Use the stdlib's compare_digest for constant time comparisons when available

    return ''.join(random.choice(allowed_chars) for i in range(length))

if hasattr(hmac, "compare_digest"):

    # Prefer the stdlib implementation, when available.

    constant_time_compare = hmac.compare_digest

else:

    def constant_time_compare(val1, val2):

        """

        Returns True if the two strings are equal, False otherwise.