Conditional Statements in Python (if, elif, else)

What Is It?

What Are Conditional Statements?

A conditional statement lets your program make decisions. Instead of executing every line of code from top to bottom, your program can choose which code to run based on whether a condition is true or false.

Think of it like making everyday decisions. When you wake up, you check: "Is it raining?" If yes, you take an umbrella. If no, you do not. This is exactly what conditional statements do in code — they check a condition and execute different blocks of code depending on the result.

In Python, conditional statements are built using three keywords: if, elif (short for "else if"), and else. Every Python program that does anything useful will use conditional statements.

The Basic Structure

The simplest conditional statement checks one condition and executes code only if that condition is True:

if condition:
    # code to execute when condition is True

The condition is any expression that evaluates to True or False (a Boolean expression). The code inside the if block is indented — this indentation is how Python knows which code belongs to the if statement.

Why Does It Matter?

Why Are Conditional Statements Essential?

1. Every Real Program Makes Decisions

Without conditionals, your program would do the exact same thing every time it runs, regardless of the input. A login system needs to check if the password is correct. A game needs to check if the player won or lost. An ATM needs to check if the account has enough balance. All of these require conditional statements.

2. Controlling Program Flow

Conditionals give you control over which parts of your code execute and in what order. Instead of writing separate programs for different scenarios, you write one program that handles all scenarios by making decisions at runtime.

3. Input Validation

When your program takes user input, you need to check if the input is valid. Is the age a positive number? Is the score between 0 and 100? Is the email address in the correct format? Conditional statements let you validate input and show appropriate error messages.

4. Foundation for Complex Logic

Conditional statements are one of the three fundamental building blocks of all programs (the other two are sequence and loops). Once you master conditionals, you can build programs of any complexity. Every algorithm, from simple grade calculators to sophisticated AI systems, relies on conditional logic at its core.

Detailed Explanation

Detailed Explanation

1. The if Statement

The if statement is the simplest conditional. It checks a condition, and if the condition is True, it executes the indented block of code below it. If the condition is False, Python skips the block entirely.

age = 16

if age >= 18:
    print("You can vote.")

print("Program continues here.")

In this example, since 16 >= 18 is False, the print statement inside the if block is skipped. The program jumps to "Program continues here."

Indentation Is Mandatory

In Python, indentation is not optional or cosmetic — it is part of the language's syntax. The standard is 4 spaces per level of indentation. All code that belongs to the if block must be indented by the same amount. If you mix indentation levels or forget to indent, Python raises an IndentationError.

# This causes an IndentationError:
if True:
print("Hello")  # Not indented — ERROR!

2. The if-else Statement

The if-else structure provides two paths: one for when the condition is True, and another for when it is False. Exactly one of the two blocks will always execute.

marks = 45

if marks >= 40:
    print("You passed!")
else:
    print("You failed.")

Since 45 >= 40 is True, the if block runs and prints "You passed!". If marks were 35, the else block would run instead. The else keyword does not have a condition — it catches everything that the if did not.

3. The if-elif-else Statement

When you need to check multiple conditions, use elif (short for "else if"). Python checks each condition from top to bottom and executes the block for the first condition that is True. If no condition is True, the else block runs.

marks = 75

if marks >= 90:
    grade = "A"
elif marks >= 80:
    grade = "B"
elif marks >= 70:
    grade = "C"
elif marks >= 60:
    grade = "D"
else:
    grade = "F"

print("Grade:", grade)

Here, Aarav scored 75. Python checks: 75 >= 90? No. 75 >= 80? No. 75 >= 70? Yes! So grade is set to "C" and all remaining elif/else blocks are skipped. This is important: once a match is found, no further conditions are checked.

Key Rules for if-elif-else

• You must start with if.
• You can have zero or more elif blocks.
• The else block is optional, and there can be at most one.
• else must come last.
• Only ONE block executes — the first one whose condition is True (or else if none are True).

4. Nested if Statements

You can place an if statement inside another if statement. This is called nesting. The inner if is only checked when the outer if condition is True.

age = 20
has_license = True

if age >= 18:
    print("You are an adult.")
    if has_license:
        print("You can drive.")
    else:
        print("Get a driving license first.")
else:
    print("You are too young to drive.")

The inner if has_license is only reached if the outer condition age >= 18 is True. Each level of nesting requires an additional level of indentation (4 more spaces).

Avoid deep nesting. If you have 3 or more levels of nesting, your code becomes hard to read. Often, you can rewrite deeply nested conditions using logical operators (and, or) to flatten the structure.

5. Short-Hand if (One-Liner)

If the if block has only one statement, you can write it on the same line as the if:

age = 20
if age >= 18: print("Adult")

This is valid Python, but use it sparingly. It can make code harder to read if overused.

6. The Ternary Operator (Conditional Expression)

Python has a compact way to write a simple if-else that assigns a value. It is called the ternary operator or conditional expression:

value_if_true if condition else value_if_false

Example:

age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)  # Output: Adult

This is equivalent to:

if age >= 18:
    status = "Adult"
else:
    status = "Minor"

The ternary operator is great for simple assignments but should not be used for complex logic. Keep it readable.

7. Combining Conditions with and, or, not

You can combine multiple conditions using logical operators:

• and — both conditions must be True
• or — at least one condition must be True
• not — reverses the condition

age = 16
has_permission = True

if age >= 13 and age <= 17 and has_permission:
    print("You can join the teen group.")

