What does a b prefix before a python string mean?

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.

The b prefix signifies a bytes string literal.

If you see it used in Python 3 source code, the expression creates a bytes object, not a regular Unicode str object. If you see it echoed in your Python shell or as part of a list, dict or other container contents, then you see a bytes object represented using this notation.

bytes objects basically contain a sequence of integers in the range 0-255, but when represented, Python displays these bytes as ASCII codepoints to make it easier to read their contents. Any bytes outside the printable range of ASCII characters are shown as escape sequences (e.g. \n, \x82, etc.). Inversely, you can use both ASCII characters and escape sequences to define byte values; for ASCII values their numeric value is used (e.g. b'A' == b'\x41')

Because a bytes object consist of a sequence of integers, you can construct a bytes object from any other sequence of integers with values in the 0-255 range, like a list:

bytes([72, 101, 108, 108, 111])

and indexing gives you back the integers (but slicing produces a new bytes value; for the above example, value[0] gives you 72, but value[:1] is b'H' as 72 is the ASCII code point for the capital letter H).

bytes model binary data, including encoded text. If your bytes value does contain text, you need to first decode it, using the correct codec. If the data is encoded as UTF-8, for example, you can obtain a Unicode str value with:

strvalue = bytesvalue.decode('utf-8')

Conversely, to go from text in a str object to bytes you need to encode. You need to decide on an encoding to use; the default is to use UTF-8, but what you will need is highly dependent on your use case:

bytesvalue = strvalue.encode('utf-8')

You can also use the constructor, bytes(strvalue, encoding) to do the same.

Both the decoding and encoding methods take an extra argument to specify how errors should be handled.

Python 2, versions 2.6 and 2.7 also support creating string literals using b'..' string literal syntax, to ease code that works on both Python 2 and 3.

bytes objects are immutable, just like str strings are. Use a bytearray() object if you need to have a mutable bytes value.