Control Flow
If Statements

“Okay okay okay, boolean variables, boolean expressions, blah blah blah, I thought I was learning how to build control flow into my code!”

You are, I promise you!

Understanding boolean variables and expressions is essential because they are the building blocks of conditional statements.

Recall the waking-up example from the beginning of this lesson. The decision-making process of “Is it raining? If so, bring an umbrella” is a conditional statement. Here it is phrased in a different way:

If it is raining then bring an umbrella.

Can you pick out the boolean expression here?

Right, "it is raining" is the boolean expression, and this conditional statement is checking to see if it is True.

If "it is raining" == True then the rest of the conditional statement will be executed and you will bring an umbrella.

This is the form of a conditional statement:

If [it is raining] then [bring an umbrella]

In Python, it looks very similar:

if is_raining: bring_umbrella()

You’ll notice that instead of “then” we have a colon, :. That tells the computer that what’s coming next is what should be executed if the condition is met. Let’s take a look at another conditional statement:

if 2 == 4 - 2: print("apple")

Will this code print apple to the terminal? Yes, because the condition of the if statement, 2 == 4 - 2 is True.

Let’s work through a couple more together:



In the workspace script.py there is a function with an if statement. I wrote this function because my coworker Dave kept using my computer without permission and he is a real doofus. It takes user_name as an input and if the user is Dave it tells him to stay off my computer.

Enter a user name in the field for user_name and try running the function.


Oh no! We got a SyntaxError! This happens when we make a small error in the syntax of the conditional statement.

Read through the error message carefully and see if you can find the error. Then, fix it, and run the code again.


Ugh! Dave got around my security and has been logging onto my computer using our coworker Angela’s user name, angela_catlady_87.

Update the function with a second if statement so it checks for this user name as well and returns

"I know it is you Dave! Go away!"

in response. That’ll teach him!

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