Abstract

This PEP proposes a new string representation form for Python 3000. In Python prior to Python 3000, the repr() built-in function converted 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 a 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.

We can use print(aJapaneseString) to get a readable string, but we don't have a similar workaround for printing strings from collections such as lists or tuples. print(listOfJapaneseStrings) uses repr() to build the string to be printed, so the resulting strings are always hex-escaped. Or when open(japaneseFilemame) raises an exception, the error message is something like IOError: [Errno 2] No such file or directory: '\u65e5\u672c\u8a9e', which isn't helpful.

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

• Add a new function to the Python C API int PY_UNICODE_ISPRINTABLE(Py_UNICODE ch). This function returns 0 if repr() should escape the Unicode character ch; otherwise it returns 1. Characters that should be escaped are:
  • Characters defined in the Unicode character database as "Other" (Cc, Cf, Cs, Co, Cn).
  • Characters defined in the Unicode character database as "Separator" (Zl, Zp, Zs) other than ASCII space (0x20).
• The algorithm to build repr() strings should be changed to:
  • Convert CR, LF, TAB and '\' to '\r', '\n', '\t', '\\'.
  • Convert 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 non-printable characters(PY_UNICODE_ISPRINTABLE() returns 0) 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.stderr to 'backslashreplace' by default.
• Set the Unicode error-handler for sys.stdout in the Python interactive session to 'backslashreplace' by default.
• Add '%a' string format operator. '%a' converts any python object to a string using repr() and then hex-escapes all non-ASCII characters. The '%a' format operator generates the same string as '%r' in Python 2.
• Add a new built-in function, ascii(). This function converts any python object to a string using repr() and then hex-escapes all non-ASCII characters. ascii() generates the same string as repr() in Python 2.
• Add an isprintable() method to the string type. str.isprintable() returns False if repr() should escape any character in the string; otherwise returns True. The isprintable() method calls the PY_UNICODE_ISPRINTABLE() function internally.

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 information in more readable form based on the HTML page's encoding.

Characters not supported by the user's console are hex-escaped on printing, by the Unicode encoder's 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('Hello ¢') will prints 'Hello \xa2'. If your encoding is ISO-8859-1, 'Hello ¢' will be printed.

For non-interactive sessions, sys.stdout's default error-handler should be set to 'strict'. Other applications reading the output might not understand hex-escaped characters, so unsupported characters should be trapped when writing.

Backwards Compatibility

Changing repr() may break some existing code, especially testing code. Five of Python's regression tests fail with this modification. If you need repr() strings without non-ASCII character as Python 2, you can use the following function.

def repr_ascii(obj):
    return str(repr(obj).encode("ASCII", "backslashreplace"), "ASCII")

For logging or for debugging, the following code can raise UnicodeEncodeError.

log = open("logfile", "w")
log.write(repr(data))     # UnicodeEncodeError will be raised
                          # if data contains unsupported characters.

To avoid exceptions being raised, you can explicitly specify the error-handler.

log = open("logfile", "w", errors="backslashreplace")
log.write(repr(data))  # Unsupported characters will be escaped.

For a console that uses a Unicode-based encoding, for example, en_US.utf8 or de_DE.utf8, the backslashescape trick doesn't work and all printable characters are not escaped. This will cause 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 have 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.) To avoid the problem, the user can adjust the terminal encoding to get a result suitable for their environment.

Open Issues

• Is the ascii() function necessary, or is it sufficient to document how to do it? If necessary, should ascii() belong to the builtin namespace?

Rejected Proposals

• Add encoding and errors arguments to the builtin print() function, with defaults of sys.getfilesystemencoding() and 'backslashreplace'.

  Complicated to implement, and in general, this is not seen as a good idea. [2]

• Use character names to escape characters, instead of hex character codes. For example, repr('\u03b1') can be converted to "\N{GREEK SMALL LETTER ALPHA}".

  Using character names can be very verbose compared to hex-escape. e.g., repr("\ufbf9") is converted to "\N{ARABIC LIGATURE UIGHUR KIRGHIZ YEH WITH HAMZA ABOVE WITH ALEF MAKSURA ISOLATED FORM}".

Reference Implementation

http://bugs.python.org/issue2630

References

Copyright

This document has been placed in the public domain.