[Python-Dev] Decoding incomplete unicode (original) (raw)

M.-A. Lemburg mal at egenix.com
Thu Aug 19 11:34:19 CEST 2004

Walter Dörwald wrote:

Martin v. Löwis wrote:

M.-A. Lemburg wrote:

I've thought about this some more. Perhaps I'm still missing something, but wouldn't it be possible to add a feeding mode to the existing stream codecs by creating a new queue data type (much like the queue you have in the test cases of your patch) and using the stream codecs on these ?

Here is the problem. In UTF-8, how does the actual algorithm tell (the application) that the bytes it got on decoding provide for three fully decodable characters, and that 2 bytes are left undecoded, and that those bytes are not inherently ill-formed, but lack a third byte to complete the multi-byte sequence? On top of that, you can implement whatever queuing or streaming APIs you want, but you need an efficient way to communicate incompleteness. We already have an efficient way to communicate incompleteness: the decode method returns the number of decoded bytes. The questions remaining are 1) communicate to whom? IMHO the info should only be used internally by the StreamReader.

Handling incompleteness should be something for the codec to deal with. The queue doesn't have to know about it in an way. However, the queue should have interfaces allowing the codec to tell whether there are more bytes waiting to be processed.

2) When is incompleteness OK? Incompleteness is of course not OK in the stateless API. For the stateful API, incompleteness has to be OK even when the input stream is (temporarily) exhausted, because otherwise a feed mode wouldn't work anyway. But then incompleteness is always OK, because the StreamReader can't distinguish a temporarily exhausted input stream from a permanently exhausted one. The only fix for this I can think of is the final argument.

A final argument may be the way to go. But it should be an argument for the .read() method (not only the .decode() method) since that's the method reading the data from the queue.

I'd suggest that we extend the existing encode and decode codec APIs to take an extra state argument that holds the codec state in whatever format the codec needs (e.g. this could be a tuple or a special object):

encode(data, errors='strict', state=None) decode(data, errors='strict', state=None)

In the case of the .read() method, decode() would be called. If the returned length_consumed does not match the length of the data input, the remaining items would have to be placed back onto the queue in non-final mode. In final mode an exception would be raised to signal the problem.

I think it's PEP time for this new extension. If time permits I'll craft an initial version over the weekend.

-- Marc-Andre Lemburg eGenix.com

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