Python string “b” prefix (byte literals)

Question

I am looking through some unit testing code and I found this:

self.assertIn(b'Hello', res.body)

I know that this means bytes in Python 3 which returns a byte array , as I found here . I believe that the code was written for Python 3.3 and am trying to figure out how it works in other versions (in my case 2.7) The related question that I found had a poorly-written accepted answer with contradictory comments that confused me.

Questions:

• In what versions of python does b'myString' "work"?
• How does it behave in python 2.x?
• How does it behave in python 3.x?
• Does that have something to do with the byte literal change ?

Answer 1

This is all described in the document you linked.

• In what versions of python does b'myString' "work"?: 2.6+.
• How does it behave in python 2.x? It creates a bytes literal—which is the exact same thing as a str literal in 2.x.
• How does it behave in python 3.x? It creates a bytes literal—which is not the same thing as a str literal in 3.x.
• Does that have something to do with the byte literal change? Yes. That's the whole point; it lets you write "future compatible" code—or code that works in both 2.6+ and 3.0+ without 2to3 .

Quoting from the first paragraph in the section you linked:

For future compatibility, Python 2.6 adds bytes as a synonym for the str type, and it also supports the b'' notation.

Note that, as it says a few lines down, Python 2.x bytes / str is not exactly the same type as Python 3.x bytes : "most notably, the constructor is completely different". But bytes literals are the same, except in the edge case where you're putting Unicode characters into a bytes literal (which has no defined meaning in 2.x, but does something arbitrary that may sometimes happen to be what you'd hoped, while in 3.x it's a guaranteed SyntaxError ).