Python 3 bytes formatting

Question

In Python 3, one can format a string like:

"{0}, {1}, {2}".format(1, 2, 3) 

But how to format bytes?

b"{0}, {1}, {2}".format(1, 2, 3) 

raises AttributeError: 'bytes' object has no attribute 'format'.

If there is no format method for bytes, how to do the formatting or "rewriting" of bytes?

Answer 1

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).

Answer 2

Another way would be:

"{0}, {1}, {2}".format(1, 2, 3).encode() 

Tested on IPython 1.1.0 & Python 3.2.3