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PEP 461 -- Adding % formatting to bytes and bytearray

Title:Adding % formatting to bytes and bytearray
Author:Ethan Furman <ethan at>
Type:Standards Track
Post-History:2014-01-14, 2014-01-15, 2014-01-17, 2014-02-22, 2014-03-25, 2014-03-27


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


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.


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.

Proposed semantics for bytes and bytearray formatting


All the numeric formatting codes (d, i, o, u, x, X, e, E, f, F, g, G, and any that are subsequently added to Python 3) will be supported, and will work as they do for str, including the padding, justification and other related modifiers (currently #, 0, -, `` `` (space), and + (plus any added to Python 3)). The only non-numeric codes allowed are c, b, a, and s (which is a synonym for b).

For the numeric codes, the only difference between str and bytes (or bytearray) interpolation is that the results from these codes will be ASCII-encoded text, not unicode. In other words, for any numeric formatting code %x:

b"%x" % val

is equivalent to:

("%x" % val).encode("ascii")


>>> b'%4x' % 10
b'   a'

>>> b'%#4x' % 10
' 0xa'

>>> b'%04X' % 10

%c will insert a single byte, either from an int in range(256), or from a bytes argument of length 1, not from a str.


>>> b'%c' % 48

>>> b'%c' % b'a'

%b will insert a series of bytes. These bytes are collected in one of two ways:

  • input type supports Py_buffer [4]? use it to collect the necessary bytes
  • input type is something else? use its __bytes__ method [5] ; if there isn't one, raise a TypeError

In particular, %b will not accept numbers nor str. str is rejected as the string to bytes conversion requires an encoding, and we are refusing to guess; numbers are rejected because:

  • what makes a number is fuzzy (float? Decimal? Fraction? some user type?)
  • allowing numbers would lead to ambiguity between numbers and textual representations of numbers (3.14 vs '3.14')
  • given the nature of wire formats, explicit is definitely better than implicit

%s is included as a synonym for %b for the sole purpose of making 2/3 code bases easier to maintain. Python 3 only code should use %b.


>>> b'%b' % b'abc'

>>> b'%b' % 'some string'.encode('utf8')
b'some string'

>>> b'%b' % 3.14
Traceback (most recent call last):
TypeError: b'%b' does not accept 'float'

>>> b'%b' % 'hello world!'
Traceback (most recent call last):
TypeError: b'%b' does not accept 'str'

%a will give the equivalent of repr(some_obj).encode('ascii', 'backslashreplace') on the interpolated value. Use cases include developing a new protocol and writing landmarks into the stream; debugging data going into an existing protocol to see if the problem is the protocol itself or bad data; a fall-back for a serialization format; or any situation where defining __bytes__ would not be appropriate but a readable/informative representation is needed [6].

%r is included as a synonym for %a for the sole purpose of making 2/3 code bases easier to maintain. Python 3 only code use %a [7].


>>> b'%a' % 3.14

>>> b'%a' % b'abc'

>>> b'%a' % 'def'

Compatibility with Python 2

As noted above, %s and %r are being included solely to help ease migration from, and/or have a single code base with, Python 2. This is important as there are modules both in the wild and behind closed doors that currently use the Python 2 str type as a bytes container, and hence are using %s as a bytes interpolator.

However, %b and %a should be used in new, Python 3 only code, so %s and %r will immediately be deprecated, but not removed from the 3.x series [7].

Proposed variations

It has been proposed to automatically use .encode('ascii','strict') for str arguments to %b.

  • Rejected as this would lead to intermittent failures. Better to have the operation always fail so the trouble-spot can be correctly fixed.

It has been proposed to have %b return the ascii-encoded repr when the value is a str (b'%b' % 'abc' --> b"'abc'").

  • Rejected as this would lead to hard to debug failures far from the problem site. Better to have the operation always fail so the trouble-spot can be easily fixed.

Originally this PEP also proposed adding format-style formatting, but it was decided that format and its related machinery were all strictly text (aka str) based, and it was dropped.

Various new special methods were proposed, such as __ascii__, __format_bytes__, etc.; such methods are not needed at this time, but can be visited again later if real-world use shows deficiencies with this solution.

A competing PEP, PEP 460 Add binary interpolation and formatting [8], also exists.


The objections raised against this PEP were mainly variations on two themes:

  • the bytes and bytearray types are for pure binary data, with no assumptions about encodings
  • offering %-interpolation that assumes an ASCII encoding will be an attractive nuisance and lead us back to the problems of the Python 2 str/unicode text model

As was seen during the discussion, bytes and bytearray are also used for mixed binary data and ASCII-compatible segments: file formats such as dbf and pdf, network protocols such as ftp and email, etc.

bytes and bytearray already have several methods which assume an ASCII compatible encoding. upper(), isalpha(), and expandtabs() to name just a few. %-interpolation, with its very restricted mini-language, will not be any more of a nuisance than the already existing methods.

Some have objected to allowing the full range of numeric formatting codes with the claim that decimal alone would be sufficient. However, at least two formats (dbf and pdf) make use of non-decimal numbers.