PEP: 263
Title: Defining Python Source Code Encodings
Author: Marc-André Lemburg <mal@lemburg.com>,
        Martin von Löwis <martin@v.loewis.de>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
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 following
regular expression::

    ^[ \t\f]*#.*?coding[:=][ \t]*([-_.a-zA-Z0-9]+)

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.  If the first line matches the second line
is ignored.

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:

   1. Missing "coding:" prefix::

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

   2. Encoding comment not on line 1 or 2::

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

   3. 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:
       https://bugs.python.org/issue526840

.. [2] Phase 2 implementation:
       https://bugs.python.org/issue534304

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.