PEP 293 – Codec Error Handling Callbacks
- Walter Dörwald <walter at livinglogic.de>
- Standards Track
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.
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 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.
Negative values for newpos are treated as being relative to
end of object. If newpos is out of bounds the encoder will raise
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
If the callback does not know how to handle the exception, it must
Decoding works similar to encoding with the following differences:
- The exception class is named
UnicodeDecodeErrorand 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
\\u00and 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
There is a third API that uses the old strict/ignore/replace error handling scheme:
The proposed patch will enhance
that it also supports the callback registry. This has the
additional side effect that
support multi-character replacement strings (see SF feature
request #403100 ).
PyUnicode_TranslateCharmap the exception class will be named
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
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
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
\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"äa"*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
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
Currently all encoding/decoding functions have arguments
const Py_UNICODE *p, int size
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
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.
A sample implementation is available as SourceForge patch #432401  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
be changed to new style classes implemented in C for
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
The errors parameter of
PyUnicode_TranslateCharmap is not
availably to Python, which makes testing of the new functionality
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
exception object so they won’t work.
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 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
now they will raise
- SF feature request #403100 “Multicharacter replacements in PyUnicode_TranslateCharmap” https://bugs.python.org/issue403100
- SF patch #432401 “unicode encoding error callbacks” https://bugs.python.org/issue432401
This document has been placed in the public domain.
Last modified: 2018-07-21 23:57:17 GMT