Conditionals in Python

Programs that make decisions

So far a program might run the same way every time. Conditionals change that: they let a program choose what to do based on what is true right now. Should the game say "You win" or "Try again"? Should the app show the daytime or the night theme? Conditionals make these decisions.

If you've written a first Python program with variables and input(), you already have everything you need to start.

The if statement

An if statement runs a block of code only when a condition is true:

age = 15
if age >= 13:
    print("You are a teenager.")

Read it as: "if age is greater than or equal to 13, print this message." Because age is 15, the condition age >= 13 is True, so the message prints.

Two things to notice:

• The condition ends with a colon :.
• The line below is indented (four spaces). Indentation tells Python which lines belong inside the if. If the condition is False, those indented lines are skipped.

True and False

A condition is really a question that Python answers with True or False — these are called boolean values. Comparisons produce them:

print(5 > 3)     # True
print(2 == 2)    # True
print(4 != 4)    # False

Here are the comparison operators:

Operator	Meaning	Example	Result
==	equal to	3 == 3	True
!=	not equal to	3 != 4	True
<	less than	2 < 5	True
>	greater than	2 > 5	False
<=	less than or equal	5 <= 5	True
>=	greater than or equal	6 >= 9	False

Watch out: = assigns a value, but == compares. Writing if x = 5: is an error; you want if x == 5:.

else: the other path

Often you want one thing to happen when the condition is true and a different thing when it's false. That's what else is for:

temperature = 28
if temperature > 25:
    print("It's hot — wear shorts.")
else:
    print("It's cool — bring a jacket.")

Exactly one of the two branches runs. The else has no condition of its own; it simply catches every case the if didn't.

elif: more than two choices

When there are several possibilities, use elif (short for "else if"). Python checks each condition in order and runs the first one that is true:

score = 82
if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
else:
    print("Grade: F")

With score = 82, the first test (>= 90) is False, the second (>= 80) is True, so it prints Grade: B and stops — the later branches are not even checked.

Combining conditions with and, or, not

You can join conditions using logical operators:

• and — true only if both sides are true
• or — true if at least one side is true
• not — flips true to false and back

age = 16
if age >= 13 and age <= 19:
    print("You are a teen.")

day = "Saturday"
if day == "Saturday" or day == "Sunday":
    print("It's the weekend!")

logged_in = False
if not logged_in:
    print("Please log in.")

In the first example, both sides must be true, so the age must be between 13 and 19 inclusive.

A complete example

Here's a small program that reacts to the user:

age = int(input("How old are you? "))

if age < 0:
    print("That's not a valid age.")
elif age < 13:
    print("You're a child.")
elif age <= 19:
    print("You're a teenager.")
else:
    print("You're an adult.")

It reads a number, then chooses exactly one message based on the value. Try running it with different ages and watch which branch wins.

Conditions inside loops

Conditionals get even more powerful inside a loop, where they make a decision on every pass:

for number in range(1, 6):
    if number % 2 == 0:
        print(number, "is even")
    else:
        print(number, "is odd")

number % 2 is the remainder after dividing by 2. If that remainder is 0, the number is even. This is the same loop idea from Loops: Making the Computer Repeat, now making a real decision each time.

Practice challenges

• Ask the user for a number and print whether it is positive, negative, or zero.
• Write a simple password check: if the typed password equals "open123", print "Access granted", otherwise "Denied".
• Ask for a year and print whether it's a leap year. (A year is a leap year if it is divisible by 4 but not by 100, unless it is also divisible by 400.)