nu — Python: First Steps

Welcome

Anatomy of a Python print() statement: function name, parentheses, and string argument labeled

Welcome to your first Python lesson.

Python is one of the most popular programming languages in the world. Scientists, engineers, artists, & students use it every day.

In this lesson you will write real Python code and run it instantly. Your code executes on a real server: not a simulation.

Let's start with the most famous program in all of computing.

Hello, World!

Your first program

Every programmer's journey begins with the same two words: Hello, World!

In Python, you print text to the screen using the print() function:

print("Hello, World!")

That's it. One line. The quotes tell Python it's text (called a string). The print() function sends it to the screen.

Write a Python program that prints exactly: Hello, World!

What Are Variables?

Variables as labeled boxes in memory: string box labeled name, integer box labeled age

Variables: giving names to values

A variable is a name that holds a value. Think of it like a labeled box.

name = "Ada"

age = 12

print(name) : prints: Ada

print(age) : prints: 12

The = sign means assign: put the value on the right into the name on the left.

Text goes in quotes (a string). Numbers do not need quotes (an integer).

Create Variables

Your turn

Create two variables & print them:

1. A variable called animal set to your favorite animal

2. A variable called count set to how many legs it has

3. Print both variables

Example output (yours will be different):

cat

4

Create variables called `animal` & `count`, then print both. Your code should output two lines.

Combining Strings

f-string anatomy: f prefix, literal text, and {variable} placeholder each labeled

String concatenation

You can join strings together with +:

greeting = "Hello" + " " + "World"

print(greeting) : prints: Hello World

f-strings (formatted strings)

A better way to mix variables into text:

name = "Ada"

print(f"My name is {name}") : prints: My name is Ada

The f before the quote activates f-string mode. Inside the string, {variable} gets replaced with the variable's value.

f-string Practice

Your turn

Create two variables:

- food: your favorite food (a string)

- rating: how much you like it from 1 to 10 (an integer)

Then use an f-string to print:

I love pizza! I rate it 9 out of 10.

(with your own food & rating)

Create `food` & `rating` variables, then print a sentence using an f-string that includes both. Your output should be one sentence.

If / Else

if/else control flow diagram: condition diamond with True/False branches, comparison operators table

Making decisions

Programs can make choices using if & else:

temperature = 35

if temperature > 30:

print("It is hot!")

else:

print("It is not hot.")

The indented code under if only runs when the condition is True.

The code under else runs when it is False.

Comparison operators: > (greater), < (less), == (equal), != (not equal), >=, <=

If/Else Challenge

Your turn

Write a program that:

1. Creates a variable score set to any number

2. If score is 60 or above, prints Pass

3. Otherwise, prints Fail

Write an if/else program with a `score` variable. Print `Pass` if score >= 60, otherwise print `Fail`.

Put It Together

Final challenge

You now know: print(), variables, f-strings, & if/else.

Combine them all in one program.

Write a program that:

1. Creates a variable name (your name, a string)

2. Creates a variable age (your age, an integer)

3. If age is 13 or older, prints: Welcome, [name]! You may enter.

4. Otherwise, prints: Sorry, [name]. You must be 13 to enter.

Use an f-string for the output.

Write the complete program described above. Use variables, f-strings, & if/else together.