Truthy and Falsy Values¶

In previous chapters we have used the boolean values True and False. We already know that expressions with operators can evaluate to True or False.

10 > 2

True

if 10 > 2:
    print('Hello World!')

Hello World!

We use these expressions a lot in if statements, while loops and so on. Now consider the following if statements where there is no use of an operator such as >, <, == etc.

x = 10
if x:
    print('Hello World')
else:
    print('Good Bye')

Hello World

x = 0
if x:
    print('Hello World')
else:
    print('Good Bye')

Good Bye

x = [1, 2, 3 ]
if x:
    print('Hello World')
else:
    print('Good Bye')

Hello World

x = []
if x:
    print('Hello World')
else:
    print('Good Bye')

Good Bye

You may be wondering how the above if statements are even evaluating to True or False. There is not a typical expression next to the if. Instead, only a variable is next to the if. In Python, specific values can evaluate to either True or False even if they are not part of a larger expression. The basic idea is that values that evaluate to False are considered Falsy whereas values that evaluate to True are considered Truthy. There are several rules we need to know to figure out what these values will evaluate to. You can checkout out the official Python documentation for these rules. We will cover them here mostly.

By default, the majority of values in Python will be truthy. That is they will evaluate to True. So you just need to remember what type of values evaluate to False. They are:

constants defined to be false: None and False.
zero of any numeric type: 0 and 0.0 for example.
empty sequences and collections: '', (), [], {}, set(), range(0)

So think of 0 and anything “empty” (having a length of 0) as evaluating to False.

if 0:
    print('I will not print')
elif []:
    print('I will not print')
elif {}:
    print('I will not print')
elif set():
    print('I will not print')
elif ():
    print('I will not print')
elif None:
    print('I will not print')
else:
    print('I will print because every value above evaluates to False.')

I will print because every value above evaluates to False.

Here are some more examples.

# once i gets to 0 it will evaluate to False and the loop will break/stop
i = 3
while i:
    print(i)
    i = i - 1

3
2
1

if 5 and 0:
    print('HEY')

if 3 and -10:
    print('HEY')

HEY

if 0 or [] or {}:
    print('HEY')

if 0 or [] or {} or 0.5:
    print('HEY')

HEY

if None:
    print('HEY')

if not None:
    print('Hey')

Hey

That is it for truthy and falsy values. It’s very important to remember these when using control flow. In the next section we will learn more about the None value type.

Learn to code with an Introduction to Python 3

Truthy and Falsy Values¶