Conditional Statements in Python
Complete Tutorial with Syntax, Examples & Output
1. Introduction
A conditional statement is used to make decisions in a Python program. It allows the program to execute different statements depending on whether a given condition is True or False.
For example, a program can check whether a student has passed an examination, whether a person is eligible to vote, or whether a number is positive or negative.
Condition → False → Execute another block
2. Types of Conditional Statements
Python provides several ways to perform conditional execution:
| Statement | Purpose |
|---|---|
if |
Executes code when a condition is True |
if-else |
Chooses between two alternatives |
if-elif-else |
Chooses among multiple alternatives |
Nested if |
Places one conditional statement inside another |
| Conditional Expression | Provides a compact one-line condition |
3. if Statement
The if statement executes a block of code only when its condition
evaluates to True.
Syntax
if condition:
statementExample
age = 20
if age >= 18:
print("Eligible to vote")Output
Eligible to vote4. Boolean Conditions
A condition generally produces one of two Boolean values:
TrueFalse
Example
age = 20
print(age >= 18)Output
True5. if-else Statement
The if-else statement is used when there are two possible
execution paths.
Syntax
if condition:
statement1
else:
statement2Example
number = 7
if number % 2 == 0:
print("Even")
else:
print("Odd")Output
Odd6. Practical Example – Pass or Fail
marks = int(input("Enter marks: "))
if marks >= 40:
print("Pass")
else:
print("Fail")Sample Output
Enter marks: 65
Pass7. if-elif-else Statement
The if-elif-else structure is used when multiple conditions need
to be checked.
Syntax
if condition1:
statement1
elif condition2:
statement2
elif condition3:
statement3
else:
statement4Example
marks = 82
if marks >= 90:
print("Grade A+")
elif marks >= 80:
print("Grade A")
elif marks >= 70:
print("Grade B")
else:
print("Grade C")Output
Grade A8. Practical Example – Student Grade
marks = int(input("Enter marks: "))
if marks >= 90:
grade = "A+"
elif marks >= 80:
grade = "A"
elif marks >= 70:
grade = "B"
elif marks >= 60:
grade = "C"
elif marks >= 40:
grade = "D"
else:
grade = "F"
print("Grade:", grade)Sample Output
Enter marks: 76
Grade: B9. Nested Conditional Statement
A nested conditional statement is a conditional statement placed inside another conditional statement.
Example
age = 22
has_id = True
if age >= 18:
if has_id:
print("Entry allowed")
else:
print("ID required")
else:
print("Entry denied")Output
Entry allowed10. Comparison Operators in Conditions
| Operator | Meaning | Example |
|---|---|---|
== |
Equal to | a == b |
!= |
Not equal to | a != b |
> |
Greater than | a > b |
< |
Less than | a < b |
>= |
Greater than or equal | a >= b |
<= |
Less than or equal | a <= b |
11. Logical Operators in Conditions
Logical operators are used to combine multiple conditions.
| Operator | Meaning |
|---|---|
and |
True only when both conditions are True |
or |
True when at least one condition is True |
not |
Reverses the Boolean result |
Example using and
age = 21
marks = 65
if age >= 18 and marks >= 40:
print("Eligible")
else:
print("Not eligible")Output
Eligible12. Using the or Operator
day = "Sunday"
if day == "Saturday" or day == "Sunday":
print("Holiday")
else:
print("Working day")Output
Holiday13. Using the not Operator
logged_in = False
if not logged_in:
print("Please login")Output
Please login14. Practical Example – Largest of Two Numbers
a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
if a > b:
print("Largest =", a)
elif b > a:
print("Largest =", b)
else:
print("Both are equal")Sample Output
Enter first number: 25
Enter second number: 40
Largest = 4015. Practical Example – Largest of Three Numbers
a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
c = int(input("Enter third number: "))
if a >= b and a >= c:
print("Largest =", a)
elif b >= a and b >= c:
print("Largest =", b)
else:
print("Largest =", c)Sample Output
Enter first number: 25
Enter second number: 70
Enter third number: 45
Largest = 7016. Practical Example – Positive, Negative or Zero
number = int(input("Enter a number: "))
if number > 0:
print("Positive")
elif number < 0:
print("Negative")
else:
print("Zero")Sample Output
Enter a number: -15
Negative17. Practical Example – Leap Year
A year is a leap year if it is divisible by 400, or if it is divisible by 4 but not divisible by 100.
