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Python if...else Statements: Choosing Between Two Outcomes

Learn how Python if...else statements choose between two code paths using conditions, comparisons, user input, age checks, and even-or-odd examples.

An if...else statement lets a Python program make a two-way decision. When a condition is true, Python runs one block of code. When the condition is false, Python runs a different block.

For example, a program can check whether someone is at least 21 years old and display one message for people who meet the requirement and another message for people who do not.

How an if...else Decision Works

An if statement is a control-flow statement that runs an indented block only when its condition is true. An else clause is the fallback branch paired with an if statement. It runs when the if condition is false.

A condition is an expression evaluated as true or false. The two possible truth values are called Boolean values: True and False.

A simple if...else statement always chooses exactly one of its two branches for each evaluation:

If the condition is true, the else block is skipped. If the condition is false, the if block is skipped.

Basic if...else Syntax

if condition:
    statements_when_true
else:
    statements_when_false

• if begins the decision.
• condition is evaluated as True or False.
• The colon after the condition is required.
• Statements under if must be indented.
• else introduces the alternative branch and does not have its own condition.
• The colon after else is also required.
• Statements under else must be indented.

Python uses indentation, or leading whitespace, to define which statements belong to a code block. Four spaces per level is the usual convention. Keep indentation consistent within both branches.

temperature = 30

if temperature >= 25:
    print("It is warm.")
else:
    print("It is cool.")

The comparison operator >= means “greater than or equal to.” If temperature is 25 or higher, the first message is printed; otherwise, the second message is printed. For more comparison operators, see Python comparison operators.

Conditions and Comparison Operators

A comparison operator compares values and produces a Boolean value. Common comparisons used with if...else include:

Do not confuse ==, which tests equality, with =, which assigns a value to a variable. For example, number % 2 == 0 asks whether a remainder equals zero.

Using User Input in a Decision

The input() function reads a response from the user and returns it as a string, which means text. Even if the user types digits, the returned value is still text.

Use int() to convert valid integer text into an integer before performing numeric comparisons:

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

This stores the converted value in the variable age. You can then use that variable in a condition. The int() function works with whole-number text such as "21", but invalid text such as "twenty-one" causes a ValueError.

For more practice reading responses, see getting user input in Python.

Example: Eligibility Based on Age

This program asks for a person's name and age. It compares the age with the threshold of 21. An age of exactly 21 meets the requirement because the condition uses >=.

name = input("Name: ")
age = int(input("Age: "))

if age >= 21:
    print(name, "meets the age requirement.")
else:
    print(name, "does not meet the age requirement.")

Sample Run: Below the Threshold

Name: Mark
Age: 19
Mark does not meet the age requirement.

Here, age >= 21 is false, so Python skips the if block and runs the else block.

Sample Run: Meeting the Threshold

Name: Mark
Age: 21
Mark meets the age requirement.

Here, age >= 21 is true, so Python runs the if block and skips the else block.

Example: Checking Whether a Number Is Even or Odd

The modulo operator, written as %, returns the remainder after division. An even number is divisible by 2 with a remainder of zero. An odd number leaves a remainder of one when divided by 2.

number = int(input("Enter an integer: "))

# A zero remainder means the number is divisible by 2.
if number % 2 == 0:
    print("The number is even.")
else:
    print("The number is odd.")

The expression number % 2 calculates the remainder. The comparison == 0 turns that calculation into a Boolean condition.

Sample Run with an Odd Number

Enter an integer: 5
The number is odd.

Sample Run with an Even Number

Enter an integer: 100
The number is even.

The comment beginning with # explains the remainder-based test. Comments do not affect program execution; Python ignores them when running the program. See using comments in Python for more about documenting code.

if...else Compared with an if Statement Alone

An if statement without an else branch runs its block only when the condition is true. If the condition is false, it simply continues with the next statement.

score = 42

if score >= 50:
    print("You passed.")

print("The check is complete.")

This is different from if...else, which provides a result for both outcomes:

score = 42

if score >= 50:
    print("You passed.")
else:
    print("You did not pass.")

Use if...else when the program must choose between two alternatives. Use an if statement alone when nothing needs to happen for a false condition.

Common Errors and Troubleshooting

Inconsistent indentation

Problem: Python reports an IndentationError, or a statement appears to run outside the intended branch.

Cause: Statements below if or else are not consistently indented.

Fix: Indent every statement in a branch by the same amount, commonly four spaces:

if age >= 21:
    print("Allowed.")
else:
    print("Not allowed.")

Missing colons

Problem: A SyntaxError points near if or else.

Fix: Add a colon after both branch headers:

if condition:
    print("True branch")
else:
    print("False branch")

Comparing text with a number

Problem: A numeric comparison involving entered input fails or behaves unexpectedly.

Cause: input() returned text instead of an integer.

Fix: Convert the response with int():

age = int(input("Age: "))

Incorrect even-number condition

Problem: The program labels numbers incorrectly.

Fix: Test whether the remainder is zero, using equality comparison:

if number % 2 == 0:

Do not replace == with the assignment operator =.

Unexpected else output at a boundary

Problem: The else message appears when an age such as 21 was expected to meet the requirement.

Fix: Check the comparison operator. age >= 21 includes 21, while age > 21 does not. Also verify that the converted age has the expected value.

Non-numeric input

Problem: Entering non-numeric text causes a ValueError.

Cause: int() can convert valid integer text only.

Fix: Enter a whole number for these examples. Input validation with exception handling can be added later.

Key Points to Remember

• if...else is a two-way decision structure.
• The condition controls which of two alternative code paths runs.
• The if suite runs for True; the else suite runs for False.
• Both if condition: and else: require colons.
• Indentation defines the statements belonging to each branch.
• else has no condition of its own; it handles the false result of the if condition.
• input() returns text, so use int() before comparing numeric input with numbers.
• Use % to calculate a remainder, and number % 2 == 0 to test for an even number.
• Exactly one branch of a simple if...else statement executes for each evaluation.