Not

Consider the "not" keyword in Python. With this keyword we change the meaning of expressions—it changes a true result to false.

With "not" we invert an expression in an elegant and clear way. We use not in if-statements. Sometimes, we want to flip or invert the value of a boolean variable.

Not in list

To start, we can use "not" as part of an in-expression. With the in-keyword we test for inclusion. We see if a value is in a list or a string or a dictionary.

Part 1 We test the list's contents with a "not in" operator. The string "red" is not an element of the list, so we print the message.

Part 2 The string "blue" is found in the list, so the "in" operator here returns true, and we print a message again.

colors = ["blue", "green"]

# Part 1: use not in on a list.
if "red" not in colors:
    print("NOT IN, red")

# Part 2: use in operator without not.
if "blue" in colors:
    print("IN, blue")NOT IN, red
IN, blue

If not example

In this example, we use an "if not" clause to test that a method returns False. So if equals() returns false, the if-block is entered.

Detail We can also use "not" inside an elif clause. The not-keyword can be added to the start of any expression.

Here The 3 "false" values are printed, as the 3 print statements are reached when this program executes in the Python interpreter.

def equals(first, second):
    # Return true if the two ints are equal.
    return first == second

value = 10
value2 = 100

if value != value2:
    # This is reached.
    print(False)

if not equals(value, value2):
    # This is reached.
    print(False)

if value == 0:
    print(0)
elif not equals(value, value2):
    # This is reached.
    print(False)False
False
False

Invert booleans

With not, we can change True to False and False to True. This may help with creating a loop that alternates between True and False.

Thus We can invert our boolean value with not—we apply "not" to get the inverse of our boolean.

value = True
print(value)

# Change True to False with not.
value = not value
print(value)

# Invert the value back to True.
value = not value
print(value)True
False
True

Not, not

This is something that is not often going to be useful. But we can chain multiple "not" keywords at the start of an expression.

Info This example tells us about how the not-operator is parsed in the language. It modifies the following expression.

valid = False
print(valid)

# We can make something "not not."
valid = not not valid
print(valid)
False

Not with equality

Instead of the "!=" operator, we can use not with the "==" operator. It is probably clearer just to use the shorter form.

# Initial animal values.
animal1 = "bird"
animal2 = "bird"

# Compare two strings.
if animal1 == animal2:
    print(1)

# Change our animals.
animal1 = "lizard"
animal2 = "fish"

# We can use "not" in front of an equals expression.
if not animal1 == animal2:
    print(2)1
2

With "not" we write Python code that sounds almost like normal English. It is expressive. It is easy to read and parse for humans.