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Comment: Corrected as per sujestions from Oleg Broytmann. Thanks!
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| The repr() in Python 3000 should be Unicode not ASCII based, just like Python 3000 strings. Also, conversion should not be affected by the locale setting, because the locale is not necessarily the same as the output device's locale. For example, it is common for a daemon process to be invoked in an ASCII setting, but writes UTF-8 to its log files. | The repr() in Python 3000 should be Unicode not ASCII based, just like Python 3000 strings. Also, conversion should not be affected by the locale setting, because the locale is not necessarily the same as the output device's locale. For example, it is common for a daemon process to be invoked in an ASCII setting, but writes UTF-8 to its log files. Also, web applications might want to report the error infomation in more readable form based on page's encoding. |
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| For interactive sessions, we can write hooks to restore hex escaped characters to the original characters. But these hooks are called only when the result of evaluating an expression entered in an interactive Python session, and doesn't work for the print() function or for non-interactive sessions. | For interactive sessions, we can write hooks to restore hex escaped characters to the original characters. But these hooks are called only when the result of evaluating an expression entered in an interactive Python session, and doesn't work for the print() function, for non-interactive sessions or for logging.debug("%r", ...), etc. |
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| - there is a problem of similarly drawing characters in Western,Greek and Cyrillic languages. These languages use similar (but different) alphabets (descended from the common ancestor) and contain letters that look similar but has different character codes. For example, it is hard to distinguish Latin 'a', 'e' and 'o' from Cyrillic 'а', 'е' and 'о'. (The visual representation, of course, very much depends on the fonts used but usually these letters are almost indistinguishable.) | |
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PEP:
Title: String representation in Python 3000 Version: $Revision$ Last-Modified: $Date$ Author: Atsuo Ishimoto <ishimoto--at--gembook.org> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: Post-History:
Abstract
This PEP proposes new string representation form for Python 3000. In Python prior to Python 3000, the repr() built-in function converts arbitrary objects to printable ASCII strings for debugging and logging. For Python 3000, a wider range of characters, based on the Unicode standard, should be considered 'printable'.
Motivation
The current repr() converts 8-bit strings to ASCII using following algorithm.
- Convert CR, LF, TAB and '\' to '\r', '\n', '\t', '\\'.
- Convert other non-printable characters(0x00-0x1f, 0x7f) and non-ASCII characters(>=0x80) to '\xXX'.
- Backslash-escape quote characters(apostrophe, ') and add the quote character at the beginning and the end.
For Unicode strings, the following additional conversions are done.
- Convert leading surrogate pair characters without trailing character(0xd800-0xdbff, but not followed by 0xdc00-0xdfff) to '\uXXXX'.
- Convert 16-bit characters(>=0x100) to '\uXXXX'.
- Convert 21-bit characters(>=0x10000) and surrogate pair characters to '\U00xxxxxx'.
This algorithm converts any string to printable ASCII, and repr() is used as handy and safe way to print strings for debugging or for logging. Although all non-ASCII characters are escaped, this does not matter when most of the string's characters are ASCII. But for other languages, such as Japanese where most characters in a string are not ASCII, this is very inconvenient. Python 3000 has a lot of nice features for non-Latin users such as non-ASCII identifiers, so it would be helpful if Python could also progress in a similar way for printable output.
Some users might be concerned that such output will mess up their console if they print binary data like images. But this is unlikely to happen in practice because bytes and strings are different types in Python 3000, so printing an image to the console won't mess it up.
This issue was once discussed by Hye-Shik Chang [1] , but was rejected.
Specification
- The algorithm to build repr() strings should be changed to:
- Convert CR, LF, TAB and '\' to '\r', '\n', '\t', '\\'.
- Convert other non-printable ASCII characters(0x00-0x1f, 0x7f) to '\xXX'.
- Convert leading surrogate pair characters without trailing character(0xd800-0xdbff, but not followed by 0xdc00-0xdfff) to '\uXXXX'.
