This PEP was just sent to the python-dev list as a method to go between unicode and C byte strings in python-3.1. It would be great to get some more input on this as we don't want the situation to get worse than it already is. -Toshio -------- Original Message -------- Subject: [Python-Dev] PEP 383: Non-decodable Bytes in System Character Interfaces Date: Wed, 22 Apr 2009 08:50:22 +0200 From: "Martin v. Löwis" <martin@xxxxxxxxxxx> To: Python-Dev <python-dev@xxxxxxxxxx>, Python List <python-list@xxxxxxxxxx> I'm proposing the following PEP for inclusion into Python 3.1. Please comment. Regards, Martin PEP: 383 Title: Non-decodable Bytes in System Character Interfaces Version: $Revision: 71793 $ Last-Modified: $Date: 2009-04-22 08:42:06 +0200 (Mi, 22. Apr 2009) $ Author: Martin v. Löwis <martin@xxxxxxxxxxx> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 22-Apr-2009 Python-Version: 3.1 Post-History: Abstract ======== File names, environment variables, and command line arguments are defined as being character data in POSIX; the C APIs however allow passing arbitrary bytes - whether these conform to a certain encoding or not. This PEP proposes a means of dealing with such irregularities by embedding the bytes in character strings in such a way that allows recreation of the original byte string. Rationale ========= The C char type is a data type that is commonly used to represent both character data and bytes. Certain POSIX interfaces are specified and widely understood as operating on character data, however, the system call interfaces make no assumption on the encoding of these data, and pass them on as-is. With Python 3, character strings use a Unicode-based internal representation, making it difficult to ignore the encoding of byte strings in the same way that the C interfaces can ignore the encoding. On the other hand, Microsoft Windows NT has correct the original design limitation of Unix, and made it explicit in its system interfaces that these data (file names, environment variables, command line arguments) are indeed character data, by providing a Unicode-based API (keeping a C-char-based one for backwards compatibility). For Python 3, one proposed solution is to provide two sets of APIs: a byte-oriented one, and a character-oriented one, where the character-oriented one would be limited to not being able to represent all data accurately. Unfortunately, for Windows, the situation would be exactly the opposite: the byte-oriented interface cannot represent all data; only the character-oriented API can. As a consequence, libraries and applications that want to support all user data in a cross-platform manner have to accept mish-mash of bytes and characters exactly in the way that caused endless troubles for Python 2.x. With this PEP, a uniform treatment of these data as characters becomes possible. The uniformity is achieved by using specific encoding algorithms, meaning that the data can be converted back to bytes on POSIX systems only if the same encoding is used. Specification ============= On Windows, Python uses the wide character APIs to access character-oriented APIs, allowing direct conversion of the environmental data to Python str objects. On POSIX systems, Python currently applies the locale's encoding to convert the byte data to Unicode. If the locale's encoding is UTF-8, it can represent the full set of Unicode characters, otherwise, only a subset is representable. In the latter case, using private-use characters to represent these bytes would be an option. For UTF-8, doing so would create an ambiguity, as the private-use characters may regularly occur in the input also. To convert non-decodable bytes, a new error handler "python-escape" is introduced, which decodes non-decodable bytes using into a private-use character U+F01xx, which is believed to not conflict with private-use characters that currently exist in Python codecs. The error handler interface is extended to allow the encode error handler to return byte strings immediately, in addition to returning Unicode strings which then get encoded again. If the locale's encoding is UTF-8, the file system encoding is set to a new encoding "utf-8b". The UTF-8b codec decodes non-decodable bytes (which must be >= 0x80) into half surrogate codes U+DC80..U+DCFF. Discussion ========== While providing a uniform API to non-decodable bytes, this interface has the limitation that chosen representation only "works" if the data get converted back to bytes with the python-escape error handler also. Encoding the data with the locale's encoding and the (default) strict error handler will raise an exception, encoding them with UTF-8 will produce non-sensical data. For most applications, we assume that they eventually pass data received from a system interface back into the same system interfaces. For example, and application invoking os.listdir() will likely pass the result strings back into APIs like os.stat() or open(), which then encodes them back into their original byte representation. Applications that need to process the original byte strings can obtain them by encoding the character strings with the file system encoding, passing "python-escape" as the error handler name. Copyright ========= This document has been placed in the public domain. _______________________________________________ Python-Dev mailing list Python-Dev@xxxxxxxxxx http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/a.badger%40gmail.com
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