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:

The if statement evaluates a condition and executes a block of code if the condition is True. If the condition is not True, the else and elif (short for "else if") statements can specify additional conditions to check and execute different code accordingly.

Here's how they work:

if

The primary conditional statement evaluates whether a condition is True. It initiates a conditional sequence. if statements can only appear once at the start of a sequence. If the condition evaluates to True, the indented block of code following the if statement will execute.

elif

Short for "else if", the elif statement provides additional conditions to check if the initial if or any preceding elif conditions were False. You can include multiple elif statements following an if statement to handle various scenarios, each with its own condition.

else

This acts as a catch-all for cases not specifically addressed by the preceding if and elif conditions. There can only be one else statement at the end of an if statement sequence, and it does not require a condition.

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:

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.

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

In the next application, we introduce nested if-else conditions to demonstrate a more granular decision-making process. By embedding one if-else structure within another, we are able to make a series of decisions based on multiple criteria.

This practical example illustrates how a grocery store might manage inventory based on product type and specific conditions, such as days since delivery or stock levels.

The following decision tree diagram visually represents the nested if-else logic used in the previous code example:

Some Syntax Notes

When writing conditional statements in Python, adhering to specific syntax rules is essential. You might have noticed the critical role of colons : and indentation in creating executable conditions.

Let's examine the role that these minor details play:

Colons

Think of the colon at the end of an if, elif, or else statement as a signal that says, "Here's what to do next if the condition I just mentioned is true." The colon acts like a signpost, clearly marking where the instructions begin.

Indentation

When the condition above an indented block of code is True, all the indented lines of code below it are executed. If it's not True, Python skips these steps and looks for the next condition.

While these may seem like minor details, they are crucial for ensuring your code executes properly. As you become more familiar with Python, these practices will become second nature.

Fantastic work on grasping these foundational aspects of Python! Your understanding of how to structure control statements correctly sets a solid base for tackling more complex programming tasks. Now, let's put your new skills to the test with a comprehensive challenge that will allow you to apply what you've learned in a practical scenario.