Sometimes you need to test strings for equality with an algorithm whose timing depends only on the length of the strings, and not on the contents of the strings themselves. If one of those strings is of constant width -- an HMAC, for example -- then it becomes a constant-time operation. This can be used to prevent some timing side-channel attacks, such as the critical vulnerability found in KeyCzar back in 2009.
This module offers a single function, equals(x, y), which takes two strings x and y and returns True if they are equal, and False if they are not. The time this function takes does not depend on what specific bytes are in these strings. Unicode strings are encoded as UTF-8 before being compared; it is recommended that you only use this on byte strings (str in Python 2, bytes in Python 3).
This works with Python 2 and 3, and PyPy. The license is Apache 2.0.