Change the treatment of positions returned by PEP293

error handers in the Unicode codecs: Negative
positions are treated as being relative to the end of
the input and out of bounds positions result in an
IndexError.

Also update the PEP and include an explanation of
this in the documentation for codecs.register_error.

Fixes a small bug in iconv_codecs: if the position
from the callback is negative *add* it to the size
instead of substracting it.

From SF patch #677429.

1 files changed, 16 insertions, 5 deletions

diff --git a/Doc/lib/libcodecs.tex b/Doc/lib/libcodecs.tex
index 355ac5d..caaaaf4 100644
--- a/Doc/lib/libcodecs.tex
+++ b/Doc/lib/libcodecs.tex
@@ -103,11 +103,22 @@ Raises a \exception{LookupError} in case the encoding cannot be found.
 Register the error handling function \var{error_handler} under the
 name \var{name}. \var{error_handler} will be called during encoding
 and decoding in case of an error, when \var{name} is specified as the
-errors parameter. \var{error_handler} will be called with an
-\exception{UnicodeEncodeError}, \exception{UnicodeDecodeError} or
-\exception{UnicodeTranslateError} instance and must return a tuple
-with a replacement for the unencodable/undecodable part of the input
-and a position where encoding/decoding should continue.
+errors parameter.
+
+For encoding \var{error_handler} will be called with a
+\exception{UnicodeEncodeError} instance, which contains information about
+the location of the error. The error handler must either raise this or
+a different exception or return a tuple with a replacement for the
+unencodable part of the input and a position where encoding should
+continue. The encoder will encode the replacement and continue encoding
+the original input at the specified position. Negative position values
+will be treated as being relative to the end of the input string. If the
+resulting position is out of bound an IndexError will be raised.
+
+Decoding and translating works similar, except \exception{UnicodeDecodeError}
+or \exception{UnicodeTranslateError} will be passed to the handler and
+that the replacement from the error handler will be put into the output
+directly.
 \end{funcdesc}
 
 \begin{funcdesc}{lookup_error}{name}