python-peps/pep-0293.txt

PEP: 293
Title: Codec Error Handling Callbacks
Version: $Revision$
Last-Modified: $Date$
Author: Walter D<>rwald
Status: Draft
Type: Standards Track
Created: 18-Jun-2002
Python-Version: 2.3
Post-History: 19-Jun-2002


Abstract

    This PEP aims at extending Python's fixed codec error handling
    schemes with a more flexible callback based approach.

    Python currently uses a fixed error handling for codec error
    handlers.  This PEP describes a mechanism which allows Python to
    use function callbacks as error handlers.  With these more
    flexible error handlers it is possible to add new functionality to
    existing codecs by e.g. providing fallback solutions or different
    encodings for cases where the standard codec mapping does not
    apply.


Specification

    Currently the set of codec error handling algorithms is fixed to
    either "strict", "replace" or "ignore" and the semantics of these
    algorithms is implemented separately for each codec.

    The proposed patch will make the set of error handling algorithms
    extensible through a codec error handler registry which maps
    handler names to handler functions.  This registry consists of the
    following two C functions:

        int PyCodec_RegisterError(const char *name, PyObject *error)

        PyObject *PyCodec_LookupError(const char *name)

    and their Python counterparts

        codecs.register_error(name, error)

        codecs.lookup_error(name)

    PyCodec_LookupError raises a LookupError if no callback function
    has been registered under this name.

    Similar to the encoding name registry there is no way of
    unregistering callback functions or iterating through the
    available functions.

    The callback functions will be used in the following way by the
    codecs: when the codec encounters an encoding/decoding error, the
    callback function is looked up by name, the information about the
    error is stored in an exception object and the callback is called
    with this object.  The callback returns information about how to
    proceed (or raises an exception).

    For encoding, the exception object will look like this:

       class UnicodeEncodeError(UnicodeError):
           def __init__(self, encoding, object, start, end, reason):
               UnicodeError.__init__(self,
                   "encoding '%s' can't encode characters " +
                   "in positions %d-%d: %s" % (encoding,
                       start, end-1, reason))
               self.encoding = encoding
               self.object = object
               self.start = start
               self.end = end
               self.reason = reason

    This type will be implemented in C with the appropriate setter and
    getter methods for the attributes, which have the following
    meaning:

      * encoding: The name of the encoding;
      * object: The original unicode object for which encode() has
        been called;
      * start: The position of the first unencodable character;
      * end: (The position of the last unencodable character)+1 (or
        the length of object, if all characters from start to the end
        of object are unencodable);
      * reason: The reason why object[start:end] couldn't be encoded.

    If object has consecutive unencodable characters, the encoder
    should collect those characters for one call to the callback if
    those characters can't be encoded for the same reason.  The
    encoder is not required to implement this behaviour but may call
    the callback for every single character, but it is strongly
    suggested that the collecting method is implemented.

    The callback must not modify the exception object.  If the
    callback does not raise an exception (either the one passed in, or
    a different one), it must return a tuple:

        (replacement, newpos)

    replacement is a unicode object that the encoder will encode and
    emit instead of the unencodable object[start:end] part, newpos
    specifies a new position within object, where (after encoding the
    replacement) the encoder will continue encoding.

    If the replacement string itself contains an unencodable character
    the encoder raises the exception object (but may set a different
    reason string before raising).

    Should further encoding errors occur, the encoder is allowed to
    reuse the exception object for the next call to the callback.
    Furthermore the encoder is allowed to cache the result of
    codecs.lookup_error.

    If the callback does not know how to handle the exception, it must
    raise a TypeError.

    Decoding works similar to encoding with the following differences:
    The exception class is named UnicodeDecodeError and the attribute
    object is the original 8bit string that the decoder is currently
    decoding.

    The decoder will call the callback with those bytes that
    constitute one undecodable sequence, even if there is more than
    one undecodable sequence that is undecodable for the same reason
    directly after the first one.  E.g. for the "unicode-escape"
    encoding, when decoding the illegal string "\\u00\\u01x", the
    callback will be called twice (once for "\\u00" and once for
    "\\u01").  This is done to be able to generate the correct number
    of replacement characters.

    The replacement returned from the callback is a unicode object
    that will be emitted by the decoder as-is without further
    processing instead of the undecodable object[start:end] part.

