What does a b prefix before a python string means ?

Question

In a python source code I stumbled upon I've seen a small b before a string like in:

b"abcdef"

I know of u prefix that means unicode and r prefix that means raw.

What does the b stand for and in which kind of source code is it useful as it seems to be exactly like a plain string without any prefix ?

Answer 1

The b stands for binary, see info about Python 3.

Answer 2

This is Python3 bytes literal. This prefix is absent in Python 2.5 and older (it is equivalent to a plain string of 2.x, while plain string of 3.x is equivalent to a literal with u prefix in 2.x). In Python 2.6+ it is equivalent to a plain string, for compatibility with 3.x.