Returns a list of random unique elements for the specified length.
        Multiple occurrences of the same value increase its probability to be in the output.

def random_sample(self, elements=('a', 'b', 'c'), length=None):
        """
        Returns a list of random unique elements for the specified length.
        Multiple occurrences of the same value increase its probability to be in the output.
        """
        return self.random_elements(elements, length, unique=True)