    There is a third API that uses the old strict/ignore/replace error
    handling scheme:

        PyUnicode_TranslateCharmap/unicode.translate

    The proposed patch will enhance PyUnicode_TranslateCharmap, so
    that it also supports the callback registry.  This has the
    additional side effect that PyUnicode_TranslateCharmap will
    support multi-character replacement strings (see SF feature
    request #403100 [1]).

    For PyUnicode_TranslateCharmap the exception class will be named
    UnicodeTranslateError.  PyUnicode_TranslateCharmap will collect
    all consecutive untranslatable characters (i.e. those that map to
    None) and call the callback with them.  The replacement returned
    from the callback is a unicode object that will be put in the
    translated result as-is, without further processing.

    All encoders and decoders are allowed to implement the callback
    functionality themselves, if they recognize the callback name
    (i.e. if it is a system callback like "strict", "replace" and
    "ignore").  The proposed patch will add two additional system
    callback names: "backslashreplace" and "xmlcharrefreplace", which
    can be used for encoding and translating and which will also be
    implemented in-place for all encoders and
    PyUnicode_TranslateCharmap.

    The Python equivalent of these five callbacks will look like this:

        def strict(exc):
            raise exc

        def ignore(exc):
            if isinstance(exc, UnicodeError):
                return (u"", exc.end)
            else:
                raise TypeError("can't handle %s" % exc.__name__)

       def replace(exc):
            if isinstance(exc, UnicodeEncodeError):
                return ((exc.end-exc.start)*u"?", exc.end)
            elif isinstance(exc, UnicodeDecodeError):
                return (u"\\ufffd", exc.end)
            elif isinstance(exc, UnicodeTranslateError):
                return ((exc.end-exc.start)*u"\\ufffd", exc.end)
            else:
                raise TypeError("can't handle %s" % exc.__name__)

       def backslashreplace(exc):
            if isinstance(exc,
                (UnicodeEncodeError, UnicodeTranslateError)):
                s = u""
                for c in exc.object[exc.start:exc.end]:
                   if ord(c)<=0xff:
                       s += u"\\x%02x" % ord(c)
                   elif ord(c)<=0xffff:
                       s += u"\\u%04x" % ord(c)
                   else:
                       s += u"\\U%08x" % ord(c)
                return (s, exc.end)
            else:
                raise TypeError("can't handle %s" % exc.__name__) 

       def xmlcharrefreplace(exc):
            if isinstance(exc,
                (UnicodeEncodeError, UnicodeTranslateError)):
                s = u""
                for c in exc.object[exc.start:exc.end]:
                   s += u"&#%d;" % ord(c)
                return (s, exc.end)
            else:
                raise TypeError("can't handle %s" % exc.__name__) 

    These five callback handlers will also be accessible to Python as
    codecs.strict_error, codecs.ignore_error, codecs.replace_error,
    codecs.backslashreplace_error and codecs.xmlcharrefreplace_error.


Rationale

    Most legacy encoding do not support the full range of Unicode
    characters.  For these cases many high level protocols support a
    way of escaping a Unicode character (e.g. Python itself supports
    the \x, \u and \U convention, XML supports character references
    via &#xxx; etc.).

    When implementing such an encoding algorithm, a problem with the
    current implementation of the encode method of Unicode objects
    becomes apparent: For determining which characters are unencodable
    by a certain encoding, every single character has to be tried,
    because encode does not provide any information about the location
    of the error(s), so

        # (1)
        us = u"xxx"
        s = us.encode(encoding)

    has to be replaced by

        # (2)
        us = u"xxx"
        v = []
        for c in us:
            try:
                v.append(c.encode(encoding))
            except UnicodeError:
                v.append("&#%d;" % ord(c))
        s = "".join(v)

    This slows down encoding dramatically as now the loop through the
    string is done in Python code and no longer in C code.

    Furthermore this solution poses problems with stateful encodings.
    For example UTF-16 uses a Byte Order Mark at the start of the
    encoded byte string to specify the byte order.  Using (2) with
    UTF-16, results in an 8 bit string with a BOM between every
    character.

