python-peps/pep-3138.txt

PEP: 3138
Title: String representation in Python 3000
Version: $Revision$
Last-Modified: $Date$
Author: Atsuo Ishimoto <ishimoto--at--gembook.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 05-May-2008
Post-History:

Abstract
========

This PEP proposes new string representation form for Python 3000. In
Python prior to Python 3000, the repr() built-in function converts
arbitrary objects to printable ASCII strings for debugging and logging.
For Python 3000, a wider range of characters, based on the Unicode
standard, should be considered 'printable'.


Motivation
==========

The current repr() converts 8-bit strings to ASCII using following
algorithm.

- Convert CR, LF, TAB and '\\' to '\\r', '\\n', '\\t', '\\\\'.

- Convert other non-printable characters(0x00-0x1f, 0x7f) and non-ASCII
  characters(>=0x80) to '\\xXX'.

- Backslash-escape quote characters(' or ") and add quote character at
  head and tail.

For Unicode strings, the following additional conversions are done.

- Convert leading surrogate pair characters without trailing character
  (0xd800-0xdbff, but not followed by 0xdc00-0xdfff) to '\\uXXXX'.

- Convert 16-bit characters(>=0x100) to '\\uXXXX'.

- Convert 21-bit characters(>=0x10000) and surrogate pair characters to
  '\\U00xxxxxx'.

This algorithm converts any string to printable ASCII, and repr() is
used as handy and safe way to print strings for debugging or for
logging. Although all non-ASCII characters are escaped, this does not
matter when most of the string's characters are ASCII. But for other
languages, such as Japanese where most characters in a string are not
ASCII, this is very inconvenient. Python 3000 has a lot of nice features
for non-Latin users such as non-ASCII identifiers, so it would be
helpful if Python could also progress in a similar way for printable
output.

Some users might be concerned that such output will mess up their
console if they print binary data like images. But this is unlikely to
happen in practice because bytes and strings are different types in
Python 3000, so printing an image to the console won't mess it up.

This issue was once discussed by Hye-Shik Chang [1]_ , but was rejected.


Specification
=============

- The algorithm to build repr() strings should be changed to:

  * Convert CR, LF, TAB and '\\' to '\\r', '\\n', '\\t', '\\\\'.

  * Convert other non-printable ASCII characters(0x00-0x1f, 0x7f) to
    '\\xXX'.

  * Convert leading surrogate pair characters without trailing character
    (0xd800-0xdbff, but not followed by 0xdc00-0xdfff) to '\\uXXXX'.

  * Convert Unicode whitespace other than ASCII space('\\x20') and
    control characters (categories Z* and C* in the Unicode database)
    to 'xXX', '\\uXXXX' or '\\U00xxxxxx'.

- Set the Unicode error-handler for sys.stdout and sys.stderr to
  'backslashreplace' by default.


Rationale
=========

The repr() in Python 3000 should be Unicode not ASCII based, just like
Python 3000 strings. Also, conversion should not be affected by the
locale setting, because the locale is not necessarily the same as the
output device's locale. For example, it is common for a daemon process
to be invoked in an ASCII setting, but writes UTF-8 to its log files.

Characters not supported by user's console are hex-escaped on printing,
by the Unicode encoders' error-handler. If the error-handler of the
output file is 'backslashreplace', such characters are hex-escaped
without raising UnicodeEncodeError. For example, if your default
encoding is ASCII, ``print('<27>')`` will prints '\\xa2'. If your encoding
is ISO-8859-1, '' will be printed.


Printable characters
--------------------

The Unicode standard doesn't define Non-printable characters, so we must
create our own definition. Here we propose to define Non-printable
characters as follows.

- Non-printable ASCII characters as Python 2.

- Broken surrogate pair characters.

- Characters defined in the Unicode character database as

  * Cc (Other, Control)
  * Cf (Other, Format)
  * Cs (Other, Surrogate)
  * Co (Other, Private Use)
  * Cn (Other, Not Assigned)
  * Zl Separator, Line ('\\u2028', LINE SEPARATOR)
  * Zp Separator, Paragraph ('\\u2029', PARAGRAPH SEPARATOR)
  * Zs (Separator, Space) other than ASCII space('\\x20'). Characters in
    this category should be escaped to avoid ambiguity.


Alternate Solutions
-------------------

To help debugging in non-Latin languages without changing repr(), other
suggestion were made.

- Supply a tool to print lists or dicts.

 Strings to be printed for debugging are not only contained by lists or
 dicts, but also in many other types of object. File objects contain a
 file name in Unicode, exception objects contain a message in Unicode,
 etc. These strings should be printed in readable form when repr()ed.
 It is unlikely to be possible to implement a tool to print all
 possible object types.

- Use sys.displayhook and sys.excepthook.

 For interactive sessions, we can write hooks to restore hex escaped
 characters to the original characters. But these hooks are called only
 when the result of evaluating an expression entered in an interactive
 Python session, and doesn't work for the print() function or for
 non-interactive sessions.

- Subclass sys.stdout and sys.stderr.

 It is difficult to implement a subclass to restore hex-escaped
 characters since there isn't enough information left by the time it's
 a string to undo the escaping correctly in all cases. For example, ``
 print("\\"+"u0041")`` should be printed as '\\u0041', not 'A'. But
 there is no chance to tell file objects apart.

- Make the encoding used by unicode_repr() adjustable.

 There is no benefit preserving the current repr() behavior to make
 application/library authors aware of non-ASCII repr(). And selecting
 an encoding on printing is more flexible than having a global setting.


Open Issues
===========

- A lot of people use UTF-8 for their encoding, for example, en_US.utf8
  and de_DE.utf8. In such cases, the backslashescape trick doesn't work.


Backwards Compatibility
=======================

Changing repr() may break some existing codes, especially testing code.
Five of Python's regression test fail with this modification. If you
need repr() strings without non-ASCII character as Python 2, you can use
following function.

::

 def repr_ascii(obj):
     return str(repr(obj).encode("ASCII", "backslashreplace"), "ASCII")


Reference Implementation
========================

http://bugs.python.org/issue2630


References
==========

.. [1] Multibyte string on string::string_print
       (http://bugs.python.org/issue479898)


Copyright
=========

This document has been placed in the public domain.