Conditional Statements: if/elif/else

Introduction to Conditional Logic

In your work as a storekeeper, you constantly make decisions: "If this product's expiry date is near, move it to the front. Otherwise, stock it normally." Or, "If an order is for a controlled substance, follow procedure A. Else, follow procedure B."

In programming, we use conditional statements to create the same kind of logic.

To see how this works in Python, let's watch a video that introduces the if, elif, and else keywords.

Now, let's break down these concepts one by one.

1. The if Statement and Conditions

The if statement is the simplest form of decision-making. It runs a block of code only if a certain condition is True.

A condition is an expression that Python can evaluate as either True or False. We create conditions using comparison operators.

Notice the == for comparison. A single equals sign (=) is for assigning a value to a variable, while a double equals sign (==) is for checking if two values are the same. This is a common point of confusion for new programmers.

Here's an example related to your experience:

stock_quantity = 35

# The code inside this 'if' block will only run if stock_quantity is greater than 0.
if stock_quantity > 0:
    print("Item is in stock.")
    print(f"We have {stock_quantity} units available.")

2. Indentation: Python's Special Rule

Since you have experience with C, you know that C uses curly braces {} to group blocks of code. Python does this differently: it uses indentation (the spaces at the beginning of a line).

Any code that should be part of an if, elif, or else block must be indented underneath it. This is not just for style; it is a strict rule of the language.

3. The else and elif Statements

What if you want to do something when the if condition is False? That's where else comes in.

stock_quantity = 0

if stock_quantity > 0:
    print(f"We have {stock_quantity} units available.")
else:
    print("This item is OUT OF STOCK.")

And what if you have more than two possibilities? You can use elif (short for "else if") to check multiple conditions in order.

Let's build a more complete inventory checker:

stock_quantity = 15

if stock_quantity > 50:
    print("Stock level is high.")
elif stock_quantity > 0:  # This is only checked if the first 'if' was False.
    print("Stock level is low. Time to re-order!")
else:  # This runs if all the above conditions were False.
    print("This item is OUT OF STOCK.")

Python checks the conditions from top to bottom and runs the code for the first one that is True. Once a match is found, it skips the rest of the elif and else blocks.

age = 25  # Try changing this value to 15 or 70

if age < 18:
    print("Access denied: too young")
elif age >= 18 and age <= 65:
    print("Welcome!")
else:
    print("Access denied: senior hours only")

4. Combining Conditions with and, or, not

Sometimes you need to check more than one thing at once. Python gives you three logical operators for this:

• and: True only if both conditions are true.
• or: True if at least one condition is true.
• not: Reverses the result (makes True into False and False into True).

Here are some practical examples:

# 'and' example: check if a product is both a specific type AND requires special handling.
product_type = "pharmaceutical"
requires_refrigeration = True

if product_type == "pharmaceutical" and requires_refrigeration == True:
    print("Handle with care: This is a refrigerated pharmaceutical product.")

# 'or' example: check if it's the weekend.
today = "Sunday"

if today == "Saturday" or today == "Sunday":
    print("It's the weekend! The warehouse is operating on reduced hours.")

# 'not' example: check if an order has not been shipped yet.
is_shipped = False

if not is_shipped:
    print("Order is pending shipment. Please prepare for dispatch.")

Let's Practice: Building an Access Control Script

Now it's time to put it all together. Open VS Code, create a new file named access_control.py, and type the following code. This script checks a user's role and decides what message to show them.

# access_control.py

print("--- System Access Control ---")
user_role = "storekeeper"  # Try changing this to "admin" or "guest"
is_active = True

if user_role == "admin" and is_active:
    print("Welcome, Admin. You have full system access.")

elif user_role == "storekeeper" and is_active:
    print("Welcome, Storekeeper. Accessing inventory management system.")

elif user_role == "guest":
    print("Welcome, Guest. You have view-only access.")

else:
    print("Access Denied. Your role is unrecognized or your account is inactive.")

print("---------------------------")

Run this script. Then, change the user_role variable to "admin" or "guest" and run it again. Also try changing is_active to False to see how the and operator affects the outcome.

Conclusion

Excellent work today! You have just learned one of the most powerful concepts in programming. By using if, elif, and else, you can make your scripts intelligent, responsive, and far more useful for automation.

Key Takeaways:

• Conditional Logic allows your program to make decisions and execute different code based on conditions.
• The if, elif, and else statements form the structure for this logic.
• A condition is an expression that results in True or False, often using comparison operators (==, >, <=, etc.).
• In Python, indentation (spacing) is used to define which code belongs to which block. It is not optional.
• You can create complex conditions by combining them with logical operators: and, or, and not.

Next Lesson:
So far, we've worked with single pieces of data in variables. But what if you need to handle a list of items, like all the products in a customer's order? In the next lesson, we will learn about lists and dictionaries, two powerful data structures that let you store and organize collections of data.