Conditionals

In the previous chapter, you learned to write expressions that evaluate to True or False. In this chapter, those expressions get put to work. An if statement tells the program to run some code only when a condition is true.

The Basic if

The simplest form has one condition and one branch.

minutes = 60

if minutes >= 60:
    print("That's a long session!")

print("Logged.")

This program prints That's a long session! and then Logged. If minutes had been 30, the program would print only Logged. — the indented line would be skipped.

Two pieces of syntax are new here. The line that starts with if ends with a colon (:). And the line that runs only when the condition is true is indented by four spaces.

Indentation Is the Syntax

In some languages, the code that belongs to an if is wrapped in curly braces or marked with keywords like then and end. In Python, the marker is indentation. The body of an if is the lines underneath it that are indented further than the if itself.

if minutes >= 60:
    print("That's a long session!")
    print("Great work.")

print("Logged.")

Both indented lines belong to the if. They run together or not at all. The last line, which is not indented, is outside the if and always runs.

Python is strict about this. If your indentation is inconsistent, Python reports an IndentationError and refuses to run the program at all. Pick four spaces per level and stick with it. Most editors do this for you automatically once you press Enter after a line ending in a colon.

A common early mistake is to forget the colon:

if minutes >= 60        # missing colon!
    print("That's a long session!")

Python responds with a SyntaxError, pointing roughly at the line with the missing colon. Read the error, add the colon, run again.

else for the Alternative

Often you want one thing to happen if the condition is true and something else to happen otherwise. The else keyword handles that case.

minutes = 30

if minutes >= 60:
    print("That's a long session!")
else:
    print("Nice short session.")

Exactly one of the two branches runs. The condition is checked once; if it is true, the if branch runs; otherwise, the else branch runs. There is no “maybe both” and no “maybe neither”.

Note the syntax: else also ends with a colon, and its body is indented to the same level as the if’s body.

75

Long session.

20

Short session.

elif for Multiple Branches

Sometimes there are more than two cases. You want to classify a study session as short, medium, or long, or you want to give different responses to different input values.

Python uses elif (short for “else if”) for additional conditions:

minutes = 45

if minutes < 15:
    print("Short session -- better than nothing.")
elif minutes <= 90:
    print("Solid session.")
else:
    print("Long session -- make sure you took breaks.")

The rules are:

1. Python checks the conditions in order, from top to bottom.
2. As soon as one of them is true, that branch runs.
3. The remaining elif and else clauses are skipped.

So in the program above, if minutes is 45, Python first checks minutes < 15. That is false, so it moves on. It checks minutes <= 90. That is true, so Solid session. is printed, and the else is skipped.

You can have as many elif clauses as you need:

if minutes < 15:
    response = "Very short."
elif minutes < 30:
    response = "Short."
elif minutes < 60:
    response = "Medium."
elif minutes < 120:
    response = "Long."
else:
    response = "Very long."

print(response)

This is a typical classification pattern. Each branch handles one range of values. The else at the end handles everything that did not match.

if minutes < 20:
    print("tiny")
elif minutes < 45:
    print("short")
else:
    print("long")

Order Matters in elif Chains

Because Python checks conditions top to bottom and stops at the first match, the order of your branches matters. Look at this program:

if minutes < 60:
    print("Under an hour.")
elif minutes < 30:
    print("Under half an hour.")

If minutes is 20, the first condition is true, so Under an hour. is printed. The second branch, even though it also matches, is never reached. The “Under half an hour.” message is unreachable code: it can never run.

A useful habit when writing elif chains is to put the most specific conditions first and the most general ones last. For threshold-based classification, that usually means listing branches in either strictly increasing or strictly decreasing order.

What Happens When No Branch Matches?

When you write an if without an else, and the condition is false, nothing happens at all. The program just moves on.

if minutes >= 60:
    print("Long session!")

print("Logged.")

If minutes is 30, this program prints only Logged. There is no error. There is no warning. The if simply did not apply.

This is fine when you only want extra behavior in one case. But it is also a quiet source of bugs. If you intended to handle two cases and only wrote one, the program will run without complaint and silently skip the case you forgot. When you write a chain of elifs, it is usually worth asking: “what if none of these match? Did I want that to be silent, or did I want an else?”

A useful default: when writing an elif chain that classifies a value, finish it with an else. Even if the else only prints “Unknown.”, you have made the gap visible rather than letting it sit in the dark.

score = 40

if score >= 80:
    message = "strong"
elif score >= 50:
    message = "practice"

print(message)

Boundaries Need Attention

Look at this classifier again:

if minutes < 15:
    print("Short.")
elif minutes <= 90:
    print("Solid.")
else:
    print("Long.")

What happens when minutes is exactly 15? The first condition minutes < 15 is false (15 < 15 is False), so Python moves on. The second condition minutes <= 90 is true, so Solid. is printed.

What about minutes = 90? The first condition is false. The second condition 90 <= 90 is true, so Solid. is printed.

What about minutes = 91? Both < and <= are false, so Long. is printed.

These are called boundary values — the exact numbers at the edges of each range. They are also one of the most common places that branching code is subtly wrong. Did you mean < or <=? Did you mean to include 15 in “Short” or in “Solid”? It is worth pausing every time you write a chain like this and asking explicitly what should happen at each boundary.

You will return to this in Verifying Behavior and Finding the Edge Cases AI Misses, where it becomes a systematic check for AI-generated code as well as your own.

90

30 to 90 minutes.

Common Pitfalls

A short list of things that trip people up at this stage:

• Missing colon. if minutes >= 60 (no :) is a syntax error.
• Wrong indentation. Mixing tabs and spaces, or indenting by different amounts on different lines, gives IndentationError.
• = instead of ==. if minutes = 60: is a syntax error. You meant if minutes == 60:.
• Branch order. As shown above, ordering if minutes < 60 before elif minutes < 30 makes the second branch unreachable.
• Forgetting else. A silent gap where one case is not handled. Sometimes intentional, often not.

When something goes wrong, read the error message before assuming the worst. Python is usually quite specific about what it could not parse.

15

FizzBuzz

10

Buzz

Check Your Understanding

1. In the following program, what is printed when minutes is 60?

if minutes < 60:
    print("Under an hour.")
elif minutes < 90:
    print("Between one and one-and-a-half hours.")
else:
    print("At least an hour and a half.")

2. Why is Python strict about indentation, and what error does it report when indentation is wrong?
3. In an if/elif/else chain, can more than one branch run for the same input? Why or why not?
4. Why is finishing an elif chain with an else often a good idea, even if the else is short?