string.printable.isprintable() returns False

[planet36]

BPO	23017
Nosy	@birkenfeld, @vstinner, @ezio-melotti, @stevendaprano, @bitdancer, @4kir4, @iritkatriel
Files	bug-string-ascii.py: Test case shows that string.printable has control characters 0001-Fix-string.printable-respect-POSIX-spec.patch docs-string.printable.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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assignee = None
closed_at = None
created_at = <Date 2014-12-09.03:52:01.009>
labels = ['type-bug', '3.9', '3.10', '3.11', 'library', 'expert-unicode', 'docs']
title = 'string.printable.isprintable() returns False'
updated_at = <Date 2021-11-29.16:17:13.755>
user = 'https://bugs.python.org/planet36'

bugs.python.org fields:

activity = <Date 2021-11-29.16:17:13.755>
actor = 'iritkatriel'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Library (Lib)', 'Unicode']
creation = <Date 2014-12-09.03:52:01.009>
creator = 'planet36'
dependencies = []
files = ['37391', '37398', '37441']
hgrepos = []
issue_num = 23017
keywords = ['patch']
message_count = 5.0
messages = ['232343', '232376', '232382', '232613', '407290']
nosy_count = 10.0
nosy_names = ['georg.brandl', 'vstinner', 'ezio.melotti', 'steven.daprano', 'r.david.murray', 'docs@python', 'akira', 'planet36', 'bru', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue23017'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[planet36]

string.printable includes all whitespace characters. However, the only whitespace character that is printable is the space (0x20).

By definition, the only ASCII characters considered printable are:
alphanumeric characters
punctuation characters
the space character (not all whitespace characters)

Source:
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap07.html#tag_07_03

7.2 POSIX Locale

Conforming systems shall provide a POSIX locale, also known as the C locale.

7.3.1 LC_CTYPE

space
Define characters to be classified as white-space characters.

In the POSIX locale, exactly <space>, <form-feed>, <newline>, <carriage-return>, <tab>, and <vertical-tab> shall be included.

cntrl
Define characters to be classified as control characters.

In the POSIX locale, no characters in classes alpha or print shall be included.

graph
Define characters to be classified as printable characters, not including the <space>.

In the POSIX locale, all characters in classes alpha, digit, and punct shall be included; no characters in class cntrl shall be included.

print
Define characters to be classified as printable characters, including the <space>.

In the POSIX locale, all characters in class graph shall be included; no characters in class cntrl shall be included.

LC_CTYPE Category in the POSIX Locale

# "print" is by default "alnum", "punct", and the <space>

[bru]

Here is a simple fix for the issue, plus a test.
It does not break any unit test but this raises a backwards-compatibility problem. Therefore I wouldn't advise using it for Python 3.4 but only 3.5+.

[bitdancer]

This is a bit of a conundrum. Our (string module) definition of printable is very clear, and it includes the other whitespace characters.

We could document that this does not match the posix definition of printable. It also does not match the RFC 5822 definition of printable (for example), which does *not* include whitespace characters (not even space), but the posix definition is a more likely source of confusion.

isprintable is a newer function than string.printable, and serves a different purpose. I suppose that when PEP-3138 was written and implemented the disconnect between the two definitions was not noticed.

For backward compatibility reasons I suspect we are stuck with the discrepancy, but perhaps others will think it worth the pain of changing string.printable. I kind of doubt it, though.

[4kir4]

C standard defines locale-specific *printing characters* that are [ -~]
in "C" locale for implementations that use 7-bit US ASCII character set
i.e., SP (space, 0x20) is a printing character in C (isprint() returns
nonzero).

There is isgraph() function that returns zero for the space but
otherwise is equivalent to isprint().

POSIX definition is aligned with the ISO C standard.

I don't know what RFC 5822 has to do with this issue but the rfc
contradicts itself e.g., in one place it has: "printable US-ASCII
characters except SP" that imlies that SP *is* printable but in other
places it considers isprint==isgraph. The authors probably meant
characters for which isgraph() is nonzero when they use "printable
US-ASCII" (that is incorrect according to C standard).

Tests from bpo-9770 show the relation between C character classes and
string constants [1]:

set(string.printable) == set(C['graph']) + set(C['space'])

where C['space'] is '\t\n\v\f\r ' (the standard C whitespace).

It is a documented behavior [2]:

This is a combination of digits, ascii_letters, punctuation,
and whitespace

where *whitespace* is C['space'].

In Python 2, *printable* is locale-dependent and it coincides with the
corresponding Python 3 definition in "C" locale with ASCII charset.

Unlike other string constants, *printable* differs from C['print'] on
both Python 2 and 3 because it includes whitespace characters other than
space.

str.isprintable [3] obeys C['print'] (in ASCII range) and considers SP
to be printable.

---

It might be too late to change string.printable to correspond to C
isprint() (for ASCII characters).

I've uploaded a documentation patch that mentions that string.printable
and str.isprintable differ.

[1] http://bugs.python.org/review/9770/diff/12212/Lib/test/test_curses_ascii.py
[2] https://hg.python.org/cpython/file/3.4/Doc/library/string.rst#l62
[3] https://docs.python.org/3.4/library/stdtypes.html#str.isprintable

[iritkatriel]

Reproduced on 3.11.