year = int(input("Enter year: "))
if year % 400 == 0:
print("Leap year")
elif year % 100 == 0:
print("Not a leap year")
elif year % 4 == 0:
print("Leap year")
else:
print("Not a leap year")Sample Output
Enter year: 2024
Leap year18. Practical Example – Voting Eligibility
age = int(input("Enter your age: "))
if age >= 18:
print("Eligible to vote")
else:
print("Not eligible to vote")Sample Output
Enter your age: 19
Eligible to vote19. Practical Example – Login Validation
username = input("Enter username: ")
password = input("Enter password: ")
if username == "admin" and password == "1234":
print("Login successful")
else:
print("Invalid username or password")Sample Output
Enter username: admin
Enter password: 1234
Login successful20. Practical Example – Calculator Using Conditions
a = float(input("Enter first number: "))
b = float(input("Enter second number: "))
op = input("Enter operator (+, -, *, /): ")
if op == "+":
print("Result =", a + b)
elif op == "-":
print("Result =", a - b)
elif op == "*":
print("Result =", a * b)
elif op == "/":
if b != 0:
print("Result =", a / b)
else:
print("Cannot divide by zero")
else:
print("Invalid operator")Sample Output
Enter first number: 20
Enter second number: 5
Enter operator (+, -, *, /): *
Result = 100.021. Conditional Expression
A conditional expression is a compact way to write an if-else
decision in a single line.
Syntax
value_if_true if condition else value_if_falseExample
age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)Output
Adult22. Membership Operators in Conditions
The operators in and not in can be used to test
whether a value exists in a sequence.
Example
subjects = ["C", "Python", "Java"]
subject = "Python"
if subject in subjects:
print("Subject found")
else:
print("Subject not found")Output
Subject found23. Identity Operators in Conditions
Python provides is and is not for identity testing.
They check whether two references refer to the same object.
Example
a = None
if a is None:
print("No value assigned")Output
No value assigned24. Truth Value in Conditions
Python allows many objects to be used directly as conditions. For example, an empty list is considered false, while a non-empty list is considered true.
Example
numbers = [10, 20, 30]
if numbers:
print("List contains elements")
else:
print("List is empty")Output
List contains elements25. Importance of Indentation
Python uses indentation to identify the statements belonging to a conditional block.
Correct Example
age = 20
if age >= 18:
print("Adult")
print("Eligible")
Both print() statements belong to the if block
because they have the same indentation.
26. Flow of Conditional Statement
START
↓
Evaluate Condition
↓
Is Condition True?
↙ ↘
YES NO
↓ ↓
Execute True Block Execute False/Next Block
↘ ↙
Continue Program
↓
END
27. Common Errors
-
Forgetting the colon:
if age >= 18: - Incorrect indentation: Statements inside the condition must be indented.
-
Using = instead of ==:
=performs assignment, while==checks equality. -
Incorrect condition order: In an
if-elifstructure, conditions should be arranged carefully. - Using too many nested conditions: Complex nesting can make programs difficult to understand and maintain.
28. if vs if-else vs if-elif-else
| Structure | Use |
|---|---|
if |
One condition |
if-else |
Two alternatives |
if-elif-else |
Multiple alternatives |
Nested if |
Decision inside another decision |
| Conditional expression | Compact one-line decision |
29. Real-Life Applications
- Student result and grading systems
- Login and authentication systems
- Banking applications
- Online shopping and discount calculation
- Voting eligibility checking
- ATM transaction validation
- Weather-based decisions
- Traffic signal control
- Game decision-making
- Machine Learning classification logic
30. Summary
Conditional statements are fundamental to Python programming. They allow a program to make decisions and execute different blocks of code depending on conditions.
ifexecutes code when a condition is True.if-elseprovides two alternative paths.if-elif-elsehandles multiple conditions.- Nested conditions allow decisions within decisions.
- Comparison operators are commonly used to construct conditions.
- Logical operators combine multiple conditions.
- Conditional expressions provide a compact one-line alternative.
- Proper indentation is essential in Python.
Created by Bijan Krishna Paul
Computer Science • Python Programming • Data Science