Lesson 5 — Selection with if and else

Estimated time: 60 minutes

Learning Objectives

By the end of this lesson you will be able to:

  1. Use if and else to make a program choose between two paths
  2. Write comparison operators correctly: ==, !=, <, >, <=, >=
  3. Understand why indentation is critical in Python
  4. Change the NeoPixel colour based on a condition

Concepts

What is Selection?

Selection is a programming concept where the program chooses between different paths depending on a condition. Think of it like a roundabout — depending on where you’re heading, you take a different exit.

Without selection, every program runs the same way every time. With selection, your program can respond to different situations.

The if Statement

An if statement checks a condition. If the condition is True, it runs the indented block underneath. If False, it skips that block.

temperature = 30

if temperature > 25:
    print("It's hot!")
    print("Drink some water.")

print("This always runs.")   # Not indented — always executes

Structure:

if condition:
    code to run if True
    (can be multiple lines)

The colon : at the end of the if line is required. The code inside the block must be indented — use 4 spaces or 1 Tab consistently.

The else Clause

else provides an alternative block that runs when the if condition is False:

score = 45

if score >= 50:
    print("You passed!")
else:
    print("Not quite — keep trying!")

Exactly one of these blocks will run — never both, never neither.

Indentation — The Critical Rule

In Python, indentation (the spaces at the start of a line) defines what code belongs inside a block. Getting indentation wrong causes IndentationError.

# CORRECT:
if x > 0:
    print("Positive")
    print("This is also inside the if")
print("This is outside — always runs")

# WRONG (will cause IndentationError):
if x > 0:
print("This isn't indented — Python won't understand this")

Thonny automatically indents for you after a :. If you press Backspace at the start of a line, it removes one level of indentation.

Comparison Operators

These operators compare two values and produce True or False:

Operator Meaning Example Result
== Equal to 5 == 5 True
!= Not equal to 5 != 3 True
< Less than 3 < 5 True
> Greater than 5 > 3 True
<= Less than or equal 5 <= 5 True
>= Greater than or equal 6 >= 5 True

= vs ==: = assigns a value to a variable. == checks if two values are equal. These are completely different! if x = 5: is an error. if x == 5: is correct.

Nested if Statements

You can put an if inside another if:

score = 85

if score >= 50:
    print("You passed!")
    if score >= 80:
        print("Excellent — that's a distinction!")
else:
    print("Better luck next time.")

Guided Walkthrough

Step 1: Basic if/else in the REPL

Try this in the Shell:

>>> x = 10
>>> if x > 5:
...     print("x is greater than 5")
... else:
...     print("x is 5 or less")
...
x is greater than 5

(In the REPL, the ... prompt means “continue this block”. Press Enter on a blank line to finish.)

Step 2: Comparison Operators 

x = 10
print(x == 10)   # True
print(x == 5)    # False
print(x != 5)    # True
print(x > 5)     # True
print(x < 5)     # False
print(x >= 10)   # True
print(x <= 9)    # False

Step 3: Score-Based LED Feedback 

import machine, neopixel, time

pin = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin, 1)

score = int(input("Enter your score (0-100): "))

if score >= 50:
    np[0] = (0, 255, 0)    # Green = pass
    np.write()
    print(f"Score: {score} — PASS!")
else:
    np[0] = (255, 0, 0)    # Red = fail
    np.write()
    print(f"Score: {score} — Not quite. Keep trying!")

time.sleep(3)
np[0] = (0, 0, 0)
np.write()

Try entering different scores and see the LED change.

Step 4: Nested if for Multiple Levels 

import machine, neopixel, time

pin = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin, 1)

score = int(input("Enter your score (0-100): "))

if score >= 50:
    print("You passed!")
    if score >= 90:
        np[0] = (255, 215, 0)  # Gold — outstanding
        print("Outstanding — gold award!")
    elif score >= 70:
        np[0] = (0, 0, 255)    # Blue — merit
        print("Merit level!")
    else:
        np[0] = (0, 255, 0)    # Green — pass
        print("Good pass!")
else:
    np[0] = (255, 0, 0)        # Red — fail
    print("You didn't pass this time.")

np.write()
time.sleep(3)
np[0] = (0, 0, 0)
np.write()

Challenges

⭐ Core

Ask the user to enter a number. If the number is even (hint: number % 2 == 0), show green on the LED and print “Even”. If odd, show blue and print “Odd”.

⭐⭐ Extension

Ask for a temperature in Celsius. Display:

  • Blue if below 10°C and print “Cold!”
  • Green if between 10 and 25°C and print “Comfortable”
  • Red if above 25°C and print “Hot!”

⭐⭐⭐ Stretch

Create a simple password checker. Define a correct password in your code. Ask the user to enter a password. If correct, show green and print “Access granted”. If wrong, show red and print “Access denied” along with how many characters they typed. Also check: if the password is shorter than 8 characters, show yellow and print a warning about short passwords (check the length before checking if it’s correct).

Common Mistakes & Debugging

SyntaxError on the if line Most likely you used = instead of ==: if x = 5: should be if x == 5:.

IndentationError: expected an indented block After if ...: the next line must be indented. Thonny does this automatically when you press Enter after the colon.

IndentationError: unexpected indent A line is indented but shouldn’t be. Check that all lines meant to be inside the block have the same indentation.

Forgetting the colon : if x > 5 (no colon) gives SyntaxError. Always end if, else, elif lines with :.

Both branches seem to run Check your indentation — code after the if block that runs regardless may look like it’s inside else but isn’t.

Key Vocabulary

Term Definition
selection A programming construct that chooses between different code paths
if statement Runs a block of code only if a condition is True
else The block that runs when the if condition is False
condition An expression that evaluates to True or False
comparison operator Compares two values: ==, !=, <, >, <=, >=
indentation Spaces at the start of a line that define code blocks in Python
boolean A True or False value — what conditions evaluate to
nested A structure placed inside another of the same kind (e.g., if inside if)

Copyright © Paul Baumgarten.