What is the use of the "not" keyword in Python?

Question

Jayant Kumar · Accepted Answer

Table of Contants
Introduction
The not keyword in Python is a logical operator used to negate a condition. It inverts the boolean value of a condition: if a condition evaluates to True, not will make it False, and if it evaluates to False, not will make it True. This operator is essential for expressing conditions that require the opposite of a given boolean value.

How the not Keyword Works
The not operator is used to reverse the truth value of a condition.
Syntax:
not condition

Example:
is_sunny = False

if not is_sunny:
    print("It is not sunny.")  # Output: It is not sunny.

In this example, is_sunny is False, so not is_sunny evaluates to True, and the if block is executed.

Using not in Conditional Statements
The not keyword is commonly used in if statements and other control flow constructs to check the negation of a condition.
Example : Simple Negation
logged_in = False

if not logged_in:
    print("Please log in to continue.")  # Output: Please log in to continue.

Here, since logged_in is False, not logged_in evaluates to True, prompting the message to be printed.
Example : Combining with Other Logical Operators
temperature = 25
is_raining = True

if not (temperature > 30 or is_raining):
    print("The weather is mild and not raining.")  # Output: The weather is mild and not raining.

In this case, temperature > 30 is False, and is_raining is True. The or condition evaluates to True, so not (True) results in False, leading to the else block being executed.

Practical Examples
1. Validating User Input
user_input = ""

if not user_input:
    print("Input cannot be empty.")  # Output: Input cannot be empty.

This example checks if user_input is empty. Since an empty string is considered False in Python, not user_input evaluates to True, triggering the error message.
2. Checking for **None** Values
data = None

if not data:
    print("Data is missing.")  # Output: Data is missing.

In this scenario, data is None, which evaluates to False. Therefore, not data evaluates to True, and the message is printed.

Conclusion
The not keyword in Python is used to invert the truth value of a condition. It is a fundamental operator for negating boolean expressions and is widely used in control flow and logical operations. By understanding how not works in conjunction with other logical operators, you can effectively manage conditions and make your Python code more expressive and flexible.

What is the use of the "not" keyword in Python?

Table of Contants

Introduction

How the `not` Keyword Works

Syntax:

Example:

Using `not` in Conditional Statements

Example : Simple Negation

Example : Combining with Other Logical Operators

Practical Examples

1. Validating User Input

2. Checking for `None` Values

Conclusion

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