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?
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).
Another way would be:
"{0}, {1}, {2}".format(1, 2, 3).encode() Tested on IPython 1.1.0 & Python 3.2.3
I found the %b working best in Python 3.6.2, it should work both for b"" and "":
print(b"Some stuff %b. Some other stuff" % my_byte_or_unicode_string) 
%b not %a for byte string, otherwise 'b' prefix is included
Interestingly .format() doesn't appear to be supported for byte-sequences; as you have demonstrated.
You could use .join() as suggested here: http://bugs.python.org/issue3982
b", ".join([b'1', b'2', b'3']) There is a speed advantage associated with .join() over using .format() shown by the BDFL himself: http://bugs.python.org/msg180449
str(some_bytes).encode()
.format() method for bytes does appear to be on track for inclusion in 3.4+
__format__ method of an object and built-in format return str. How will that work with bytes and encodings?For Python 3.6+ you can use this nice and clean syntax:
f'foo {bar}'.encode() # a byte string 
str, not bytes. For example, it won't give the intended result if bar is a bytes object. Or if you want to put non-ASCII byte values in the format string.
I've found this to work.
a = "{0}, {1}, {2}".format(1, 2, 3) b = bytes(a, encoding="ascii") >>> b b'1, 2, 3' 