    To work around this problem, a stream writer - which keeps state
    between calls to the encoding function - has to be used:

        # (3)
        us = u"xxx"
        import codecs, cStringIO as StringIO
        writer = codecs.getwriter(encoding)

        v = StringIO.StringIO()
        uv = writer(v)
        for c in us:
            try:
                uv.write(c)
            except UnicodeError:
                uv.write(u"&#%d;" % ord(c))
        s = v.getvalue()

    To compare the speed of (1) and (3) the following test script has
    been used:

        # (4)
        import time
        us = u"<22>a"*1000000
        encoding = "ascii"
        import codecs, cStringIO as StringIO

        t1 = time.time()

        s1 = us.encode(encoding, "replace")

        t2 = time.time()

        writer = codecs.getwriter(encoding)

        v = StringIO.StringIO()
        uv = writer(v)
        for c in us:
            try:
                uv.write(c)
            except UnicodeError:
                uv.write(u"?")
        s2 = v.getvalue()

        t3 = time.time()

        assert(s1==s2)
        print "1:", t2-t1
        print "2:", t3-t2
        print "factor:", (t3-t2)/(t2-t1)

    On Linux this gives the following output (with Python 2.3a0):

        1: 0.274321913719
        2: 51.1284689903
        factor: 186.381278466

    i.e. (3) is 180 times slower than (1).

    Callbacks must be stateless, because as soon as a callback is
    registered it is available globally and can be called by multiple
    encode() calls.  To be able to use stateful callbacks, the errors
    parameter for encode/decode/translate would have to be changed
    from char * to PyObject *, so that the callback could be used
    directly, without the need to register the callback globally.  As
    this requires changes to lots of C prototypes, this approach was
    rejected.

    Currently all encoding/decoding functions have arguments

        const Py_UNICODE *p, int size

    or

        const char *p, int size

    to specify the unicode characters/8bit characters to be
    encoded/decoded.  So in case of an error the codec has to create a
    new unicode or str object from these parameters and store it in
    the exception object.  The callers of these encoding/decoding
    functions extract these parameters from str/unicode objects
    themselves most of the time, so it could speed up error handling
    if these object were passed directly.  As this again requires
    changes to many C functions, this approach has been rejected.

    For stream readers/writers the errors attribute must be changeable
    to be able to switch between different error handling methods
    during the lifetime of the stream reader/writer. This is currently
    the case for codecs.StreamReader and codecs.StreamWriter and
    all their subclasses. All core codecs and probably most of the
    third party codecs (e.g. JapaneseCodecs) derive their stream
    readers/writers from these classes so this already works,
    but the attribute errors should be documented as a requirement.


Implementation Notes

    A sample implementation is available as SourceForge patch #432401
    [2] including a script for testing the speed of various
    string/encoding/error combinations and a test script.

    Currently the new exception classes are old style Python
    classes. This means that accessing attributes results
    in a dict lookup. The C API is implemented in a way
    that makes it possible to switch to new style classes
    behind the scene, if Exception (and UnicodeError) will
    be changed to new style classes implemented in C for
    improved performance.

    The class codecs.StreamReaderWriter uses the errors parameter for
    both reading and writing.  To be more flexible this should
    probably be changed to two separate parameters for reading and
    writing.

    The errors parameter of PyUnicode_TranslateCharmap is not
    availably to Python, which makes testing of the new functionality
    of PyUnicode_TranslateCharmap impossible with Python scripts.  The
    patch should add an optional argument errors to unicode.translate
    to expose the functionality and make testing possible.

    Codecs that do something different than encoding/decoding from/to
    unicode and want to use the new machinery can define their own
    exception classes and the strict handlers will automatically work
    with it. The other predefined error handlers are unicode specific
    and expect to get a Unicode(Encode|Decode|Translate)Error
    exception object so they won't work.


Backwards Compatibility

    The semantics of unicode.encode with errors="replace" has changed:
    The old version always stored a ? character in the output string
    even if no character was mapped to ?  in the mapping.  With the
    proposed patch, the replacement string from the callback callback
    will again be looked up in the mapping dictionary.  But as all
    supported encodings are ASCII based, and thus map ? to ?, this
    should not be a problem in practice.

    Illegal values for the errors argument raised ValueError before,
    now they will raise LookupError.


References

    [1] SF feature request #403100
        "Multicharacter replacements in PyUnicode_TranslateCharmap"
        http://www.python.org/sf/403100

    [2] SF patch #432401 "unicode encoding error callbacks"
        http://www.python.org/sf/432401


Copyright

    This document has been placed in the public domain.



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