PEP: 0263 Title: Defining Python Source Code Encodings Version: $Revision$ Author: mal@lemburg.com (Marc-André Lemburg) Status: Draft Type: Standards Track Python-Version: 2.3 Created: 06-Jun-2001 Last-Modified: 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 favourite 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 repect to the handling of Python source code data. Defining the Encoding Just as in coercion of strings to Unicode, Python will default to the interpreter's default encoding (which is ASCII in standard Python installations) 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: #!/usr/bin/python # -*- coding: -*- More precise, 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. 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. 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. tokenize the Unicode content 4. compile it, creating Unicode objects from the given Unicode data and creating string objects from the Unicode literal data by first reencoding the Unicode data into 8-bit string data using the given file encoding 5. variable names and other identifiers will be reencoded into 8-bit strings using the file encoding to assure backward compatibility with the existing implementation Note that Python identifiers are restricted to the ASCII subset of the encoding. Implementation Since changing the Python tokenizer/parser combination will require major changes in the internals of the interpreter and enforcing the use of magic comments in source code files which place non-default encoding characters in string literals, comments and Unicode literals, the proposed solution should be implemented in two phases: 1. Implement the magic comment detection and default encoding handling, but only apply the detected encoding to Unicode literals in the source file. In addition to this step and to aid in the transition to explicit encoding declaration, the tokenizer must check the complete source file for compliance with the default encoding (which usually is ASCII). If the source file does not properly decode, a single warning is generated per file. 2. Change the tokenizer/compiler base string type from char* to Py_UNICODE* and apply the encoding to the complete file. Source files which fail to decode cause an error to be raised during compilation. 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 decsribed above. Martin v. Loewis is working on a patch which implements phase 1. See [1] for details. Scope This PEP intends to provide an upgrade path from th current (more-or-less) undefined source code encoding situation to a more robust and portable definition. References [1] Phase 1 implementation: http://sourceforge.net/tracker/?func=detail&atid=305470&aid=526840&group_id=5470 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. 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