Some typo fixes in PEP 3138; also add variables footer.
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Georg Brandl
parent
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commit
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273
pep-3138.txt
273
pep-3138.txt
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@ -13,11 +13,11 @@ Post-History: 05-May-2008, 05-Jun-2008
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Abstract
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========
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This PEP proposes a new string representation form for Python 3000. In
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Python prior to Python 3000, the repr() built-in function converted
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arbitrary objects to printable ASCII strings for debugging and logging.
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For Python 3000, a wider range of characters, based on the Unicode
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standard, should be considered 'printable'.
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This PEP proposes a new string representation form for Python 3000.
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In Python prior to Python 3000, the repr() built-in function converted
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arbitrary objects to printable ASCII strings for debugging and
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logging. For Python 3000, a wider range of characters, based on the
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Unicode standard, should be considered 'printable'.
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Motivation
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@ -28,8 +28,8 @@ algorithm.
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- Convert CR, LF, TAB and '\\' to '\\r', '\\n', '\\t', '\\\\'.
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- Convert other non-printable characters(0x00-0x1f, 0x7f) and non-ASCII
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characters(>=0x80) to '\\xXX'.
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- Convert other non-printable characters(0x00-0x1f, 0x7f) and
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non-ASCII characters (>= 0x80) to '\\xXX'.
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- Backslash-escape quote characters (apostrophe, ') and add the quote
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character at the beginning and the end.
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@ -39,177 +39,184 @@ For Unicode strings, the following additional conversions are done.
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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 16-bit characters(>=0x100) to '\\uXXXX'.
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- Convert 16-bit characters (>= 0x100) to '\\uXXXX'.
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- Convert 21-bit characters(>=0x10000) and surrogate pair characters to
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'\\U00xxxxxx'.
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- Convert 21-bit characters (>= 0x10000) and surrogate pair characters
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to '\\U00xxxxxx'.
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This algorithm converts any string to printable ASCII, and repr() is
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used as a handy and safe way to print strings for debugging or for
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logging. Although all non-ASCII characters are escaped, this does not
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matter when most of the string's characters are ASCII. But for other
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logging. Although all non-ASCII characters are escaped, this does not
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matter when most of the string's characters are ASCII. But for other
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languages, such as Japanese where most characters in a string are not
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ASCII, this is very inconvenient.
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We can use ``print(aJapaneseString)`` to get a readable string, but we
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don't have a similar workaround for printing strings from collections
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such as lists or tuples. ``print(listOfJapaneseStrings)`` uses repr() to
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build the string to be printed, so the resulting strings are always
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hex-escaped. Or when ``open(japaneseFilemame)`` raises an exception, the
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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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such as lists or tuples. ``print(listOfJapaneseStrings)`` uses repr()
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to build the string to be printed, so the resulting strings are always
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hex-escaped. Or when ``open(japaneseFilemame)`` raises an exception,
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the error message is something like ``IOError: [Errno 2] No such file
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or 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 progress
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in a similar way for printable output.
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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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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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happen in practice because bytes and strings are different types in
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console if they print binary data like images. But this is unlikely
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to happen in practice because bytes and strings are different types in
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Python 3000, so printing an image to the console won't mess it up.
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This issue was once discussed by Hye-Shik Chang [1]_ , but was rejected.
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This issue was once discussed by Hye-Shik Chang [1]_, but was rejected.
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Specification
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=============
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- Add a new function to the Python C API ``int Py_UNICODE_ISPRINTABLE
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(Py_UNICODE ch)``. This function returns 0 if repr() should escape the
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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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(Py_UNICODE ch)``. This function returns 0 if repr() should escape
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the Unicode character ``ch``; otherwise it returns 1. Characters
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that should be escaped are defined in the Unicode character database
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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
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('\\u2029').
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* Zs (Separator, Space) other than ASCII space ('\\x20'). Characters
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in 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
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'\\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
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character (0xd800-0xdbff, but not followed by 0xdc00-0xdfff) to
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'\\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
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0) 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 a
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quote 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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- Set the Unicode error-handler for sys.stderr to 'backslashreplace'
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by 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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(PyObject *o)``. This function converts any python object to a
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string using PyObject_Repr() and then hex-escapes all non-ASCII
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characters. ``PyObject_ASCII()`` generates the same string as
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``PyObject_Repr()`` 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 new built-in function, ``ascii()``. This function converts
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any python object to a string using repr() and then hex-escapes all
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non-ASCII characters. ``ascii()`` generates the same string as
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``repr()`` in Python 2.
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- Add ``'%a'`` string format operator. ``'%a'`` converts any python
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- Add a ``'%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. Also, add ``'!a'`` conversion flags to the
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characters. The ``'%a'`` format operator generates the same string
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as ``'%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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PyUnicode_FromFormat(). They convert 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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- Add an ``isprintable()`` method to the string type.
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``str.isprintable()`` returns False if repr() would escape any
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character in the string; otherwise returns True. The
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``isprintable()`` method calls the ``Py_UNICODE_ISPRINTABLE()``
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function internally.
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Rationale
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=========
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The repr() in Python 3000 should be Unicode not ASCII based, just like
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Python 3000 strings. Also, conversion should not be affected by the
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locale setting, because the locale is not necessarily the same as the
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output device's locale. For example, it is common for a daemon process
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to be invoked in an ASCII setting, but writes UTF-8 to its log files.
