As of Python 3.5, % formatting will work for bytes, too!

This was part of PEP 461, authored by Ethan Furman:

PEP: 461
Title: Adding % formatting to bytes and bytearray
Version: $Revision$
Last-Modified: $Date$
Author: Ethan Furman <ethan at stoneleaf.us>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 2014-01-13
Python-Version: 3.5
Post-History: 2014-01-14, 2014-01-15, 2014-01-17, 2014-02-22, 2014-03-25,
               2014-03-27
Resolution:


Abstract
========

This PEP proposes adding % formatting operations similar to Python 2's ``str``
type to ``bytes`` and ``bytearray`` [1]_ [2]_.


Rationale
=========

While interpolation is usually thought of as a string operation, there are
cases where interpolation on ``bytes`` or ``bytearrays`` make sense, and the
work needed to make up for this missing functionality detracts from the overall
readability of the code.


Motivation
==========

With Python 3 and the split between ``str`` and ``bytes``, one small but
important area of programming became slightly more difficult, and much more
painful -- wire format protocols [3]_.

This area of programming is characterized by a mixture of binary data and
ASCII compatible segments of text (aka ASCII-encoded text).  Bringing back a
restricted %-interpolation for ``bytes`` and ``bytearray`` will aid both in
writing new wire format code, and in porting Python 2 wire format code.

Common use-cases include ``dbf`` and ``pdf`` file formats, ``email``
formats, and ``FTP`` and ``HTTP`` communications, among many others.

PEP 461 was accepted by Guido van Rossum on March 27, 2014:

Accepted. Congrats with marshalling yet another quite contentious discussion, and putting up with my last-minute block-headedness!

From this, we can obviously conclude that % is no longer scheduled for deprecation (as was announced with Python 3.1).

Decode the bytes object to produce a string:

>>> b"abcde".decode("utf-8")
'abcde'

The above example assumes that the bytes object is in UTF-8, because it is a common encoding. However, you should use the encoding your data is actually in!

No. The idea is explicitly dismissed in the PEP:

For the same reason that we don't support bytes.format(), you may not combine 'f' with 'b' string literals. The primary problem is that an object's __format__() method may return Unicode data that is not compatible with a bytes string.

Binary f-strings would first require a solution for bytes.format(). This idea has been proposed in the past, most recently in PEP 461. The discussions of such a feature usually suggest either

• adding a method such as __bformat__() so an object can control how it is converted to bytes, or

• having bytes.format() not be as general purpose or extensible as str.format().

Both of these remain as options in the future, if such functionality is desired.