Flow Control

Conditional Logic: Booleans, True and False

Booleans are one of the most important concepts in programming because they allow us to execute code based on a condition. The answer is always one of two things: True or False.

For example, imagine a variable called is_student. If is_student is set to True, we can use that to trigger specific code , like printing a welcome message:

The Two Rules You Must Never Forget

Python doesn't use curly braces {} to mark the beginning and end of a code block. Instead, Python relies entirely on two things:

1. The colon : at the end of your if statement , this signals to Python that a block of code is coming.
2. Indentation (4 spaces or one tab) , this tells Python which lines belong inside that block.

To understand why indentation matters so much, you need to understand how Python's interpreter reads your code. It works through your file line by line, top to bottom. When it hits an if statement followed by a colon, it asks: "Which lines are my instructions?" , and it answers that question entirely by looking at indentation.

Any line that is indented after the colon is considered inside the block and will only run if the condition is True. The moment the interpreter encounters a line that is back at the original indentation level, it knows the block is over , and that line will always run, regardless of the condition.

A single misplaced space can crash your program or, worse, silently change what it does , so indentation is never optional in Python.

Try it yourself , run the code below, then change is_student to False and run it again to see exactly which lines are affected.

if / else

An if statement alone only handles one outcome, what happens when the condition is True. But what about when it's False? That's where else comes in.

Think of it as a two-way road. Python checks the condition, if it's True, it takes the first road. If it's False, it takes the second. One of the two blocks will always run.

Using the same is_student example, we can now handle both outcomes:

Notice that else follows the exact same rules, it ends with a colon : and its code block must be indentedNote: else does not need a condition. A common mistake is writing else False: which will cause an error. else simply means "in every other case.".

Try it yourself: run the code below, then toggle is_student between True and False to see both paths in action.

elif

So far our code has handled two outcomes: True or False. But what if you need to check more than one condition? That is where elif comes in. Short for "else if", it lets you chain multiple conditions together. Python checks each one in order and runs the first block that evaluates to True.

Try changing is_student and is_enrolled to see all four possible outcomes:

• True, True
• True, False
• False, True
• False, False

We can also combine conditions inside a single if statement using the andThere is also an or keyword coming up soon! keyword. When using and, both conditions must be True for the block to run. If either one is False, the whole expression is False.

As a general rule, a conditional block is structured like this: one if to start, one or more elif blocks to handle additional conditions, and one else at the end as the catch-all. Python works through them in order and stops at the first one that is True.

Exercise 1: Basic if/else

• Task: Create a variable is_logged_in = False.
• Goal: Write an if/else block that prints "Welcome back!" if the user is logged in, and "Please log in to continue." if they are not. Test both outcomes.

Exercise 2: elif Chain

• Task: Create a variable score = 72.
• Goal: Write a conditional block that prints "A" if the score is 90 or above, "B" if it is 80 or above, "C" if it is 70 or above, and "Failed" for anything below. Test by changing the score value.

Exercise 3: The and Keyword

• Task: Create two variables has_ticket = True and is_over_18 = False.
• Goal: Write a conditional block using and that prints "Entry granted." only if both conditions are True. If either is False, print "Entry denied." Test all four combinations of True and False.

Exercise 4: Real World Logic

• Task: Create two variables is_member = True and has_paid = False.
• Goal: Write a block with if, elif, and else that prints "Full access granted." if both are True, "Payment required." if they are a member but have not paid, "Please sign up." if they have paid but are not a member, and "No access." if neither is True.

Exercise 5: Spot the Bug

• Task: The following code has two errors. Find and fix them:
• is_admin = True
if is_admin
print("Welcome, admin.")
else:
print("Access denied.")
• Goal: Correct the errors and explain in a comment why each one caused a problem.