Python: Negation

Along with the logical operators AND and OR, there is also an operation called “negation” It changes the logical meaning to the opposite. In programming, negation corresponds to the unary operator not:

not True   # False
not False  # True

For example, if there's a function that checks if a number is even, then you can use negation to check if a number is odd:

def is_even(number):
    return number % 2 == 0

print(is_even(10))      # => True
print(not is_even(10))  # => False

In the example above, we added not to the left of the function call and got the opposite action.

Negation is a tool with which you can express intended rules in code without writing new functions.

If you write not is_even(10), the code will still work:

print(not not is_even(10))  # => True

In logic, double negation means positive:

not not True   # True
not not False  # False

print(not not is_even(10))  # => True
print(not not is_even(11))  # => False

Now you know what the operators AND, OR and not mean. They allow you to specify compound conditions with two or more logical expressions.

1. Implement a function called is_palindrome() that determines whether a word is a palindrome or not. A palindrome is a word that reads the same way in both directions. Words may be passed to the function in any case, so you must first convert the word to lowercase: word.lower().

   is_palindrome('hut') # true
   is_palindrome('hexlet') # false
   is_palindrome('Argument') # true
   is_palindrome('Function') # false

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2. Implement a function called is_not_palindrome() which checks if a word is NOT a palindrome:

   is_not_palindrome('шалаш') # false
   is_not_palindrome('Ага') # false
   is_not_palindrome('хекслет') # true

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   To do this, call is_palindrome() inside is_not_palindrome() and apply negation.