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Effortlessly Check if a Variable is Not in Python


Using the “not” Boolean Operator in Python

Working With Boolean Logic in Python

George Boole put together what is now known as Boolean algebra, which relies on true and false values. It also defines a set of Boolean operations: AND, OR, and NOT. These Boolean values and operators are helpful in programming because they help you decide the course of action in your programs.

In Python, the Boolean type, bool, is a subclass of int:

issubclass(bool, int)
# Output: True

This type has two possible values, True and False, which are built-in constants in Python and must be capitalized. Internally, Python implements them as integer numbers:

# Output: <class 'bool'>
# Output: <class 'bool'>
isinstance(True, int)
# Output: True
isinstance(False, int)
# Output: True
# Output: 1
# Output: 0

Python internally implements its Boolean values as 1 for True and 0 for False.

Getting Started With Python’s not Operator

Python’s not operator allows you to invert the truth value of Boolean expressions and objects. The not operator takes a single operand and returns False if the operand is true, and True if the operand is false.

Let’s see some examples of using the not operator:

x = True
y = False
print(not x)
# Output: False
print(not y)
# Output: True

In the first example, x is True. When we apply the not operator to x, it returns False. In the second example, y is False, and applying the not operator to y returns True.

Using the not Operator in Boolean Contexts

The not operator is commonly used in Boolean contexts such as if statements and while loops.

if Statements

In an if statement, the not operator allows you to check if a condition is not true. Here’s an example:

x = 5
if not x > 10:
print("x is not greater than 10")
print("x is greater than 10")

In this example, the not operator negates the condition x > 10. Since x is not greater than 10, the output will be “x is not greater than 10”.

while Loops

You can also use the not operator in the condition of a while loop to execute the loop as long as a condition is not met. Here’s an example:

x = 0
while not x > 5:
x += 1

In this example, the while loop will continue to execute as long as x is not greater than 5. It will print the value of x and increment it by 1 until x becomes greater than 5.

Using the not Operator in Non-Boolean Contexts

The not operator in Python can also be used in non-Boolean contexts, allowing you to invert the truth value of your variables. In this case, the not operator performs logical negation by converting the operand to a Boolean value and then negating it.

Here’s an example:

x = 42
if not x:
print("x is false")
print("x is true")

In this example, the not operator converts x to a Boolean value and then negates it. Since x is a non-zero number, it is considered True in a Boolean context. Therefore, the output will be “x is true”.

Using the Function-Based not Operator

In addition to the not operator, Python provides the operator.not_() function, which performs logical negation in a similar way. This function can be useful in situations where you need to programmatically negate an expression.

Here’s an example:

import operator
x = True
result = operator.not_(x)
# Output: False

In this example, we import the operator module and use the not_() function to negate the value of x. The result is printed as False.

Working With Python’s not Operator: Best Practices

When using the not operator in your code, it’s important to follow some best practices to ensure readability and avoid unnecessary negative logic.

Test for Membership

The not operator can be used to test for the absence of an element in a sequence. Instead of using the in operator together with the not operator, you can use the not in operator directly.

Here’s an example:

numbers = [1, 2, 3, 4, 5]
if 6 not in numbers:
print("6 is not in the list")

In this example, the not in operator checks if 6 is not present in the numbers list. If it is not present, the message “6 is not in the list” is printed.

Check the Identity of Objects

When checking the identity of objects, it’s better to use the is operator instead of the not operator together with the == operator. The is operator tests for object identity, while the == operator tests for object equality.

Here’s an example:

x = [1, 2, 3]
y = [1, 2, 3]
if x is not y:
print("x and y are not the same object")

In this example, the is not operator checks if x and y refer to different objects. If they refer to different objects, the message “x and y are not the same object” is printed.

Avoid Unnecessary Negative Logic

While the not operator is useful for inverting Boolean values and expressions, it’s important to use it judiciously and avoid unnecessary negative logic. Negative logic can make your code harder to read and understand.

Here’s an example of unnecessary negative logic:

if not condition1 and not condition2:
print("Both condition1 and condition2 are false")

In this example, the use of not operators can make the conditions more difficult to understand. A better approach would be to use the positive form of the conditions:

if not condition1 and not condition2:
print("Both condition1 and condition2 are false")

This version of the code reads more naturally and is easier to understand.


Python’s not operator allows you to invert the truth value of Boolean expressions and objects. Whether you’re using it in Boolean contexts or non-Boolean contexts, understanding how the not operator works and following best practices will help you write accurate and readable code.

In this tutorial, you learned how to use the not operator in different contexts, how to use the operator.not_() function, and best practices to follow when working with the not operator. You also coded several examples to solidify your understanding.

Now that you have a good grasp of the not operator in Python, you can confidently use it in your own programs to control the flow of execution and make your code more expressive.