Booleans

In the if-else lesson we briefly saw True and False, and in the loops we even used while True. Time to look at them more carefully, because they are everywhere in programming

A boolean is a value that can only be one of two things: True or False. That's it. No other options

is_online = True
has_key = False
print(is_online)
print(has_key)

Be careful: True and False are written with a capital letter. true and false will give an error in Python

We already know that conditions like age < 18 or x == 5 are checked by if. Well, those conditions are actually booleans in disguise

age = 20
print(age < 18)     # False
print(age >= 18)    # True

Try it. Python literally prints True or False

So far so good. But what if we want to combine multiple conditions? Imagine a story: in order to drive a car, you need to be at least 18 years old AND have a license. Both must be true at the same time

age = 20
has_license = True

if age >= 18 and has_license == True:
    print("You can drive")
else:
    print("You must be at least 18 and have a license to drive")

The keyword and says: both conditions must be True. If even one of them is False, the whole thing is False

In other words, if we have left and right, both left and right must be True for the whole expression to be True

age = 20
has_license = False

if age >= 18 and has_license == True:
    print("You can drive")
else:
    print("You must be at least 18 and have a license to drive")

Here age >= 18 is True, but has_license == True is False. True and False = False, so we go to else

By the way, has_license == True is the same as just writing has_license, since has_license is already a boolean. So we can shorten it

if age >= 18 and has_license:
    print("You can drive")

Programmers often prefer to drop the == True because if has_license already means if has_license == True. You can write it either way, both are equally correct

There is also or. Imagine: you can enter the club if you are a VIP OR you have a special invitation. Just one of them is enough

is_vip = False
has_invitation = True

if is_vip or has_invitation:
    print("Welcome to the club")
else:
    print("Access denied")

Even though is_vip is False, has_invitation is True, and that's enough

And finally, not. not flips a boolean: not True becomes False, not False becomes True

is_logged_in = False
if not is_logged_in:
    print("Please log in first")

Reads almost like English: if not logged in, please log in

Quick summary, the truth tables

True  and True  = True
True  and False = False
False and True  = False
False and False = False

True  or  True  = True
True  or  False = True
False or  True  = True
False or  False = False

not True  = False
not False = True

Booleans in loops

We've already seen a boolean as a condition: while True runs as long as the condition is... True. But we can also use a boolean stored in a variable to control a loop. Such a variable is often called a flag

running = True
number = 0
while running:
    print(number)
    number = number + 1
    if number == 3:
        running = False
print("Done")

Output

0
1
2
Done

As long as running is True, the loop continues. When the if makes it False, the condition while running becomes false and the loop stops at the next check. It's a clean alternative to break: instead of jumping out abruptly, we let the condition close itself

Mission: Command Console

The station has a console that runs as long as it's powered on. Use a boolean variable powered_on (starts as True) as the condition for a while loop. On every step, read a command with input():

if the user types shutdown → set powered_on to False (the loop will stop here)
if the user types status → print System active
otherwise → print Unknown command

After the loop ends, print Closing console....

Example

If the user types status, then hello, then shutdown in turn, the program prints

System active
Unknown command
Closing console...