Conditions and if Statements

Programs constantly need to make decisions: Is the password correct? Is the item in stock? Has the user passed the exam? In Python, these decisions are made with conditional statements, and the most basic one is the if statement.

An if statement runs a block of code only when a condition is true. If the condition is false, Python skips that block and continues with the rest of the program.

Conditions and Comparison Operators

A condition is an expression that evaluates to True or False. Conditions are usually built with comparison operators, which come from basic mathematics and logic.

These operators are used mainly in if statements, loops, and logical expressions to control the flow of a program.

Remember: A single = assigns a value. A double == compares two values.

The if Statement

Syntax

if condition:
    # code that runs only if the condition is True

The structure has three parts:

1. The keyword if
2. A condition, followed by a colon :
3. An indented block of code — the body of the if statement

Example: Comparing Two Values

x = 45
y = 30

if x > y:
    print("x is larger than y")

Expected output:

x is larger than y

The condition x > y is 45 > 30, which is True, so Python runs the indented print() statement.

If you change y to 50, the condition becomes False, and nothing is printed.

How the if Statement Works

1. Python evaluates the condition.
2. The condition produces True or False.
3. If the result is True, the indented block runs.
4. If the result is False, Python skips the block.
5. In either case, the program continues with the first line after the block.

temperature = 18

if temperature < 20:
    print("It's cool today.")

print("Have a nice day!")

Expected output:

It's cool today.
Have a nice day!

The last print() is not indented, so it is not part of the if block. It runs no matter what the condition is.

Example: Checking Whether a Number Is Even

num = 8

if num % 2 == 0:
    print("The number is even")

Expected output:

The number is even

How it works:

• num % 2 gives the remainder when num is divided by 2.
• For an even number, the remainder is 0.
• 8 % 2 is 0, so 0 == 0 is True, and the message is printed.

The Importance of Indentation

Python uses indentation to define code blocks. Many other languages use curly braces { }; Python does not.

Incorrect Indentation (Error)

marks = 75
if marks > 60:
print("You passed the exam")

Output:

IndentationError: expected an indented block after 'if' statement on line 2

Python requires at least one indented line after the colon.

Correct Indentation

marks = 75
if marks > 60:
    print("You passed the exam")

Expected output:

You passed the exam

The standard indentation is 4 spaces.

Multiple Statements Inside an if Block

An if block can contain any number of statements. All of them must have the same indentation.

score = 92

if score >= 90:
    print("Excellent performance!")
    print("Grade: A")
    print("Keep up the great work")

Expected output:

Excellent performance!
Grade: A
Keep up the great work

All three lines belong to the if block, so they run together when the condition is True — and are all skipped when it is False.

Using Variables Directly in Conditions

A Boolean variable already holds True or False, so it can be used directly as a condition without a comparison.

Example: Boolean Condition

has_permission = False

if has_permission:
    print("Access granted")

Expected output: (nothing is printed)

has_permission is False, so the block is skipped. Writing if has_permission == True: works too, but if has_permission: is cleaner and is the standard Python style.

Truthy and Falsy Values in Conditions

Any value can be used as a condition. Empty values (0, "", [], {}, None) are treated as False; non-empty values are treated as True.

username = "asha_k"

if username:
    print("Welcome,", username)

Expected output:

Welcome, asha_k

If username were an empty string "", nothing would be printed. This is a common way to check that input was provided.

Combining Conditions

Use the logical operators and, or, and not to build more complex conditions.

age = 25
has_ticket = True

if age >= 18 and has_ticket:
    print("You may enter the event")

Expected output:

You may enter the event

Both conditions must be True for and to succeed.

day = "Sunday"

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

Expected output:

It's the weekend

At least one condition must be True for or to succeed.

Practical Example: Free Shipping Check

cart_total = 1250
free_shipping_limit = 999

if cart_total >= free_shipping_limit:
    print("You qualify for free shipping!")

print("Cart total:", cart_total)

Expected output:

You qualify for free shipping!
Cart total: 1250

Common Mistakes

What's Next?

An if statement on its own handles only the "true" case. In the next lessons, you will learn how to:

• Check several conditions in order with elif
• Handle the "otherwise" case with else
• Write compact one-line conditions with shorthand if
• Place if statements inside other if statements with nested if

Related Concepts

• Python Booleans — True, False, and truthy/falsy values
• Python Operators — comparison and logical operators
• Python Syntax — indentation rules