The and & or keywords

Summary of this lecture

The and and or keywords behave slightly differently than many learners believe. Every value in Python can be turned to a boolean (True/False), so we say values can be "truthy" or "falsy" based on what boolean they return.

Zero or empty values, such as 0, "", or empty lists and tuples, are "falsy". Almost every other value is "truthy".

The and keyword returns the first value if it is falsy. Otherwise it returns the second value. The or keyword returns the first value if it is truthy. Otherwise it returns the second value.

Using and & or with Boolean values

The and keyword

The and and or keywords behave logically with Boolean values. If you say "the door is open and the window is closed", everyone understands that both things are true.

Let's say you've asked the user for their age, they've entered 25, and you want to see if the age is over 18 and under 65:

age = 25  # could be int(input("Enter your age: "))

result = age > 18 and age < 65  # True and True
print(result)  # True

If we make a change to check the age is under 18 and over 65, then we get False:

age = 25

result = age < 18 and age > 65  # False and False
print(result)  # False

If we do something like age < 18 and age < 65, we still get False because both conditions aren't True. age < 18 is False:

age = 25

result = age < 18 and age < 65  # False and True
print(result)  # False

Here's a table of the potential options for the and keyword.

First value	Second value	Operation	Result
True	False	True and False	False
True	True	True and True	True
False	True	False and True	False
False	False	False and False	False

Something interesting to note is that the and keyword returns the first value if it is False, otherwise it returns the second value.

This may seem confusing, but it actually simplifies the way the keyword works. For example, here:

age = 25

result = age < 18 and age < 65  # False and ??? doesn't matter
print(result)  # False

Python doesn't have to calculate age < 65 because it knows that the result will be False, since False and anything is always False.

Here's a diagram to help you remember how the and keyword works:

The or keyword

The or keyword is more or less the opposite!

That's because if the first value is True, then it doesn't matter what the second value is, True or anything will always be True.

For example, let's look at this code:

age = 25

result = age > 18 or age < 65  # True and ??? doesn't matter
print(result)  # True

Have a play around with the and and or keywords, and try out all the different combinations or True and False!

Here's a table of the or keyword:

First value	Second value	Operation	Result
True	False	True or False	True
True	True	True or True	True
False	True	False or True	True
False	False	False or False	False

Using and & or with non-Boolean values

Truthy and Falsy values (Truth Value Testing)

In the diagrams earlier I wrote the values must be "truthy" or "falsy". I didn't write True and False on purpose.

Almost every value in Python can be converted into a Boolean value using the bool() function, even if it doesn't seem to make "sense". For example:

bool(0)  # False, zero
bool(13)  # True

bool("")  # False, empty string
bool("Hello")  # True

bool([])  # False, empty list
bool([1, 3, 5])  # True

Generally, zero or empty values evaluate to False when passed through bool(). Non-zero or non-empty values evaluate to True. For a more complete list, see the official documentation.

So if you use the and or or keywords with any value, even if they are not Booleans, the principle still applies.

Here's an example:

default_age = 30
age = 0

user_age = age or default_age
print(user_age)  # 30

Since age is 0, and that is a "falsy" value, then or returns the second value, which is 30.

This may not seem very useful just now, but it's actually a super-common pattern. Here's another, more complete example:

default_greeting = "there"
name = input("What's your name? (optional) ")
user_name = name or default_greeting

print(f"Hello, {user_name}!")  # Hello, there or Hello, Bob (example)

It's less common to use the and keyword in this way, but it can still be done. It's worth knowing about it!

Examples of and and or on non-Boolean values

Here are a few questions, see if you can calculate the correct result!

Questions

1. 34 and 10
2. 0 and "hello"
3. 10 and "world"
4. -3 and 87.5

Answers

1. 34 and 10 ➡️ 34 is truthy, so and returns the second value. The result is 10.
2. 0 and "hello" ➡️ 0 is falsy, so and returns it. The result is 0.
3. 10 and "world" ➡️ 10 is truthy, so and returns the second value. The result is "world".
4. -3 and 87.5 ➡️ -3 is truthy, so and returns the second value. The result is 87.5.

And now for the or keyword:

Questions

1. None or 35
2. 0 or "hello"
3. 10 or "world"
4. -3 or 87.5

Answers

1. None or 35 ➡️ None is falsy, so or returns the second value. The result is 35.
2. 0 or "hello" ➡️ 0 is falsy, so or returns the second value. The result is "hello".
3. 10 or "world" ➡️ 10 is truthy, so or returns it. The result is 10.
4. -3 or 87.5 ➡️ -3 is truthy, so or returns it. The result is -3.