You can also use chained comparisons: 13 <= age <= 17 is equivalent to age >= 13 and age <= 17.

8. Truthiness and Falsiness

In Python, every value has a Boolean equivalent. When you use a non-Boolean value in an if condition, Python automatically converts it to True or False. This is called truthiness and falsiness.

Values That Are Falsy (Treated as False)

• False (the Boolean)
• 0 and 0.0 (zero in any numeric type)
• "" (empty string)
• [] (empty list)
• () (empty tuple)
• {} (empty dictionary)
• None

Everything Else Is Truthy (Treated as True)

• Non-zero numbers: 1, -5, 3.14
• Non-empty strings: "hello", "0", " "
• Non-empty collections: [1, 2], (0,), {"a": 1}

This allows elegant code like:

name = input("Enter your name: ")
if name:  # True if name is not empty
    print("Hello,", name)
else:
    print("You did not enter a name.")

9. Chaining Comparisons

Python supports chained comparisons that match mathematical notation:

score = 75
if 60 <= score <= 100:
    print("Valid score")

This is equivalent to score >= 60 and score <= 100. Chains can include any comparison operators: a < b < c, a == b == c, a < b >= c. Each pair of adjacent comparisons is combined with and.

An important detail: in a < b < c, b is evaluated only once. In the equivalent a < b and b < c, b is evaluated twice. This matters if b is an expression with side effects (like a function call).

Code Examples

temperature = 38

if temperature > 37:
    print("You have a fever.")
    print("Please see a doctor.")

print("Program done.")

marks = int(input("Enter your marks: "))  # Assume user enters 35

if marks >= 40:
    print("Congratulations! You passed.")
    print("Keep up the good work!")
else:
    print("You did not pass.")
    print("Study harder for next time.")

print("Total marks:", marks)

marks = int(input("Enter marks: "))  # Assume user enters 82

if marks >= 90:
    print("Grade: A")
elif marks >= 80:
    print("Grade: B")
elif marks >= 70:
    print("Grade: C")
elif marks >= 60:
    print("Grade: D")
elif marks >= 40:
    print("Grade: E")
else:
    print("Grade: F (Fail)")

age = 25
citizenship = "Indian"
has_voter_id = True

if age >= 18:
    if citizenship == "Indian":
        if has_voter_id:
            print("You are eligible to vote.")
        else:
            print("Please get a voter ID card.")
    else:
        print("Only Indian citizens can vote in Indian elections.")
else:
    print("You must be at least 18 to vote.")

# Flat version (preferred over deep nesting)
age = 25
citizenship = "Indian"
has_voter_id = True

if age >= 18 and citizenship == "Indian" and has_voter_id:
    print("You are eligible to vote.")
elif age < 18:
    print("You must be at least 18 to vote.")
elif citizenship != "Indian":
    print("Only Indian citizens can vote.")
else:
    print("Please get a voter ID card.")

age = 16

# Ternary operator
status = "Adult" if age >= 18 else "Minor"
print("Status:", status)

# Can be used directly in print
print("You are", "old enough" if age >= 18 else "too young", "to vote.")

# Nested ternary (avoid this - hard to read)
grade = "A" if age > 20 else "B" if age > 15 else "C"
print("Grade:", grade)

# Falsy values
print("0 is", "truthy" if 0 else "falsy")
print("'' is", "truthy" if "" else "falsy")
print("None is", "truthy" if None else "falsy")
print("[] is", "truthy" if [] else "falsy")
print("{} is", "truthy" if {} else "falsy")

# Truthy values
print("1 is", "truthy" if 1 else "falsy")
print("'hello' is", "truthy" if "hello" else "falsy")
print("'0' is", "truthy" if "0" else "falsy")
print("[0] is", "truthy" if [0] else "falsy")
print("-1 is", "truthy" if -1 else "falsy")

score = 75

# Chained comparison
if 0 <= score <= 100:
    print("Valid score:", score)

# Equivalent to:
if score >= 0 and score <= 100:
    print("Also valid:", score)

# Chaining works with any comparison operators
x = 5
print("1 < 5 < 10:", 1 < x < 10)
print("1 < 5 > 3:", 1 < x > 3)
print("5 == 5 == 5:", 5 == x == 5)
print("10 > 5 > 3 > 1:", 10 > x > 3 > 1)

Common Mistakes

age = 18
if age >= 18
    print("Adult")

age = 18
if age >= 18:
    print("Adult")

score = 85
if score >= 40:
print("Pass")
    print("Well done")

score = 85
if score >= 40:
    print("Pass")
    print("Well done")

x = 10
if x = 10:
    print("x is 10")

x = 10
if x == 10:
    print("x is 10")

marks = 95
if marks >= 40:
    print("Grade: E")
elif marks >= 60:
    print("Grade: D")
elif marks >= 90:
    print("Grade: A")

marks = 95
if marks >= 90:
    print("Grade: A")
elif marks >= 60:
    print("Grade: D")
elif marks >= 40:
    print("Grade: E")

user_input = input("Enter 0 to exit: ")  # User types 0
if not user_input:  # Expecting this to be True
    print("Exiting...")
else:
    print("Continuing...")

user_input = input("Enter 0 to exit: ")
if user_input == "0":  # Compare with the string '0'
    print("Exiting...")
else:
    print("Continuing...")

Summary