- Convert Unicode whitespace other than ASCII space('\x20') and control characters (categories Z* and C* in the Unicode database) to 'xXX', '\uXXXX' or '\U00xxxxxx'.
- Backslash-escape quote characters(apostrophe, ') and add quote character at the beginning and the end.
- Set the Unicode error-handler for sys.stdout and sys.stderr to 'backslashreplace' by default.
Rationale
The repr() in Python 3000 should be Unicode not ASCII based, just like Python 3000 strings. Also, conversion should not be affected by the locale setting, because the locale is not necessarily the same as the output device's locale. For example, it is common for a daemon process to be invoked in an ASCII setting, but writes UTF-8 to its log files. Also, web applications might want to report the error infomation in more readable form based on page's encoding.
Characters not supported by user's console are hex-escaped on printing, by the Unicode encoders' error-handler. If the error-handler of the output file is 'backslashreplace', such characters are hex-escaped without raising UnicodeEncodeError. For example, if your default encoding is ASCII, print('¢') will prints '\xa2'. If your encoding is ISO-8859-1, '¢' will be printed.
Printable characters
The Unicode standard doesn't define Non-printable characters, so we must create our own definition. Here we propose to define Non-printable characters as follows.
- Non-printable ASCII characters as Python 2.
- Broken surrogate pair characters.
- Characters defined in the Unicode character database as
- Cc (Other, Control)
- Cf (Other, Format)
- Cs (Other, Surrogate)
- Co (Other, Private Use)
- Cn (Other, Not Assigned)
- Zl Separator, Line ('\u2028', LINE SEPARATOR)
- Zp Separator, Paragraph ('\u2029', PARAGRAPH SEPARATOR)
- Zs (Separator, Space) other than ASCII space('\x20'). Characters in this category should be escaped to avoid ambiguity.
Alternate Solutions
To help debugging in non-Latin languages without changing repr(), other suggestion were made.
Supply a tool to print lists or dicts.
Strings to be printed for debugging are not only contained by lists or dicts, but also in many other types of object. File objects contain a file name in Unicode, exception objects contain a message in Unicode, etc. These strings should be printed in readable form when repr()ed. It is unlikely to be possible to implement a tool to print all possible object types.
Use sys.displayhook and sys.excepthook.
For interactive sessions, we can write hooks to restore hex escaped characters to the original characters. But these hooks are called only when the result of evaluating an expression entered in an interactive Python session, and doesn't work for the print() function, for non-interactive sessions or for logging.debug("%r", ...), etc.
Subclass sys.stdout and sys.stderr.
It is difficult to implement a subclass to restore hex-escaped characters since there isn't enough information left by the time it's a string to undo the escaping correctly in all cases. For example, print("\\"+"u0041") should be printed as '\u0041', not 'A'. But there is no chance to tell file objects apart.
Make the encoding used by unicode_repr() adjustable.
There is no benefit preserving the current repr() behavior to make application/library authors aware of non-ASCII repr(). And selecting an encoding on printing is more flexible than having a global setting.
Open Issues
- A lot of people use UTF-8 for their encoding, for example, en_US.utf8 and de_DE.utf8. In such cases, the backslashescape trick doesn't work.
- there is a problem of similarly drawing characters in Western,Greek and Cyrillic languages. These languages use similar (but different) alphabets (descended from the common ancestor) and contain letters that look similar but has different character codes. For example, it is hard to distinguish Latin 'a', 'e' and 'o' from Cyrillic 'а', 'е' and 'о'. (The visual representation, of course, very much depends on the fonts used but usually these letters are almost indistinguishable.)
Backwards Compatibility
Changing repr() may break some existing codes, especially testing code. Five of Python's regression test fail with this modification. If you need repr() strings without non-ASCII character as Python 2, you can use following function.
def repr_ascii(obj):
return str(repr(obj).encode("ASCII", "backslashreplace"), "ASCII")Reference Implementation
References
| [1] | Multibyte string on string::string_print (http://bugs.python.org/issue479898) |
Copyright
This document has been placed in the public domain.