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How to Negate Booleans in Python

Negating a boolean value (changing True to False and vice-versa) is a fundamental operation in Python.

This guide explains how to negate booleans using the not operator, how to apply negation to lists of booleans, and how to work with boolean NumPy arrays. We'll also touch on why you should avoid the bitwise NOT operator (~) for boolean negation.

Negating a Single Boolean Value with not​

The not operator is the correct and only way to negate a boolean value in Python:

my_bool = True
result = not my_bool
print(result)       # Output: False

print(not True)     # Output: False
print(not False)    # Output: True
  • not True evaluates to False.
  • not False evaluates to True.
  • The not operator is not only used for boolean values, but it works by checking if a value is falsy.
note

All values that are not truthy are considered falsy. The falsy values in Python are:

  • constants defined to be falsy: None and False.
  • 0 (zero) of any numeric type.
  • empty sequences and collections: "" (empty string), () (empty tuple), [] (empty list), {} (empty dictionary), set() (empty set), range(0) (empty range).

Negating Booleans in a List​

To negate all boolean values within a list, use a list comprehension or the map function.

List comprehensions provide a concise and readable way to apply not to each element:

my_list = [True, True, False, False]
new_list = [not item for item in my_list]
print(new_list)  # Output: [False, False, True, True]
  • [not item for item in my_list]: This creates a new list where each element is the negation of the corresponding element in my_list.

Negating Booleans Using map() (Less Readable)​

You can use the map() function, but it's generally less readable than a list comprehension for this simple case:

import operator
my_list = [True, True, False, False]
new_list = list(map(operator.not_, my_list))
print(new_list) # Output: [False, False, True, True]
  • The map() function can be used to apply the not operator to a list of booleans.
  • The operator.not_ is the function equivalent of the not keyword, and we can use it as an argument in map().
  • The list() is used to convert the result of map into a list.

Negating Boolean Values in a NumPy Array​

If you're working with NumPy arrays, use np.logical_not() or the bitwise NOT operator (~):

import numpy as np

arr = np.array([True, True, False, False], dtype=bool)

result = np.logical_not(arr)
print(result)  # Output: [False False  True  True]

#  Using the bitwise NOT operator (~):
result2 = ~arr
print(result2) # Output: [False False  True  True]

result3 = np.invert(arr)
print(result3) # Output: [False False  True  True]

result4 = np.bitwise_not(arr)
print(result4) # Output: [False False  True  True]
  • np.logical_not(arr): This is the most explicit and readable way to negate a boolean NumPy array.
  • ~arr: The bitwise NOT operator (~) works correctly on boolean NumPy arrays in this specific case. However, it's generally clearer to use np.logical_not().
  • np.invert() can also be used.
  • np.bitwise_not() can also be used.

Avoid the Bitwise NOT Operator (~) for Booleans​

Avoid using the bitwise operator on boolean values, as the bool class is a subclass of int.

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