What does the b'' sentinel mean in Python iter()?

Question

Let's say I have a process that returns a bunch of lines, which I want to iterate through:

import subprocess

myCmd = ['foo', '--bar', '--baz']
myProcess = subprocess.Popen(myCmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
for myLine in iter(myProcess.stdout.readline, b''):
    print myLine

What does the sentinel argument to iter() do in this example, where I pass it the value b''? I think I understand '' by itself — I stop iterating on an empty line — but I don't know what b'' means.

Answer 1

In python3 the string is a byteliteral. Ignored in python 2.

Answer 2

I'll try my best to be more precise than what I read so far. (Slightly revised version)

The Python notation b'string' denotes a byte string literal in Python versions which supports it. The notation has been introduced with PEP 358 for Python 2 as a step towards the migration to unicode support. The current Python syntax for 2.6 does not mention the b prefix while the Python syntax for 2.7 does mention the b prefix. However, PEP 358 was intended for Python 2.6 and all Python 2.6 interpreters which I have seen accept it.

With the migration to Python 3, the default for string literals changed from byte to unicode. However, the __future__ module was introduced to ease migration. The following illustrates the effect with respect to string literals.

Python 2.6.6 (r266:84292, Dec 26 2010, 22:31:48) 
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 'abc'
'abc'
>>> b'abc'
'abc'
>>> from __future__ import unicode_literals
>>> 'abc'
u'abc'
>>> b'abc'
'abc'

I hope I could motivate that the following statements have a limited validity:

• In Python 3 this denotes a byte literal: Yes, but also in Python 2 at least since version 2.6.
• This notation is ignored in Python 2: It is more that it has no effect if you have a Python 2 version which already supports it and unless you (or the foreign code you look at) imports __future__ and only if you do not care about compatibility with Python 3 (which everyone should consider and which is often aimed at in third party software).

The shortest valid answer to the OP's question, in my view, would be:

This is a prefix for string literals, enforcing byte representation as opposed to unicode representation, introduced in Python 2.6, not changing the Python default for Python 2 (but deserves attention if from __future__ import can be seen).