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PEP: 0263
Title: Defining Python Source Code Encodings
Version: 982904d11574
Last-Modified:  2009-06-04 19:44:37 +0000 (Thu, 04 Jun 2009)
Author: Marc-André Lemburg <mal at lemburg.com>, Martin von Löwis <martin at v.loewis.de>
Status: Final
Type: Standards Track
Created: 06-Jun-2001
Python-Version: 2.3
Post-History: 

Abstract

    This PEP proposes to introduce a syntax to declare the encoding of
    a Python source file. The encoding information is then used by the
    Python parser to interpret the file using the given encoding. Most
    notably this enhances the interpretation of Unicode literals in
    the source code and makes it possible to write Unicode literals
    using e.g. UTF-8 directly in an Unicode aware editor.

Problem

    In Python 2.1, Unicode literals can only be written using the
    Latin-1 based encoding "unicode-escape". This makes the
    programming environment rather unfriendly to Python users who live
    and work in non-Latin-1 locales such as many of the Asian 
    countries. Programmers can write their 8-bit strings using the
    favorite encoding, but are bound to the "unicode-escape" encoding
    for Unicode literals.

Proposed Solution

    I propose to make the Python source code encoding both visible and
    changeable on a per-source file basis by using a special comment
    at the top of the file to declare the encoding.

    To make Python aware of this encoding declaration a number of
    concept changes are necessary with respect to the handling of
    Python source code data.

Defining the Encoding

    Python will default to ASCII as standard encoding if no other
    encoding hints are given.

    To define a source code encoding, a magic comment must
    be placed into the source files either as first or second
    line in the file, such as:

          # coding=<encoding name>

    or (using formats recognized by popular editors)

          #!/usr/bin/python
          # -*- coding: <encoding name> -*-

    or

          #!/usr/bin/python
          # vim: set fileencoding=<encoding name> :

    More precisely, the first or second line must match the regular
    expression "coding[:=]\s*([-\w.]+)". The first group of this
    expression is then interpreted as encoding name. If the encoding
    is unknown to Python, an error is raised during compilation. There
    must not be any Python statement on the line that contains the
    encoding declaration.

    To aid with platforms such as Windows, which add Unicode BOM marks
    to the beginning of Unicode files, the UTF-8 signature
    '\xef\xbb\xbf' will be interpreted as 'utf-8' encoding as well
    (even if no magic encoding comment is given).

    If a source file uses both the UTF-8 BOM mark signature and a
    magic encoding comment, the only allowed encoding for the comment
    is 'utf-8'.  Any other encoding will cause an error.

Examples

    These are some examples to clarify the different styles for
    defining the source code encoding at the top of a Python source
    file:

    1. With interpreter binary and using Emacs style file encoding
       comment:

          #!/usr/bin/python
          # -*- coding: latin-1 -*-
          import os, sys
          ...

          #!/usr/bin/python
          # -*- coding: iso-8859-15 -*-
          import os, sys
          ...

          #!/usr/bin/python
          # -*- coding: ascii -*-
          import os, sys
          ...

    2. Without interpreter line, using plain text:

          # This Python file uses the following encoding: utf-8
          import os, sys
          ...

    3. Text editors might have different ways of defining the file's
       encoding, e.g.

          #!/usr/local/bin/python
          # coding: latin-1
          import os, sys
          ...

    4. Without encoding comment, Python's parser will assume ASCII
       text:

          #!/usr/local/bin/python
          import os, sys
          ...

    5. Encoding comments which don't work:

       Missing "coding:" prefix:

          #!/usr/local/bin/python
          # latin-1
          import os, sys
          ...

       Encoding comment not on line 1 or 2:

          #!/usr/local/bin/python
          #
          # -*- coding: latin-1 -*-
          import os, sys
          ...

       Unsupported encoding:

          #!/usr/local/bin/python
          # -*- coding: utf-42 -*-
          import os, sys
          ...

Concepts

    The PEP is based on the following concepts which would have to be
    implemented to enable usage of such a magic comment:

    1. The complete Python source file should use a single encoding.
       Embedding of differently encoded data is not allowed and will
       result in a decoding error during compilation of the Python
       source code.

       Any encoding which allows processing the first two lines in the
       way indicated above is allowed as source code encoding, this
       includes ASCII compatible encodings as well as certain
       multi-byte encodings such as Shift_JIS. It does not include
       encodings which use two or more bytes for all characters like
       e.g. UTF-16. The reason for this is to keep the encoding
       detection algorithm in the tokenizer simple.

    2. Handling of escape sequences should continue to work as it does 
       now, but with all possible source code encodings, that is
       standard string literals (both 8-bit and Unicode) are subject to 
       escape sequence expansion while raw string literals only expand
       a very small subset of escape sequences.

    3. Python's tokenizer/compiler combo will need to be updated to
       work as follows:

       1. read the file

       2. decode it into Unicode assuming a fixed per-file encoding

       3. convert it into a UTF-8 byte string

       4. tokenize the UTF-8 content

       5. compile it, creating Unicode objects from the given Unicode data
          and creating string objects from the Unicode literal data
          by first reencoding the UTF-8 data into 8-bit string data
          using the given file encoding

       Note that Python identifiers are restricted to the ASCII
       subset of the encoding, and thus need no further conversion
       after step 4.

Implementation

    For backwards-compatibility with existing code which currently
    uses non-ASCII in string literals without declaring an encoding,
    the implementation will be introduced in two phases:

    1. Allow non-ASCII in string literals and comments, by internally
       treating a missing encoding declaration as a declaration of
       "iso-8859-1". This will cause arbitrary byte strings to
       correctly round-trip between step 2 and step 5 of the
       processing, and provide compatibility with Python 2.2 for
       Unicode literals that contain non-ASCII bytes.

       A warning will be issued if non-ASCII bytes are found in the
       input, once per improperly encoded input file.

    2. Remove the warning, and change the default encoding to "ascii".

    The builtin compile() API will be enhanced to accept Unicode as
    input. 8-bit string input is subject to the standard procedure for
    encoding detection as described above.

    If a Unicode string with a coding declaration is passed to compile(),
    a SyntaxError will be raised.

    SUZUKI Hisao is working on a patch; see [2] for details. A patch
    implementing only phase 1 is available at [1].

Phases

    Implementation of steps 1 and 2 above were completed in 2.3,
    except for changing the default encoding to "ascii".

    The default encoding was set to "ascii" in version 2.5.
   

Scope

    This PEP intends to provide an upgrade path from the current
    (more-or-less) undefined source code encoding situation to a more
    robust and portable definition.

References

    [1] Phase 1 implementation:
        http://python.org/sf/526840
    [2] Phase 2 implementation:
        http://python.org/sf/534304

History

    1.10 and above: see CVS history
    1.8: Added '.' to the coding RE.
    1.7: Added warnings to phase 1 implementation. Replaced the
         Latin-1 default encoding with the interpreter's default
         encoding. Added tweaks to compile().
    1.4 - 1.6: Minor tweaks
    1.3: Worked in comments by Martin v. Loewis: 
         UTF-8 BOM mark detection, Emacs style magic comment,
         two phase approach to the implementation

Copyright

    This document has been placed in the public domain.