Learn Conditional Expressions | Conditional Statements

In this chapter, we dive into one of the most fundamental aspects of programming in Python — conditional expressions using if, else, and elif operators.

These operators allow you to execute different blocks of code based on specific conditions, giving you the power to make decisions within your programs dynamically. Understanding these expressions is crucial for developing applications that can react to various inputs and situations effectively.

Watch as Alex demonstrates the versatility of these conditional expressions:

Conditional expressions in Python let you control which parts of your code run, based on whether certain conditions are met.

Here is how each part works:

if

Use if to start a conditional check. The if statement tests a condition. If the condition is True, Python runs the block of code directly beneath it. Only one if statement can start a conditional chain.

elif

elif stands for "else if." It lets you check more conditions if the first if was not True. You can add as many elif statements as you need, each with its own condition. Python checks each elif in order until one is True or until it reaches the end of the chain.

else

The else statement goes at the end of your conditional chain. It does not use a condition. If none of the previous if or elif conditions were True, the code under else will run. There can only be one else in a chain.

Together, these statements help you guide your program to make decisions and respond to different situations.

if condition1:
    code_block
elif condition2:
    code_block
else:
    code_block

Think of it like standing at a series of doors in a restaurant:

The first door (the if statement) checks if you have a ticket. If you do, you go through and ignore the rest;
If not, the next door (the elif statement) checks if your name is on a guest list. If it is, you enter there;
If neither condition is met, you reach the last door (else). This one lets in anyone who didn't qualify for the first two, so nobody is left standing outside.

Conditional expressions in Python work just like these doors — your code chooses only one path based on the first condition that matches.

Example Applications

Let's start with a simple if / else statement to make a decision based on a single condition. Here, we are trying to determine if a grocery item needs to be restocked based on its current stock level:


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# Stock level of an item
stock_level = 15

# Simple `if`/`else` to check stock levels
if stock_level < 20: # condition
    print("Stock is low")
else:
    print("Stock is okay")

This flowchart demonstrates the decision paths that your Python interpreter would take in the previous if-else example:

Next, let's utilize the elif statement in a slightly more complex application. In this application, we use if / elif / else statements to apply discount rates based on total costs to encourage larger sales.


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# Initial conditions
totalCost = 150

# Applying discounts based on purchase amount
if totalCost >= 200: # condition
    print("20% discount applied")
elif totalCost >= 100: # additional condition
    print("10% discount applied")
else:
    print("No discount for purchases under $100")

This flowchart visualizes the process of applying discounts based on total cost using if, elif, else statements:

Conditional statements allow your Python programs to make choices and execute different code paths based on various conditions. This is the basic conditional structure in Python. Now, let's move on to practice applying these concepts with some interactive tasks.

Task

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Write a program that prints a discount message based on the product type and the day of the week.

Use the variables product_type and day_of_week.
If product_type is "Fruits" and day_of_week is "Monday", print Fruits discount!
If product_type is "Vegetables" and day_of_week is "Tuesday", print Vegetables discount!
Otherwise, print No discount.

Output Requirements:

Print exactly one line based on the input values.
Valid outputs are:
- Fruits discount!
- Vegetables discount!
- No discount.

Solution

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Section 3. Chapter 6

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