New cleaned-up version from Atsuo.
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Guido van Rossum
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126
pep-3138.txt
126
pep-3138.txt
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@ -7,7 +7,7 @@ Status: Accepted
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 05-May-2008
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Post-History:
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Post-History: 05-May-2008, 05-Jun-2008
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Abstract
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@ -60,8 +60,8 @@ error message is something like ``IOError: [Errno 2] No such file or
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directory: '\u65e5\u672c\u8a9e'``, which isn't helpful.
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Python 3000 has a lot of nice features for non-Latin users such as
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non-ASCII identifiers, so it would be helpful if Python could also
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progress in a similar way for printable output.
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non-ASCII identifiers, so it would be helpful if Python could also progress
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in a similar way for printable output.
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Some users might be concerned that such output will mess up their
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console if they print binary data like images. But this is unlikely to
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@ -79,48 +79,57 @@ Specification
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Unicode character ``ch``; otherwise it returns 1. Characters that should
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be escaped are defined in the Unicode character database as:
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* Cc (Other, Control)
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* Cf (Other, Format)
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* Cs (Other, Surrogate)
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* Co (Other, Private Use)
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* Cn (Other, Not Assigned)
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* Zl (Separator, Line), refers to LINE SEPARATOR ('\\u2028').
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* Zp (Separator, Paragraph), refers to PARAGRAPH SEPARATOR ('\\u2029').
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* Zs (Separator, Space) other than ASCII space('\\x20'). Characters in
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this category should be escaped to avoid ambiguity.
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* Cc (Other, Control)
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* Cf (Other, Format)
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* Cs (Other, Surrogate)
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* Co (Other, Private Use)
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* Cn (Other, Not Assigned)
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* Zl (Separator, Line), refers to LINE SEPARATOR ('\\u2028').
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* Zp (Separator, Paragraph), refers to PARAGRAPH SEPARATOR ('\\u2029').
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* Zs (Separator, Space) other than ASCII space('\\x20'). Characters in
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this category should be escaped to avoid ambiguity.
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- The algorithm to build repr() strings should be changed to:
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* Convert CR, LF, TAB and '\\' to '\\r', '\\n', '\\t', '\\\\'.
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* Convert CR, LF, TAB and '\\' to '\\r', '\\n', '\\t', '\\\\'.
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* Convert non-printable ASCII characters(0x00-0x1f, 0x7f) to '\\xXX'.
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* Convert non-printable ASCII characters(0x00-0x1f, 0x7f) to '\\xXX'.
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* Convert leading surrogate pair characters without trailing character
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(0xd800-0xdbff, but not followed by 0xdc00-0xdfff) to '\\uXXXX'.
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* Convert leading surrogate pair characters without trailing character
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(0xd800-0xdbff, but not followed by 0xdc00-0xdfff) to '\\uXXXX'.
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* Convert non-printable characters(Py_UNICODE_ISPRINTABLE() returns 0)
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to '\\xXX', '\\uXXXX' or '\\U00xxxxxx'.
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* Convert non-printable characters(Py_UNICODE_ISPRINTABLE() returns 0)
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to 'xXX', '\\uXXXX' or '\\U00xxxxxx'.
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* Backslash-escape quote characters (apostrophe, 0x27) and add quote
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character at the beginning and the end.
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* Backslash-escape quote characters (apostrophe, 0x27) and add quote
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character at the beginning and the end.
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- Set the Unicode error-handler for sys.stderr to 'backslashreplace' by
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default.
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- Add a new function to the Python C API ``PyObject *PyObject_ASCII
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(PyObject *o)``. This function converts any python object to a string
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using PyObject_Repr() and then hex-escapes all non-ASCII characters.
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``PyObject_ASCII()`` generates the same string as ``PyObject_Repr()``
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in Python 2.
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- Add a new built-in function, ``ascii()``. This function converts any
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python object to a string using repr() and then hex-escapes all non-ASCII
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characters. ``ascii()`` generates the same string as ``repr()`` in
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Python 2.
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- Add ``'%a'`` string format operator. ``'%a'`` converts any python
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object to a string using repr() and then hex-escapes all non-ASCII
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characters. The ``'%a'`` format operator generates the same string as
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``'%r'`` in Python 2.
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- Add a new built-in function, ``ascii()``. This function converts any
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python object to a string using repr() and then hex-escapes all non-
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ASCII characters. ``ascii()`` generates the same string as ``repr()``
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in Python 2.
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``'%r'`` in Python 2. Also, add ``'!a'`` conversion flags to the
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``string.format()`` method and add ``'%A'`` operator to the
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PyUnicode_FromFormat(). They converts any object to an ASCII string
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as ``'%a'`` string format operator.
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- Add an ``isprintable()`` method to the string type. ``str.isprintable()``
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returns False if repr() should escape any character in the string;
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otherwise returns True. The ``isprintable()`` method calls the
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`` Py_UNICODE_ISPRINTABLE()`` function internally.
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``Py_UNICODE_ISPRINTABLE()`` function internally.
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Rationale
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@ -136,18 +145,21 @@ more readable form based on the HTML page's encoding.
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Characters not supported by the user's console could be hex-escaped on
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printing, by the Unicode encoder's error-handler. If the error-handler
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of the output file is 'backslashreplace', such characters are hex-
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escaped without raising UnicodeEncodeError. For example, if your default
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encoding is ASCII, ``print('Hello ¢')`` will prints 'Hello \\xa2'. If
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of the output file is 'backslashreplace', such characters are
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hex-escaped without raising UnicodeEncodeError. For example, if your default
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encoding is ASCII, ``print('Hello ¢')`` will print 'Hello \\xa2'. If
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your encoding is ISO-8859-1, 'Hello ¢' will be printed.
