def _check_password(password):
        """Check the strength of a password.
        A password is considered strong if
            8 characters length or more
            1 digit or more
            1 uppercase letter or more
            1 lowercase letter or more
            1 symbol (excluding whitespace characters) or more

        Parameters
        ----------
        password: str

        Returns
        -------
        bool
            True if the password is strong enough, False otherwise.
        """
        # Check the length.
        length_error = len(password) < 8

        # Search for digits.
        digit_error = re.search(r'\d', password) is None

        # Search for uppercase letters.
        uppercase_error = re.search(r'[A-Z]', password) is None

        # Search for lowercase letters.
        lowercase_error = re.search(r'[a-z]', password) is None

        # Search for symbols (excluding whitespace characters).
        symbol_error = re.search(r'[^A-Za-z\d\s]', password) is None

        return not (length_error or digit_error or uppercase_error or\
                    lowercase_error or symbol_error)