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Also, web applications might want to report the error information in
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more readable form based on the HTML page's encoding.
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The repr() in Python 3000 should be Unicode, not ASCII based, just
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like Python 3000 strings. Also, conversion should not be affected by
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the locale setting, because the locale is not necessarily the same as
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the output device's locale. For example, it is common for a daemon
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process to be invoked in an ASCII setting, but writes UTF-8 to its log
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files. Also, web applications might want to report the error
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information in 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
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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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printing, by the Unicode encoder's error-handler. If the
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error-handler of the output file is 'backslashreplace', such
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characters are hex-escaped without raising UnicodeEncodeError. For
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example, if the default encoding is ASCII, ``print('Hello ¢')`` will
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print 'Hello \\xa2'. If the encoding is ISO-8859-1, 'Hello ¢' will be
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printed.
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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 explicitly change the
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error-handler. Unlike sys.stdout, sys.stderr doesn't raise
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The default error-handler for sys.stdout is 'strict'. Other
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applications reading the output might not understand hex-escaped
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characters, so unsupported characters should be trapped when writing.
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If unsupported characters must be escaped, the error-handler should be
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changed explicitly. 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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'backslashreplace'. So printing error messages containing non-ASCII
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characters to sys.stderr will not raise an exception. Also,
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information about uncaught exceptions (exception object, traceback) is
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printed by the interpreter without raising exceptions.
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Alternate Solutions
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-------------------
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To help debugging in non-Latin languages without changing repr(), other
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suggestions were made.
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To help debugging in non-Latin languages without changing repr(),
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other suggestions were made.
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- Supply a tool to print lists or dicts.
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Strings to be printed for debugging are not only contained by lists or
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dicts, but also in many other types of object. File objects contain a
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file name in Unicode, exception objects contain a message in Unicode,
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etc. These strings should be printed in readable form when repr()ed.
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It is unlikely to be possible to implement a tool to print all
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possible object types.
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Strings to be printed for debugging are not only contained by lists
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or dicts, but also in many other types of object. File objects
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contain a file name in Unicode, exception objects contain a message
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in Unicode, etc. These strings should be printed in readable form
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when repr()ed. It is unlikely to be possible to implement a tool to
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print all possible object types.
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- Use sys.displayhook and sys.excepthook.
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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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characters to the original characters. But these hooks are called
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only when printing the result of evaluating an expression entered in
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an interactive Python session, and don't work for the ``print()``
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function, for non-interactive sessions or for ``logging.debug("%r",
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...)``, 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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there is no chance to tell file objects apart.
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characters since there isn't enough information left by the time
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it's a string to undo the escaping correctly in all cases. For
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example, ``print("\\"+"u0041")`` should be printed as '\\u0041', not
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'A'. But 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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existing repr() the default.
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With adjustable repr(), the result of using repr() is unpredictable
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and would make it impossible to write correct code involving repr().
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And if current repr() is the default, then the old convention remains
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intact and users may expect ASCII strings as the result of repr().
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Third party applications or libraries could be confused when a custom
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repr() function is used.
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And if current repr() is the default, then the old convention
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remains intact and users may expect ASCII strings as the result of
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repr(). Third party applications or libraries could be confused
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when a custom repr() function is used.
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Backwards Compatibility
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=======================
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Changing repr() may break some existing code, especially testing code.
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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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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
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use 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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@ -221,25 +228,25 @@ UnicodeEncodeError. ::
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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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To avoid exceptions being raised, you can explicitly specify the
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error-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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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 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 '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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For a console that uses a Unicode-based encoding, for example,
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en_US.utf8 or de_DE.utf8, the backslashreplace trick doesn't work and
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all printable characters are not escaped. This will cause a problem
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of similarly drawing characters in Western, Greek and Cyrillic
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languages. These languages use similar (but different) alphabets
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(descended from a common ancestor) and contain letters that look
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similar but have different character codes. For example, it is hard
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to distinguish Latin 'a', 'e' and 'o' from Cyrillic 'а', 'е' and 'о'.
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(The visual representation, of course, very much depends on the fonts
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used but usually these letters are almost indistinguishable.) To
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avoid the problem, the user can adjust the terminal encoding to get a
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result suitable for their environment.
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Rejected Proposals
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@ -252,20 +259,21 @@ 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 ``"\N{GREEK
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SMALL LETTER ALPHA}"``.
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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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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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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
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UIGHUR KIRGHIZ YEH WITH HAMZA ABOVE WITH ALEF MAKSURA ISOLATED
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FORM}"``.
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- Default error-handler of sys.stdout should be 'backslashreplace'.
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Stuff written to stdout might be consumed by another program that
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might misinterpret the \ escapes. For interactive session, it is
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possible to make 'backslashreplace' error-handler to default, but may
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add confusion of the kind "it works in interactive mode but not when
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redirecting to a file".
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might misinterpret the \\ escapes. For interactive sessions, it is
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possible to make the 'backslashreplace' error-handler the default,
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but this may add confusion of the kind "it works in interactive mode
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but not when redirecting to a file".
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Implementation
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@ -288,3 +296,14 @@ 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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