Python Conditional Statements

Introduction

Conditional statements are the decision-makers in programming. They allow your program to behave differently based on different conditions, essentially enabling your code to make decisions. In Python, these decisions are primarily implemented using if, elif (short for "else if"), and else statements.

Think of conditional statements as the crossroads in your code's journey. Based on certain conditions, your program will take different paths, executing specific blocks of code while skipping others.

Basic Syntax of Conditional Statements

The if Statement

The most fundamental conditional statement in Python is the if statement. It evaluates a condition and executes a block of code if that condition is True.

python

if condition:
    # Code to execute if the condition is True

Here's a simple example:

python

age = 18

if age >= 18:
    print("You are eligible to vote!")

Output:

You are eligible to vote!

In this example, the code checks if age is greater than or equal to 18. Since the condition is True, the print statement executes.

The else Statement

The else statement provides an alternative block of code to execute when the if condition is False.

python

if condition:
    # Code to execute if the condition is True
else:
    # Code to execute if the condition is False

Let's extend our voting example:

python

age = 16

if age >= 18:
    print("You are eligible to vote!")
else:
    print("Sorry, you are not eligible to vote yet.")

Output:

Sorry, you are not eligible to vote yet.

The elif Statement

Short for "else if," the elif statement allows you to check multiple conditions in sequence.

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 with multiple conditions:

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

Nested Conditional Statements

You can place conditional statements inside other conditional statements, creating nested conditions.

python

temperature = 25
humidity = 80

if temperature > 30:
    print("It's hot outside!")
    if humidity > 60:
        print("And it's humid too!")
else:
    print("Temperature is moderate.")
    if humidity > 60:
        print("But it's quite humid.")

Output:

Temperature is moderate.
But it's quite humid.

Logical Operators with Conditionals

Python provides logical operators to combine multiple conditions:

• and: Both conditions must be True
• or: At least one condition must be True
• not: Inverts the condition (makes True into False and vice versa)

python

username = "admin"
password = "12345"

if username == "admin" and password == "12345":
    print("Login successful")
else:
    print("Invalid credentials")

Output:

Login successful

Using or:

python

day = "Saturday"

if day == "Saturday" or day == "Sunday":
    print("It's the weekend!")
else:
    print("It's a weekday.")

Output:

It's the weekend!

Using not:

python

is_raining = False

if not is_raining:
    print("No need for an umbrella!")
else:
    print("Take an umbrella!")

Output:

No need for an umbrella!

Conditional Expressions (Ternary Operator)

Python offers a shorthand way to write simple conditional statements using the ternary operator:

python

# Syntax: 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)

Real-World Applications

1. Authentication System

python

def authenticate_user():
    username = input("Enter username: ")
    password = input("Enter password: ")
    
    stored_username = "user123"
    stored_password = "pass456"
    
    if username == stored_username and password == stored_password:
        print("Authentication successful!")
        return True
    else:
        print("Invalid username or password!")
        return False

# Testing the function
authenticate_user()

If you enter "user123" as username and "pass456" as password, the output will be:

Authentication successful!

Otherwise:

Invalid username or password!

2. Temperature Converter with Validation

python

def convert_temperature():
    temp_str = input("Enter temperature (e.g., '32F' or '0C'): ")
    
    if len(temp_str) < 2:
        print("Invalid format. Use format like '32F' or '0C'.")
        return
    
    try:
        temp = float(temp_str[:-1])
        unit = temp_str[-1].upper()
        
        if unit == 'C':
            result = (temp * 9/5) + 32
            print(f"{temp}°C is equal to {result:.1f}°F")
        elif unit == 'F':
            result = (temp - 32) * 5/9
            print(f"{temp}°F is equal to {result:.1f}°C")
        else:
            print("Invalid unit. Use 'C' for Celsius or 'F' for Fahrenheit.")
    except ValueError:
        print("Invalid temperature. Please enter a number followed by 'C' or 'F'.")

# Testing the function
convert_temperature()

If you enter "32F", the output will be:

32.0°F is equal to 0.0°C

3. Simple Menu System

python

def display_menu():
    print("\n=== Main Menu ===")
    print("1. View Profile")
    print("2. Edit Settings")
    print("3. Help")
    print("4. Exit")
    
    choice = input("\nEnter your choice (1-4): ")
    
    if choice == '1':
        print("Viewing your profile...")
    elif choice == '2':
        print("Opening settings menu...")
    elif choice == '3':
        print("Displaying help documentation...")
    elif choice == '4':
        print("Exiting program. Goodbye!")
    else:
        print("Invalid choice. Please select a number between 1 and 4.")

# Testing the function
display_menu()

Additional Techniques

Using in with Conditionals

The in operator checks if a value exists in a sequence (like a list, tuple, or string):

python

fruits = ["apple", "banana", "cherry"]

if "banana" in fruits:
    print("Yes, banana is a fruit!")

Output:

Yes, banana is a fruit!

Truth Value Testing

In Python, values are inherently considered True or False in a boolean context:

• Empty sequences (lists, tuples, strings, etc.) are considered False
• Zero in any numeric type is considered False
• None is considered False
• Everything else is generally considered True

python

# Examples
if []:
    print("This won't print because an empty list is False")
    
if 0:
    print("This won't print because 0 is False")
    
if "Hello":
    print("This will print because a non-empty string is True")

Output:

This will print because a non-empty string is True

Common Pitfalls and Best Practices

1. Using == vs. is

The == operator compares values, while is compares identities (if they're the same object):

python

a = [1, 2, 3]
b = [1, 2, 3]

print(a == b)  # True - same values
print(a is b)  # False - different objects

Output:

True
False

2. Be Careful with Mutable Default Arguments

python

# Problematic
def add_item(item, list=[]):
    list.append(item)
    return list

print(add_item("apple"))  # ['apple']
print(add_item("banana"))  # ['apple', 'banana'] (not what you might expect)

# Better approach
def add_item_better(item, list=None):
    if list is None:
        list = []
    list.append(item)
    return list

print(add_item_better("apple"))  # ['apple']
print(add_item_better("banana"))  # ['banana']

3. Using Multiple Conditions Efficiently

python

# Less efficient (evaluates all conditions)
if x > 0 and x < 10 and x % 2 == 0:
    print("x is a positive even number less than 10")

# More efficient (short-circuit evaluation)
if 0 < x < 10 and x % 2 == 0:
    print("x is a positive even number less than 10")

Summary

Conditional statements are fundamental building blocks in Python programming that allow your code to make decisions based on different conditions:

• if statements execute code when a condition is true
• elif statements provide additional conditions to check
• else statements provide a fallback when no conditions are met
• Logical operators (and, or, not) allow combining or inverting conditions
• The ternary operator provides a concise way to write simple conditional expressions

Mastering conditional statements is essential for writing dynamic and responsive programs that can adapt to different inputs and situations.

Practice Exercises

1. Basic Condition: Write a program that checks if a number is positive, negative, or zero.
2. Grade Calculator: Create a function that takes a numerical score and returns the corresponding letter grade.
3. Leap Year Checker: Write a program that determines if a given year is a leap year.
4. Discount Calculator: Create a function that calculates the final price after applying discounts based on the purchase amount.
5. Simple Calculator: Implement a basic calculator that performs different operations (+, -, *, /) based on user input.

Additional Resources

• Official Python Documentation on Control Flow
• Python Conditional Statements (W3Schools)
• Python Practice Problems (HackerRank)
• Python Principles: Conditionals

Remember, practice is key to mastering conditional statements. Start with simple conditions and gradually work your way up to more complex decision-making logic in your programs.