* the blockif num > 3:    print("num is greater than 3")elif num > 1:    print("num is greater than 1")else:    print("num is 1 or less")num = 5

# No parentheses or braces: the colon opens the block and the indentation *is* the block
if num > 3:
    print("num is greater than 3")
elif num > 1:
    print("num is greater than 1")
else:
    print("num is 1 or less")

Comparison and logic operators to know: ==, !=, <, >= work as in JavaScript; &&, ||, ! are spelled and, or, not; and there is no === because == never coerces types in Python.

While Loops#

In Javascript

let counter = 0 //This will loop 10 timeswhile(counter < 10){    console.log(counter)    counter += 1}let counter = 0

//This will loop 10 times
while(counter < 10){
    console.log(counter)
    counter += 1
}

In Python

counter = 0 # this will loop 10 times (Python has no ++, use += 1)while counter < 10:    print(counter)    counter += 1counter = 0

# this will loop 10 times (Python has no ++, use += 1)
while counter < 10:
    print(counter)
    counter += 1

Python's for loop is the one you'll use most. It walks any iterable; range(10) produces 0–9:

for counter in range(10):    print(counter)for counter in range(10):
    print(counter)

10 Minute Exercise#

Write the code to do the following:

Loop 10 times starting the counter at 0
On each loop if the counter is even print "it's even"
If odd, print "meow" if the number is divisible by 3
Otherwise print nothing

Collections#

In Javascript we have arrays and objects

const myArray = [1,2,3] console.log(myArray) //logging the entire arrayconsole.log(myArray[0]) // logging an individual item const myObject = {    cheese: "gouda",    bread: "rye"} console.log(myObject) //logging the entire objectconsole.log(myObject.cheese) // logging a property using dot notationconsole.log(myObject["bread"]) // logging a property using square bracket notationconst myArray = [1,2,3]

console.log(myArray) //logging the entire array
console.log(myArray[0]) // logging an individual item

const myObject = {
    cheese: "gouda",
    bread: "rye"
}

console.log(myObject) //logging the entire object
console.log(myObject.cheese) // logging a property using dot notation
console.log(myObject["bread"]) // logging a property using square bracket notation

In Python we have lists and dictionaries

my_list = [1, 2, 3]print(my_list)     # printing the whole listprint(my_list[0])  # printing an individual itemprint(my_list[-1]) # negative indexes count from the end my_dictionary = {"cheese": "gouda", "bread": "rye"}print(my_dictionary)            # printing the whole dictionaryprint(my_dictionary["cheese"])  # one value; a missing key raises KeyErrorprint(my_dictionary.get("jam", "none"))  # .get returns a default instead of raisingmy_list = [1, 2, 3]
print(my_list)     # printing the whole list
print(my_list[0])  # printing an individual item
print(my_list[-1]) # negative indexes count from the end

my_dictionary = {"cheese": "gouda", "bread": "rye"}
print(my_dictionary)            # printing the whole dictionary
print(my_dictionary["cheese"])  # one value; a missing key raises KeyError
print(my_dictionary.get("jam", "none"))  # .get returns a default instead of raising

There is no dot access for dictionary keys (my_dictionary.cheese is an error) — square brackets or .get() only. Python also has tuples (immutable lists, (1, 2, 3)) and sets ({1, 2, 3}, unique values).

10 Minute Exercise#

Look up Python's list methods in the tutorial and apply them to the following list.

[1,2,3,4,5,6,7,8,9,10][1,2,3,4,5,6,7,8,9,10]

How to find a list's length (len)
How to loop over a list (for item in my_list:)
How to add and remove items from a list (append, insert, pop, remove)

Functions#

in Javascript

const addNums = (x, y) => {    return x + y} console.log(addNums(5,5))const addNums = (x, y) => {
    return x + y
}

console.log(addNums(5,5))

In Python

def add_nums(x, y):    return x + y print(add_nums(5, 5))def add_nums(x, y):
    return x + y

print(add_nums(5, 5))

Two things JavaScript doesn't give you for free: default values and keyword arguments.

def greet(name, greeting="Hello"):    return f"{greeting}, {name}!" print(greet("Arthur"))                      # Hello, Arthur!print(greet("Arthur", greeting="Welcome"))  # Welcome, Arthur!def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

print(greet("Arthur"))                      # Hello, Arthur!
print(greet("Arthur", greeting="Welcome"))  # Welcome, Arthur!

Python's lambda x, y: x + y is the equivalent of a one-expression arrow function, used mostly as an argument to sorted, map and friends.

10 minute exercise#

Write the following functions

sub_nums that takes two arguments and returns their difference
say_hello that takes a name as an argument and says hello to that name
say_hello_adv that takes a dictionary with a name and age property and prints "hello {name}, how does it feel to be {age} years old"
looper takes one list as an argument, loops over the list and prints each item individually

Resources to Learn More#

The Python Tutorial — the official walk-through; chapters 3–5 cover everything on this page
PEP 8 — Style Guide for Python Code — why it's snake_case and four spaces
Built-in functions — print, input, len, range, int, str and the rest

Creator

Written by Arthur Bernier Jr (Big Poppa Code). Arthur Bernier Jr — Big Poppa Code — is a software engineer, educator and entrepreneur: a former CTO at VC-backed startups and engineering lead at a billion-dollar company, mentor to 800+ engineers, now Senior Director of AI, Computer Programming and Cloud Computing at Per Scholas, and the host of the Deeper Than Code podcast. Everything on this page is his original teaching material.

bigpoppacode.io

Verified: Checked 2026-11-02 against docs.python.org/3/tutorial/introduction.html docs.python.org/3/tutorial/controlflow.html docs.python.org/3/tutorial/datastructures.html docs.python.org/3/library/functions.html#input peps.python.org/pep-0008 nodejs.org/api/readline.html#promises-api · exec: ran
Every Python block ran under CPython 3.10.12 (the build machine's newest; nothing here uses syntax newer than 3.6 f-strings) and every JavaScript block under Node 24.13. Both input() examples were fed stdin.