Hello, Python

Step 1: Your first program

A program is a list of instructions. The computer reads them from top to bottom and does each one. The first instruction most people learn is print(), which shows something on the screen.

print("Hello, world!")

Hello, world!

The words inside the quotation marks are the text you want to show. Change them to your own name and run it again.

Try it: Print two lines, one after the other. Each print() starts a new line.

Step 2: Variables are labeled boxes

Often you want to keep a value so you can use it later. A variable is a name that holds a value. The = sign puts the value on the right into the name on the left.

name = "Amara"
age = 12
print(name)
print(age)

Amara
12

Read age = 12 as “age gets 12”. Good variable names are lowercase, use underscores instead of spaces (favorite_color), and say what the value is for.

Variables can change. If you write age = 13 later, the old value is replaced.

Step 3: Kinds of data

Python treats different kinds of information differently. The type() function tells you which kind you have.

print(type("hi"))
print(type(7))
print(type(3.5))
print(type(True))

<class 'str'>
<class 'int'>
<class 'float'>
<class 'bool'>

• str is text, always in quotes.
• int is a whole number.
• float is a number with a decimal point.
• bool is either True or False.

This matters because "2" + "3" joins text and gives 23, while 2 + 3 adds numbers and gives 5.

print("2" + "3")
print(2 + 3)

23
5

Step 4: Asking the user a question

input() pauses the program and waits for the user to type. Whatever they type comes back as text, so you can store it in a variable.

name = input("What is your name? ")
print("Nice to meet you, " + name + "!")

What is your name? Amara
Nice to meet you, Amara!

Joining text with + works, but an f-string is easier. Put an f before the opening quote and write variables inside curly braces:

age = 12
print(f"Next year you will be {age + 1}.")

Next year you will be 13.

Because input() always gives text, you must convert it before doing math. int() turns text into a whole number.

age = int(input("How old are you? "))
print(f"In 5 years you will be {age + 5}.")

How old are you? 12
In 5 years you will be 17.

Step 5: Comments and reading errors

A line that starts with # is a comment. Python ignores it, so it is a place to leave notes for humans.

# Ask the player's name
name = input("Name? ")

Everyone’s programs have errors, including experienced programmers. The trick is to read the last line of the message first. It says what went wrong.

print(Hello)

NameError: name 'Hello' is not defined

Python thought Hello was a variable because it had no quotes. Adding quotes fixes it: print("Hello").

Remember: an error message is a hint, not a punishment. Read the last line, then look at the line number it points to.