Truthy vs Falsy

Truthy and Falsy in Action

In the previous example we used True and False directly. Below, we have replaced is_enrolled with a string. Python will evaluate whether that string is truthy or falsy and use that as the condition.

Try changing is_enrolled to an empty string "" and run it again to see the difference.

Behind the scenes, Python is checking the data type using bool(). When Python evaluates a condition, it is essentially wrapping the value in a bool() call and asking "does this return True or False?"

Checking the bool Value

You can check the truthy or falsy value of anything by wrapping it in bool(). This is exactly what Python is doing internally when it evaluates your conditions.

The box below contains one example to get you started. Try replacing the value inside bool() with each of the following and run it each time to see what Python returns:

• 0
• 'hello'
• ''
• [1, 2, 3]
• []
• None

Real World Use: Form Validation

One of the most common real world uses of truthy and falsy is checking whether a user has filled in a form. Instead of checking if a value is explicitly True, we can simply check if it exists.

Think about a login form. If the user submits the form without filling in their username or password, those fields will be empty strings. An empty string is falsy, so Python will treat them as False. If the fields are filled in, the values are truthy and Python will treat them as True.

Try setting username or password to an empty string "" and run it again to simulate a user leaving a field blank. What happens if you just enter a "space"?

A single space " " is considered truthy. An empty string "" is falsy because it contains nothing at all. A space is a character, and a string with any character in it has a value, so Python treats it as truthy. This means a user could technically submit a single space as a password and pass the check, which is why real applications add extra validation on top of this.

Exercise 1: Spot the Falsy

• Task: Create five variables: a = 0, b = "", c = [], d = None, e = "hello".
• Goal: Use bool() to print the Boolean value of each one. Write a comment next to each print statement explaining why it is truthy or falsy.

Exercise 2: The Empty Check

• Task: Create a variable shopping_cart = [].
• Goal: Write an if/else block that prints "You have items in your cart." if the list has items in it, and "Your cart is empty." if it does not. Test both outcomes by adding items to the list and running it again.

Exercise 3: The Space Trap

• Task: Create a variable username = " " (a single space).
• Goal: Write an if/else block that prints "Username accepted." if the username is truthy and "Please enter a username." if it is falsy. Run it and note the result. Write a comment explaining why this could be a problem in a real application.

Exercise 4: Form Validation

• Task: Create three variables: name = "Sarah", email = "", phone = 0.
• Goal: Write a conditional block that checks all three fields. If all three are truthy print "Form submitted successfully.". If any one of them is falsy print "Please fill in all fields.". Test by filling in and clearing each field.

Exercise 5: Zero is not Nothing

• Task: Create a variable score = 0.
• Goal: Write an if/else block that prints "Score recorded." if the score is truthy and "No score entered." if it is falsy. Run it and note the result. Write a comment explaining why using truthy and falsy to check a score could cause a bug when the score is legitimately zero.