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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
  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

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

The exercise doesn't pass checking. What to do? 😶

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

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In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

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I've read the lessons but nothing is clear 🙄

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