100 lines
3.6 KiB
ReStructuredText
100 lines
3.6 KiB
ReStructuredText
PEP: 3120
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Title: Using UTF-8 as the default source encoding
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Version: $Revision$
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Last-Modified: $Date$
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Author: Martin von Löwis <martin@v.loewis.de>
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Status: Final
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 15-Apr-2007
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Python-Version: 3.0
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Post-History:
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Specification
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=============
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This PEP proposes to change the default source encoding from ASCII to
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UTF-8. Support for alternative source encodings (:pep:`263`) continues to
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exist; an explicit encoding declaration takes precedence over the
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default.
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A Bit of History
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================
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In Python 1, the source encoding was unspecified, except that the
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source encoding had to be a superset of the system's basic execution
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character set (i.e. an ASCII superset, on most systems). The source
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encoding was only relevant for the lexis itself (bytes representing
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letters for keywords, identifiers, punctuation, line breaks, etc).
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The contents of a string literal was copied literally from the file
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on source.
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In Python 2.0, the source encoding changed to Latin-1 as a side effect
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of introducing Unicode. For Unicode string literals, the characters
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were still copied literally from the source file, but widened on a
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character-by-character basis. As Unicode gives a fixed interpretation
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to code points, this algorithm effectively fixed a source encoding, at
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least for files containing non-ASCII characters in Unicode literals.
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:pep:`263` identified the problem that you can use only those Unicode
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characters in a Unicode literal which are also in Latin-1, and
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introduced a syntax for declaring the source encoding. If no source
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encoding was given, the default should be ASCII. For compatibility
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with Python 2.0 and 2.1, files were interpreted as Latin-1 for a
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transitional period. This transition ended with Python 2.5, which
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gives an error if non-ASCII characters are encountered and no source
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encoding is declared.
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Rationale
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=========
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With :pep:`263`, using arbitrary non-ASCII characters in a Python file is
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possible, but tedious. One has to explicitly add an encoding
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declaration. Even though some editors (like IDLE and Emacs) support
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the declarations of :pep:`263`, many editors still do not (and never
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will); users have to explicitly adjust the encoding which the editor
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assumes on a file-by-file basis.
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When the default encoding is changed to UTF-8, adding non-ASCII text
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to Python files becomes easier and more portable: On some systems,
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editors will automatically choose UTF-8 when saving text (e.g. on Unix
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systems where the locale uses UTF-8). On other systems, editors will
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guess the encoding when reading the file, and UTF-8 is easy to
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guess. Yet other editors support associating a default encoding with a
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file extension, allowing users to associate .py with UTF-8.
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For Python 2, an important reason for using non-UTF-8 encodings was
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that byte string literals would be in the source encoding at run-time,
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allowing then to output them to a file or render them to the user
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as-is. With Python 3, all strings will be Unicode strings, so the
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original encoding of the source will have no impact at run-time.
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Implementation
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==============
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The parser needs to be changed to accept bytes > 127 if no source
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encoding is specified; instead of giving an error, it needs to check
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that the bytes are well-formed UTF-8 (decoding is not necessary,
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as the parser converts all source code to UTF-8, anyway).
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IDLE needs to be changed to use UTF-8 as the default encoding.
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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