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Python If Else

Introduction​

In programming, we often need to make decisions based on certain conditions. For example, a program might need to do one thing if a user is logged in and another if they're not. Python's conditional statements allow us to implement this decision-making logic in our code.

Conditional statements, primarily built with if, else, and elif (short for "else if"), are fundamental control flow tools that execute different blocks of code depending on whether specified conditions are True or False.

Basic If Statement​

The most basic form of conditional execution in Python is the if statement. It executes a block of code only if a specified condition evaluates to True.

Syntax​

python
if condition:
    # code to execute if condition is True

Example​

python
age = 18

if age >= 18:
    print("You are an adult.")

# Output: You are an adult.

In this example, the message prints because the condition age >= 18 evaluates to True.

If-Else Statement​

What if we want to execute one block of code when a condition is True and another when it's False? That's where the if-else statement comes in.

Syntax​

python
if condition:
    # code to execute if condition is True
else:
    # code to execute if condition is False

Example​

python
temperature = 15

if temperature > 25:
    print("It's warm outside!")
else:
    print("It's not very warm today.")

# Output: It's not very warm today.

Since the temperature is 15, which is not greater than 25, the code in the else block executes.

If-Elif-Else Statement​

Often, we need to check multiple conditions. The elif (short for "else if") statement allows us to check additional conditions if the previous conditions are not met.

Syntax​

python
if condition1:
    # code to execute if condition1 is True
elif condition2:
    # code to execute if condition1 is False and condition2 is True
else:
    # code to execute if both condition1 and condition2 are False

Example​

python
score = 85

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

# Output: Grade: B

In this example, the program checks multiple conditions to determine the appropriate grade based on the score.

Nested If Statements​

You can also place an if statement inside another if statement, creating what's called a "nested if" statement.

Syntax​

python
if condition1:
    # code to execute if condition1 is True
    if condition2:
        # code to execute if both condition1 and condition2 are True
    else:
        # code to execute if condition1 is True and condition2 is False
else:
    # code to execute if condition1 is False

Example​

python
is_weekend = True
weather = "sunny"

if is_weekend:
    if weather == "sunny":
        print("Let's go to the beach!")
    else:
        print("Let's watch a movie at home.")
else:
    print("It's a weekday, time to work.")

# Output: Let's go to the beach!

Conditional Expressions (Ternary Operators)​

Python also supports a more concise way to express simple if-else statements called conditional expressions or ternary operators.

Syntax​

python
value_if_true if condition else value_if_false

Example​

python
age = 20
status = "adult" if age >= 18 else "minor"
print(status)

# Output: adult

This is equivalent to:

python
age = 20
if age >= 18:
    status = "adult"
else:
    status = "minor"
print(status)

# Output: adult

Real-World Applications​

User Authentication​

python
username = input("Enter username: ")
password = input("Enter password: ")

# In a real application, you'd check against a database
if username == "admin" and password == "password123":
    print("Login successful!")
else:
    print("Invalid username or password.")

# Input: admin, password123
# Output: Login successful!

Weather App​

python
temperature = 28

if temperature > 30:
    print("It's very hot! Stay hydrated.")
elif temperature > 20:
    print("It's a pleasant day.")
elif temperature > 10:
    print("It's a bit cool.")
else:
    print("It's cold outside!")

# Output: It's a pleasant day.

Discount Calculator​

python
total_purchase = 120
is_member = True

if is_member:
    if total_purchase > 100:
        discount = 0.2  # 20% discount
    else:
        discount = 0.1  # 10% discount
else:
    if total_purchase > 100:
        discount = 0.1  # 10% discount
    else:
        discount = 0.05  # 5% discount

final_price = total_purchase * (1 - discount)
print(f"Final price after discount: ${final_price}")

# Output: Final price after discount: $96.0

Common Mistakes and Best Practices​

Indentation​

Python uses indentation to define code blocks. Incorrect indentation will cause syntax errors or unexpected behavior.

python
# Correct indentation
if True:
    print("This is properly indented.")
    
# Incorrect indentation - will cause an IndentationError
if True:
print("This will cause an error.")

Comparison vs. Assignment​

Be careful not to use = (assignment) when you mean == (comparison).

python
# Incorrect - assigns 10 to x and always evaluates to True
if x = 10:
    print("x is 10")
    
# Correct - checks if x equals 10
if x == 10:
    print("x is 10")

Boolean Evaluation​

Remember that in Python, empty containers, zero, None, and False itself all evaluate to False.

python
if 0:
    print("This won't print because 0 is falsy")
    
if []:
    print("This won't print because an empty list is falsy")
    
if "Hello":
    print("This will print because a non-empty string is truthy")

# Output: This will print because a non-empty string is truthy

Summary​

Conditional statements (if, else, and elif) are essential tools in Python programming that allow your code to make decisions based on conditions. They enable your programs to execute different code blocks based on whether certain conditions are met.

Key points to remember:

  • Use if to execute code when a condition is True
  • Use else to execute code when the if condition is False
  • Use elif to check additional conditions
  • Pay attention to indentation as it defines the code blocks
  • Use nested if statements for more complex conditional logic
  • Consider using conditional expressions for simple if-else statements

Exercises​

  1. Write a program that takes a user's age and prints whether they are a child (under 13), teenager (13-19), or adult (20+).

  2. Create a simple calculator that takes two numbers and an operator (+, -, *, /) and returns the result of the operation.

  3. Write a program that determines whether a year entered by the user is a leap year.

  4. Create a program that categorizes a person's BMI (Body Mass Index) as underweight, normal, overweight, or obese based on standard BMI ranges.

  5. Implement a simple grading system that takes a numerical score and outputs a letter grade (A, B, C, D, F) based on the standard grading scale.

Additional Resources​

Happy coding!


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