Booleans and Conditionals

Ask yes/no questions and choose different paths with if, elif, and else.

• Level: Beginner
• Estimated time: 25–40 minutes
• You will learn: Ask yes/no questions and choose different paths with if, elif, and else.
• Practice in: Google Colab, Jupyter, or VS Code

Open this lesson in Google Colab

Questions

• What problem does Booleans and Conditionals help us solve in a small Python program?
• What should we predict before running the example?
• What value, output, or error should we inspect after changing one line?

Objectives

• Run a complete example for Booleans and conditionals in Colab.
• Explain the example line by line using plain language.
• Change one part of the code and predict the result before running it.
• Recognize one common mistake and use the error message as evidence.

Hands-on episode: Booleans and Conditionals

A Boolean is either True or False. A conditional uses a Boolean expression to decide which indented block of code should run.

We will learn this by running code, not by memorizing a definition first. Open the Colab notebook from the button above, find this section, and run each cell in order. Keep a small note beside the notebook with three columns: prediction, actual result, and what changed.

Example 1.1

Predict which message appears for this score and explain which comparison made the decision.

score = 82

if score >= 90:
print("Excellent")
elif score >= 70:
print("Passed")
else:
print("Review and try again")

Run the cell once without editing it. If the result is different from your prediction, leave the prediction visible and write one sentence about the difference. That sentence is more useful than a perfect first guess.

Explain Example 1.1

• Python checks score >= 90 first. With 82, that question is false.
• Python then checks score >= 70. With 82, that question is true.
• Only the indented block under the first true branch runs.
• The else block is skipped because Python already found a matching branch.

Now explain the example out loud or in a Markdown cell. Use short sentences: “this line creates…”, “this name stores…”, “this output appears because…”. If you cannot explain a line yet, run only the lines above it and inspect the values that exist at that moment.

Challenge 1.1

NoteChallenge

Change the score to 95, then 65, and trace the branches again. The code did not change, but the data changed, so a different path through the program becomes active.

Show a safe way to approach the challenge

1. Copy Example 1.1 into a new Colab cell.
2. Change exactly one value, name, condition, or line.
3. Write the expected output before running the cell.
4. Run the cell and compare the actual result with your prediction.
5. If the result surprises you, undo the change and try a smaller one.

Suggested first move: Change the score to 95, then 65, and trace the branches again.

Debugging checkpoint 1.1

WarningDebugging checkpoint

Branch order matters. If you check score >= 70 before score >= 90, then a score of 95 will match the earlier branch and never reach the excellent case. When a conditional surprises you, write each condition as a yes-or-no sentence and test them in order.

Do not debug by rewriting the whole example. Read the error type or surprising output, inspect the closest value with print(...) or type(...), then change one thing. This is the same routine you will use in larger projects.

Apply it

Create a small recommendation program in Colab: if the number of study minutes is under 10, suggest a short review; if it is under 30, suggest one exercise; otherwise suggest a mini-project.

Finish by adding a Markdown cell that answers: What did this example teach me that I can reuse in a project?

Key points

• Learn the concept by running a complete, small example first.
• Predict before execution so your thinking becomes visible.
• Change one thing at a time so cause and effect stay clear.
• Treat errors as clues about the exact line or value Python could not handle.

The problem this lesson solves

Programs often need to make decisions. A weather program decides whether to show “bring a jacket”. A form decides whether a password is long enough. A game decides whether the player has won.

Python decisions start with Boolean values: True and False. A conditional statement then uses those values to choose which block of code runs.

Learning goals

By the end of this lesson, you should be able to:

• read comparison expressions such as score >= 70;
• explain the difference between assignment = and comparison ==;
• write if, elif, and else blocks with correct indentation;
• trace which branch Python will run;
• debug common conditional mistakes.

How conditionals choose a path

A conditional is a branching path. Python asks yes/no questions in order, then runs only the first block whose condition is true.

flowchart TD
  start([Start]) --> q1{"score >= 90?"}
  q1 -- True --> excellent["print('excellent')"]
  q1 -- False --> q2{"score >= 70?"}
  q2 -- True --> passing["print('passing')"]
  q2 -- False --> practice["print('keep practicing')"]
  excellent --> stop([Continue after conditional])
  passing --> stop
  practice --> stop

When debugging conditionals, trace the questions in order and stop at the first true branch.

