arrays - Python string "b" prefix (byte literals) -

i looking through unit testing code , found this:

self.assertin(b'hello', res.body) 

i know means bytes in python 3 returns byte array, found here. believe code written python 3.3 , trying figure out how works in other versions (in case 2.7) related question found had poorly-written accepted answer contradictory comments confused me.

questions:

• in versions of python b'mystring' "work"?
• how behave in python 2.x?
• how behave in python 3.x?
• does have byte literal change?

this described in document linked.

• in versions of python b'mystring' "work"?: 2.6+.
• how behave in python 2.x? it creates bytes literal—which exact same thing str literal in 2.x.
• how behave in python 3.x? it creates bytes literal—which not same thing str literal in 3.x.
• does have byte literal change? yes. that's whole point; lets write "future compatible" code—or code works in both 2.6+ , 3.0+ without 2to3.

quoting first paragraph in section linked:

for future compatibility, python 2.6 adds bytes synonym str type, , supports b'' notation.

note that, says few lines down, python 2.x bytes/str not same type python 3.x bytes: "most notably, constructor different". bytes literals same, except in edge case you're putting unicode characters bytes literal (which has no defined meaning in 2.x, arbitrary may happen you'd hoped, while in 3.x it's guaranteed syntaxerror).