Lecture 9 — Input, Variables & Identifiers⌗

Instructor: Laksh Budhrani

Review / Recap⌗

Turtle Commands You Already Know⌗

Any questions about the previous assignment?⌗

Ask now before we move on.

Objective⌗

Understand how to collect user input, store it in variables, and name variables correctly using identifiers.

1. What is a variable
2. How to name variables (identifiers)
3. How input() collects information

Objective 1 — Variables⌗

A variable is a container that stores a value.

Examples:

car_length = 120
color = "blue"
wheel_radius = 20

Key Ideas:
• Variables store information
• You can use them later

Objective 2 — Identifiers (Variable Names)⌗

Identifiers are the names you give to variables.

Rules⌗

✔ Must start with a letter or underscore
✔ Can contain letters, numbers, underscores
✔ No spaces
✔ No special characters
✔ Case‑sensitive
✔ Should be meaningful

Correct Examples⌗

pizza_size
car_length
wheel_radius
crust_color

Wrong Examples⌗

2size        # cannot start with a number
car length   # spaces are not allowed
wheel-radius # hyphens are not allowed
@color       # special characters not allowed

Objective 3 — The input() Function⌗

What input() Does⌗

• Pauses the program
• Waits for the user to type something
• Continues after Enter is pressed

Basic Syntax:

input("Enter something: ")

VERY IMPORTANT — Rule of Thumb for Numbers⌗

When you ask the user for a whole number, Python does NOT automatically treat it as a number.

To make sure Python understands it correctly:

RULE OF THUMB⌗

Whenever you want a whole number, ALWAYS use:

int(input("Enter a number: "))

Examples⌗

car_length = int(input("Enter car length: "))
wheel_radius = int(input("Enter wheel radius: "))
pizza_size = int(input("Enter pizza size: "))

This keeps your program correct and prevents errors later.

Combine input + variables + identifiers⌗

Example:

color = input("Enter a color: ")
size = int(input("Enter a size: "))

Now you can use these values in Turtle.

Quick MCQs (Check Your Understanding)⌗

1. Which of the following is a correct identifier?
A. 2car
B. car length
C. car_length
D. car-length

2. What does input() do?
A. Draws a shape
B. Pauses and waits for user typing
C. Changes turtle color
D. Runs the program faster

3. If you want to ask the user for a whole number, what should you use?
A. input()
B. input() + number
C. int(input())
D. whole(input())

4. Which identifier is wrong?
A. pizza_size
B. crustColor
C. size2
D. pizza size

Guided Activity — Customizable Car⌗

Create a new Python file in VS Code named:
turtle_class9.py

Image⌗

Requirements⌗

Ask the user for:

1. Car length → use int()
2. Car color → string
3. Wheel radius → use int()

Then draw a car using Turtle:

• Rectangle body (use car length)
• Filled color (use car color)
• Two wheels (use wheel radius)

Boilerplate Code⌗

import turtle as trtl

p = trtl.Turtle()
p.speed(0)

# --- ENTER YOUR CODE HERE ---
# (Write your drawing code in this section)



# --------------------------------------------------------------
# (Stop writing code above this line)

wn = trtl.Screen()
wn.mainloop()

Independent Activity — Customizable Pizza Maker⌗

Pizza Image⌗

Your Task⌗

Ask the user for:

• Pizza size → use int()
• Crust color
• Cheese color

Then draw the pizza using:

• Two concentric circles
• p.circle()
• p.begin_fill()
• p.end_fill()
• p.fillcolor()
• p.goto()
• penup()
• pendown()
• etc.

Use the Turtle boilerplate code provided earlier in this lecture.

Create Your Own File⌗

Create a new Python file in VS Code named:

turtle_class9_independent.py

Inside that file, use the boilerplate and write your pizza code in the section marked:

# --- ENTER YOUR CODE HERE ---

Submission Instructions⌗

Submit your work in Google Classroom by:

1. Creating a Google Doc
2. Pasting your code into the document
3. Adding a screenshot of the output window showing your pizza drawing
4. Turn in the Google Doc to the assignment titled In‑Class Exercise 9