print(), variables, f-strings and input(): your first program that talks back

In lesson 1 you met Python's data types. Now you'll put them to work: show values with print(), keep them in variables and change them, build sentences with f-strings, and ask the user a question with input(). By the end you'll write a small program that talks to its user. Every example runs right here: press Run, then press Edit and change it.

Your first line: print()

print() shows whatever you put in its brackets. Text goes in quotes:

print("Hello, world!")
print("I'm learning Python.")
print(7 + 3)

Three things to notice:

• Text in quotes is a string. You can use "double" or 'single' quotes, as long as both ends match.
• Numbers have no quotes. 7 + 3 is maths, so Python prints 10.
• Each print() starts a new line.

You can give print() several values, separated by commas. It puts a space between them:

print("Apples:", 4, "Pears:", 2)

Variables: a name for a value

A variable keeps a value under a name, so you can use it later. You make one with =:

name = "Ada"
age = 36

print(name)
print(age)
print(name, "is", age, "years old")

Unlike JavaScript, there's no let, const or var. You just write the name, =, and the value. You can give a variable a new value whenever you like:

score = 0
print(score)

score = score + 10
print(score)

score += 5   # the short way to write: score = score + 5
print(score)

Everything after # is a comment: a note for people, which Python skips.

A quick reminder: strings and numbers

From lesson 1: "7" in quotes is a str, 7 without quotes is an int. That matters as soon as you use +:

print(7 + 7)       # 14: adding numbers
print("7" + "7")   # 77: joining two strings
print("ha" * 3)    # hahaha: a string repeated

Keep this in mind: it's the reason for the most common input() mistake further down.

f-strings: values inside a sentence

Joining text with + gets messy fast. Python has a better way, the f-string: put f before the quotes, and write variables in {curly brackets}:

name = "Ada"
age = 36

print(f"My name is {name} and I am {age} years old.")
print(f"Next year I'll be {age + 1}.")

Anything inside {} is real Python, so you can even do sums there, like {age + 1}. This is the way most Python code builds text today.

input(): ask the user something

input() stops, waits for the user to type an answer, and gives it back to you. The text in the brackets is the question.

Here the answers come from the Input box under the code: one line for each input(). Type your name there, then press Run:

name = input("What's your name? ")
print(f"Nice to meet you, {name}!")

input() always gives you a string

This is the most common surprise for beginners. Even if the user types 12, input() gives you the text "12", not the number. To do maths with it, turn it into a number with int() (whole numbers) or float() (decimals).

Put two lines in the Input box, for example Ada and 36, and run it:

name = input("Name: ")
age = int(input("Age: "))

print(f"{name}, in ten years you'll be {age + 10}.")

int(input("Age: ")) reads from the inside out: first input() gets the text, then int() turns it into a number.

Common mistakes

Exercise

product = input("Product: ")
price = float(input("Price: "))
amount = int(input("How many: "))

total = price * amount
print(f"{amount} x {product} = {total} euros")

Quick check

a = "5"
b = "5"
print(a + b)