06-conditionals

06 · Conditionals

Part 1 — Fundamentals · Estimated time: 25–35 min · Prerequisite: 04 · Booleans & Comparison

A conditional lets your program make decisions: if something is true, do one thing; otherwise, do another. This is where code stops being a fixed list of steps and starts to react. You already know how to make True/False values — now you'll act on them.

What you'll learn

• The if statement, and why indentation and the colon matter
• Adding an else for the "otherwise" case
• Chaining choices with elif
• Combining conditions with and / or / not
• The one-line conditional expression (a if condition else b)

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1. The if statement

temperature = 30

if temperature > 25:
    print("It's warm out.")
    print("Wear a t-shirt.")

Two things are essential:

• the line ends with a colon :
• the body is indented (4 spaces). The indentation is how Python knows which lines belong "inside" the if. Lines that aren't indented run regardless.

▶️ Run it: python examples/01_first_if.py

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2. else — the otherwise case

age = 16

if age >= 18:
    print("You may enter.")
else:
    print("Sorry, 18 and over only.")

Exactly one of the two blocks runs — never both.

▶️ Run it: python examples/02_if_else.py

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3. elif — choosing between many options

elif (short for "else if") lets you check several conditions in order. Python runs the first one that's true and skips the rest.

score = 85

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
else:
    print("F")

Order matters: because 85 >= 80 is checked before 85 >= 70, you get B.

▶️ Run it: python examples/03_elif_chain.py

💡 Try it yourself: change score to 72 and predict the output before running it.

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4. Combining conditions

Use and, or, not to build richer tests. You can also nest an if inside another.

age = 25
has_ticket = True

if age >= 18 and has_ticket:
    print("Enjoy the show!")

# A one-line conditional expression for simple either/or values:
weather = "rain"
plan = "stay in" if weather == "rain" else "go out"
print(plan)   # stay in

▶️ Run it: python examples/04_combining_conditions.py

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Common mistakes

Mistake	What happens	Fix
Forgetting the :	SyntaxError	End the if/elif/else line with a colon.
Inconsistent indentation	IndentationError	Indent the body by 4 spaces, consistently.
if x = 5:	SyntaxError	Compare with ==, not =.
Using many ifs instead of elif	every condition is checked; more than one block can run	Use elif when the cases are alternatives.

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Recap / cheat-sheet

if condition:
    ...            # runs when condition is True
elif other:
    ...            # checked only if the ones above were False
else:
    ...            # runs when nothing above matched

# combine: if a and b:  / if a or b:  / if not a:
value = x if condition else y    # one-line conditional expression

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Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_even_or_odd.py — decide if a number is even or odd.
2. exercises/02_grade.py — turn a score into a letter grade with an elif chain.
3. exercises/03_ticket_price.py — pick a ticket price based on age.

Try each yourself before opening the solution.

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Next → 07 · Loops (coming soon)