M.-A. Lemburg wrote:
[...] def decode_stateful(data, state=None): ... decode and modify state ... return (decoded_data, length_consumed, state)
Another option might be that the decode function changes the state object in place.
But that's totally up to the implementor.
where the object type and contents of the state variable is defined per codec (e.g. could be a tuple, just a single integer or some other special object).
If a tuple is passed and returned this makes it possible from Python code to mangle the state. IMHO this should be avoided if possible.
Otherwise we'll end up having different interface signatures for all codecs and extending them to accomodate for future enhancement will become unfeasable without introducing yet another set of APIs.
We already have slightly different decoding functions: utf_16_ex_decode() takes additional arguments.
Right - it was a step in the wrong direction. Let's not use a different path for the future.
utf_16_ex_decode() serves a purpose: it help implement the UTF16 decoder, which has to switch to UTF-16-BE or UTF-16-LE according to the BOM, so utf_16_ex_decode() needs a way to comunicate that back to the caller.
Let's discuss this some more and implement it for Python 2.5. For Python 2.4, I think we can get away with what we already have:
OK, I've updated the patch.
[...] The buffer logic should only be used for streams that do not support the interface to push back already read bytes (e.g. .unread()).
From a design perspective, keeping read data inside the codec is the wrong thing to do, simply because it leaves the input stream in an undefined state in case of an error and there's no way to correlate the stream's read position to the location of the error.
With a pushback method on the stream, all the stream data will be stored on the stream, not the codec, so the above would no longer be a problem.
On the other hand this requires special stream. Data already read is part of the codec state, so why not put it into the codec?
Ideally, the codec should not store data,
I consider the remaining undecoded bytes to be part of the codec state once the have been read from the stream.
but only reference it. It's better to keep things well separated which is why I think we need the .unread() interface and eventually a queue interface to support the feeding operation.
However, we can always add the .unread() support to the stream codecs at a later stage, so it's probably ok to default to the buffer logic for Python 2.4.
That still leaves the issue of the last read operation, which I'm tempted to leave unresolved for Python 2.4. No matter what the solution is, it would likely require changes to all codecs, which is not good.
We could have a method on the codec which checks whether the codec buffer or the stream still has pending data left. Using this method is an application scope consideration, not a codec issue.
But this mean that the normal error handling can't be used for those trailing bytes.
Right, but then: missing data (which usually causes the trailing bytes) is really something for the application to deal with, e.g. by requesting more data from the user, another application or trying to work around the problem in some way. I don't think that the codec error handler can practically cover these possibilities.
But in many cases the user might want to use "ignore" or "replace" error handling.
Bye, Walter Dörwald