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Default error-handler of sys.stdout is 'strict'. Other applications
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The default error-handler for sys.stdout is 'strict'. Other applications
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reading the output might not understand hex-escaped characters, so
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unsupported characters should be trapped when writing. If you need to
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escape unsupported characters, you should change error-handler
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explicitly. For sys.stderr, default error-handler is set to
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'backslashreplace' and printing exceptions or error messages won't
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be failed.
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escape unsupported characters, you should explicitly change the
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error-handler. Unlike sys.stdout, sys.stderr doesn't raise
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UnicodeEncodingError by default, because the default error-handler is
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'backslashreplace'. So printing error messeges containing non-ASCII
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characters to sys.stderr will not raise an exception. Also, information
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about uncaught exceptions (exception object, traceback) are printed by
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the interpreter without raising exceptions.
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Alternate Solutions
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-------------------
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@ -169,15 +181,15 @@ suggestions were made.
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For interactive sessions, we can write hooks to restore hex escaped
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characters to the original characters. But these hooks are called only
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when printing the result of evaluating an expression entered in an
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interactive Python session, and doesn't work for the print() function,
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for non-interactive sessions or for logging.debug("%r", ...), etc.
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interactive Python session, and doesn't work for the ``print()`` function,
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for non-interactive sessions or for ``logging.debug("%r", ...)``, etc.
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- Subclass sys.stdout and sys.stderr.
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It is difficult to implement a subclass to restore hex-escaped
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characters since there isn't enough information left by the time it's
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a string to undo the escaping correctly in all cases. For example, ``
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print("\\"+"u0041")`` should be printed as '\\u0041', not 'A'. But
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a string to undo the escaping correctly in all cases. For example,
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``print("\\"+"u0041")`` should be printed as '\\u0041', not 'A'. But
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there is no chance to tell file objects apart.
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- Make the encoding used by unicode_repr() adjustable, and make the
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@ -199,45 +211,37 @@ Five of Python's regression tests fail with this modification. If you
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need repr() strings without non-ASCII character as Python 2, you can use
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the following function. ::
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def repr_ascii(obj):
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return str(repr(obj).encode("ASCII", "backslashreplace"), "ASCII")
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def repr_ascii(obj):
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return str(repr(obj).encode("ASCII", "backslashreplace"), "ASCII")
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For logging or for debugging, the following code can raise
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UnicodeEncodeError. ::
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log = open("logfile", "w")
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log.write(repr(data)) # UnicodeEncodeError will be raised
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# if data contains unsupported characters.
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log = open("logfile", "w")
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log.write(repr(data)) # UnicodeEncodeError will be raised
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# if data contains unsupported characters.
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To avoid exceptions being raised, you can explicitly specify the error-
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handler. ::
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log = open("logfile", "w", errors="backslashreplace")
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log.write(repr(data)) # Unsupported characters will be escaped.
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log = open("logfile", "w", errors="backslashreplace")
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log.write(repr(data)) # Unsupported characters will be escaped.
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For a console that uses a Unicode-based encoding, for example, en_US.
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utf8 or de_DE.utf8, the backslashescape trick doesn't work and all
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printable characters are not escaped. This will cause a problem of
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similarly drawing characters in Western, Greek and Cyrillic languages.
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These languages use similar (but different) alphabets (descended from
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the common ancestor) and contain letters that look similar but have
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These languages use similar (but different) alphabets (descended from a
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common ancestor) and contain letters that look similar but have
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different character codes. For example, it is hard to distinguish Latin
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'a', 'e' and 'o' from Cyrillic '\u0430', '\u0435' and '\u043e'. (The visual
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'a', 'e' and 'o' from Cyrillic 'а', 'е' and 'о'. (The visual
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representation, of course, very much depends on the fonts used but
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usually these letters are almost indistinguishable.) To avoid the
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problem, the user can adjust the terminal encoding to get a result
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suitable for their environment.
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Open Issues
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===========
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- Is the ``ascii()`` function necessary, or is it sufficient to document
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how to do it? If necessary, should ``ascii()`` belong to the builtin
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namespace?
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Rejected Proposals
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==================
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@ -248,10 +252,10 @@ Rejected Proposals
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idea. [2]_
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- Use character names to escape characters, instead of hex character
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codes. For example, ``repr('\u03b1')`` can be converted to
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``"\N{GREEK SMALL LETTER ALPHA}"``.
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codes. For example, ``repr('\u03b1')`` can be converted to ``"\N{GREEK
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SMALL LETTER ALPHA}"``.
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Using character names can be very verbose compared to hex-escape.
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Using character names can be very verbose compared to hex-escape.
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e.g., ``repr("\ufbf9")`` is converted to ``"\N{ARABIC LIGATURE UIGHUR
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KIRGHIZ YEH WITH HAMZA ABOVE WITH ALEF MAKSURA ISOLATED FORM}"``.
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@ -273,7 +277,7 @@ http://bugs.python.org/issue2630
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References
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==========
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.. [1] Multibyte string on string::string_print
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.. [1] Multibyte string on string\::string_print
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(http://bugs.python.org/issue479898)
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.. [2] [Python-3000] Displaying strings containing unicode escapes
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