True and False

Programs constantly ask questions. Is the stock empty? Is the order big enough for free delivery? Python answers every such question with one of two values: True or False.

print(10 > 3)
print(10 < 3)
print(10 == 10)

True and False are values like 42 or "bread". They are called booleans. You can print them, name them, and return them from functions.

Comparisons

Written	Meaning
a == b	equal
a != b	not equal
a < b, a > b	less than, greater than
a <= b, a >= b	less than or equal, greater than or equal

Equality uses two equals signs. One = gives a name a value; two == ask whether two values are the same. Mixing them up is the most common mistake at this stage.

stock = 0
print(stock == 0)
print(stock != 0)

Comparisons work on text too. == checks the text matches exactly, including capital letters.

item = "Bread"
print(item == "bread")
print(item == "Bread")

Combining questions

and is true only if both sides are true. or is true if either side is. not flips a boolean.

quantity = 3
price = 40

print(quantity > 0 and price < 50)
print(quantity > 5 or price < 50)
print(not (quantity > 0))

Read quantity > 0 and price < 50 as "quantity is positive and price is under 50". Python works out the left side first, and only looks at the right side if it still needs to: for and, a false left side already settles it; for or, a true left side does.

The parentheses in not (quantity > 0) are optional, because not applies to the whole comparison, but they make it clear what is being flipped.

Functions that answer questions

A function can return a boolean. By convention their names read like a question: is_empty, has_discount, can_deliver.

def is_in_stock(quantity):
    return quantity > 0

print(is_in_stock(5))
print(is_in_stock(0))

There is no if here. quantity > 0 already is True or False, so it is returned directly.

Try it

Free delivery?

Delivery is free for orders of 50 or more. Write is_free_delivery(total) that returns True when delivery is free and False otherwise.

is_free_delivery(60)  ->  True
is_free_delivery(50)  ->  True
is_free_delivery(49)  ->  False

def is_free_delivery(total):
    ...

Valid quantity?

A customer can order between 1 and 20 of an item. Write is_valid_quantity(q) that returns True if q is in that range, inclusive, and False otherwise.

is_valid_quantity(1)   ->  True
is_valid_quantity(20)  ->  True
is_valid_quantity(0)   ->  False
is_valid_quantity(21)  ->  False

def is_valid_quantity(q):
    ...

Same item?

Write same_item(a, b) that returns True if the two item names are the same, ignoring whether letters are capitals or not.

same_item("bread", "bread")  ->  True
same_item("Bread", "bread")  ->  True
same_item("bread", "milk")   ->  False

You can turn a string into all lowercase with .lower(), like a.lower(). Methods get a full page later; for now, use that one.

def same_item(a, b):
    ...

Next: Making decisions.