1. Boolean values and comparisons

A Boolean is a value with only two possibilities: True or False. Booleans are useful because they answer yes/no questions.

print(10 > 3)
print(10 == 3)
print("Ada" == "Ada")

Comparison operators

Operator	Meaning	Example
==	equal to	score == 10
!=	not equal to	name != ""
<	less than	age < 18
<=	less than or equal to	temperature <= 0
>	greater than	total > 100
>=	greater than or equal to	score >= 70

A comparison expression produces a Boolean value.

Example 1

Predict each printed value before running the code.

temperature = 18
print(temperature < 20)
print(temperature == 18)
print(temperature != 30)

Step-by-step explanation

1. temperature = 18
   • This is assignment. The name temperature now refers to the integer 18.
2. temperature < 20
   • Python checks: “Is 18 less than 20?”
   • Yes, so the expression becomes True.
3. temperature == 18
   • Python checks: “Is 18 equal to 18?”
   • Yes, so the expression becomes True.
4. temperature != 30
   • Python checks: “Is 18 not equal to 30?”
   • Yes, so the expression becomes True.

Assignment is not comparison

This line stores a value:

score = 10

This expression asks a question:

score == 10

Beginners often mix these up because in everyday math we say “equals” for both. In Python, one equals sign assigns; two equals signs compare.

Combining questions with and, or, and not

You can combine Boolean expressions:

age = 20
has_ticket = True

print(age >= 18 and has_ticket)
print(age < 18 or has_ticket)
print(not has_ticket)

• and is true only when both sides are true.
• or is true when at least one side is true.
• not flips a Boolean value.

Common mistakes

WarningCommon mistake

Mistake: using = when you meant ==.

if score = 10:
    print("perfect")

Python raises SyntaxError because assignment is not a question. Use == for comparison:

if score == 10:
    print("perfect")

Check your understanding

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2. Conditionals choose a path

A conditional runs one block of code when a condition is true. The most common form is if:

if condition:
    code_to_run_when_true

The indented lines belong to the if block. Indentation is not decoration in Python; it is how Python knows which lines belong together.

Example 2

score = 82

if score >= 90:
    print("excellent")
elif score >= 70:
    print("passing")
else:
    print("keep practicing")

Step-by-step explanation

1. score = 82
   • The name score stores the integer 82.
2. if score >= 90:
   • Python asks: “Is 82 greater than or equal to 90?”
   • The answer is False, so Python skips the indented block under this if.
3. elif score >= 70:
   • elif means “else, if this new condition is true”.
   • Python asks: “Is 82 greater than or equal to 70?”
   • The answer is True, so Python runs print("passing").
4. else:
   • Python does not run the else block because an earlier branch already ran.

Branch order matters

Python checks branches from top to bottom and stops at the first true branch. This code has a bug:

score = 95

if score >= 70:
    print("passing")
elif score >= 90:
    print("excellent")

The output is passing, not excellent, because score >= 70 is already true. Put the more specific condition first:

if score >= 90:
    print("excellent")
elif score >= 70:
    print("passing")

Common mistakes

WarningCommon mistake

Mistake: forgetting the colon.

if score >= 70
    print("passing")

The colon tells Python that an indented block is coming. Add it after the condition:

if score >= 70:
    print("passing")

WarningCommon mistake

Mistake: inconsistent indentation.

if score >= 70:
print("passing")

The print line must be indented so Python knows it belongs to the if block.

Practice

Try this in Colab. Change only age first, then only has_permission.

age = 16
has_permission = True

if age >= 18 or has_permission:
    print("You can enter.")
else:
    print("Ask an adult for help.")

Before each run, write down which branch you expect and why.

Check your understanding

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Key points

TipKey points

• Boolean values are True and False.
• Comparisons such as score >= 70 produce Boolean values.
• = assigns; == compares.
• if, elif, and else let a program choose one path.
• Python uses indentation to decide which lines belong to a block.
• Branches are checked from top to bottom.

References

• Python Tutorial: control flow: https://docs.python.org/3/tutorial/controlflow.html
• Python Boolean operations: https://docs.python.org/3/library/stdtypes.html#boolean-operations-and-or-not