[Python-Dev] PEP 393 Summer of Code Project (original) (raw)

Stephen J. Turnbull stephen at xemacs.org
Thu Sep 1 09:13:03 CEST 2011

Where I cut your words, we are in 100% agreement. (FWIW :-)

Guido van Rossum writes:

On Tue, Aug 30, 2011 at 11:03 PM, Stephen J. Turnbull <stephen at xemacs.org> wrote:

Well, that's why I wrote "intended to be suggestive".  The Unicode Standard does not specify at all what the internal representation of characters may be, it only specifies what their external behavior must be when two processes communicate.  (For "process" as used in the standard, think "Python modules" here, since we are concerned with the problems of folks who develop in Python.)  When observing the behavior of a Unicode process, there are no UTF-16 arrays or UTF-8 arrays or even UTF-32 arrays; only arrays of characters.

Hm, that's not how I would read "process". IMO that is an intentionally vague term,

I agree. I'm sorry that I didn't make myself clear. The reason I read "process" as "module" is that some modules of Python, and therefore Python as a whole, cannot conform to the Unicode standard. Eg, anything that inputs or outputs bytes. Therefore only "modules" and "types" can be asked to conform. (I don't think it makes sense to ask anything lower level to conform. See below where I comment on your .lower() example.)

What I am advocating (for the long term) is provision of one module (or type) such that if the text processing done by the application is done entirely in terms of this module (type), it will conform (to some specified degree, chosen to balance user wants with implementation and support costs). It may be desireable to provide others for sufficiently important particular use cases, but at present I see a clear need for one. Unicode conformance is going to be a common requirement for apps used by global enterprises.

I oppose trying to make str into that type. We need str, just as it is, for many reasons.

and we are free to decide how to interpret it. I don't think it will work very well to define a process as a Python module; what about Python modules that agree about passing along array of code units (or streams of UTF-8, for that matter)?

Certainly a group of cooperating modules could form a conforming process, just as you describe it for one example. The "one module" mentioned above need not implement everything internally, but it would take responsiblity for providing guarantees (eg, unit tests) of whatever conformance claims it makes.

Thus, according to the rules of handling a UTF-16 stream, it is an error to observe a lone surrogate or a surrogate pair that isn't a high-low pair (Unicode 6.0, Ch. 3 "Conformance", requirements C1 and C8-C10).  That's what I mean by "can't tell it's UTF-16".

But if you can observe (valid) surrogate pairs it is still UTF-16.

In the concrete implementation I have in mind, surrogate pairs are represented by a str containing 2 code units. But in that case s[i][1] is an error, and s[i][0] == s[i]. print(s[i][0]) and print(s[i]) will print the same character to the screen. If you decode it to bytes, well, it's not a str any more so what have you proved? Ie, what you will see is code points not in the BMP.

You don't have to agree that such "surrogate containment" behavior is so valuable as I think it is, but that's what I have in mind as one requirement for a "conforming implementation of UTF-16".

At the same time I think it would be useful if certain string operations like .lower() worked in such a way that if the input were valid UTF-16, then the output would also be, while if the input contained an invalid surrogate, the result would simply be something that is no worse (in particular, those are all mapped to themselves).

I don't think that it's a good idea to go for conformance at the method level. It would be a feature for apps that don't claim full conformance because they nevertheless give good results in more cases. The downside will be Python apps using str that will pass conformance tests written for, say Western Europe, but end users in Kuwait and Kuala Lumpur will report bugs.

An analogy is actually found in .lower() on 8-bit strings in Python 2: it assumes the string contains ASCII, and non-ASCII characters are mapped to themselves. If your string contains Latin-1 or EBCDIC or UTF-8 it will not do the right thing. But that doesn't mean strings cannot contain those encodings, it just means that the .lower() method is not useful if they do. (Why ASCII? Because that is the system encoding in Python 2.)

Sure. I think that approach is fine for str, too, except that I would hope it looks up BMP base characters in the case-mapping database. The fact is that with very few exceptions non-BMP characters are going to be symbols (mathematical operators and emoticons, for example). This is good enough, except when it's not---but when it's not, only 100% conformance is really a reasonable target. IMO, of course.

I think we should just document how it behaves and not get hung up on what it is called. Mentioning UTF-16

If you also say, "this type can represent all characters in Unicode, as well as certain non-characters", why mention UTF-16 at all?

Let's call those things graphemes (Tom C's term, I quite like leaving "character" ambiguous)

OK, but those definitions need to be made clear, as "grapheme cluster" and "combined character" are defined in the Unicode standard, and in fact mean slightly different things from each other.

-- they are sequences of multiple code points that represent a single "visual squiggle" (the kind of thing that you'd want to be swappable in vim with "xp" :-). I agree that APIs are needed to manipulate (match, generate, validate, mutilate, etc.) things at the grapheme level. I don't agree that this means a separate data type is required.

Clear enough.

There are ever-larger units of information encoded in text strings, with ever farther-reaching (and more vague) requirements on valid sequences. Do you want to have a data type that can represent (only valid) words in a language? Sentences? Novels?

No, and I can tell you why! The difference between characters and words is much more important than that between code point and grapheme cluster for most users and the developers who serve them. Even small children recognize typographical ligatures as being composite objects, while at least this Spanish-as-a-second-language learner was taught that ñ' is an atomic character represented by a discontiguous glyph, like i', and it is no more related to n' than m' is. Users really believe that characters are atomic. Even in the cases of Han characters and Hangul, users think of the characters as being "atomic," but in the sense of Bohr rather than that of Democritus.

I think the situation for text processing is analogous to chemistry where the atom, with a few fairly gross properties (the outer electron orbitals) is the fundamental unit, not the elementary particles like electrons and protons and structures like inner orbitals. Sure, there are higher order structures like molecules, phases, and crystals, but it is elements that have the most regular and simply described behavior for the chemist, and it does not become any simpler for the chemist if you decompose the atom. The composed character or grapheme cluster is the analogue of the atom for most processing at the level of "text". The only real exceptions I can imagine are in the domain of linguistics.

I think that at this point in time the best we can do is claim that Python (the language standard) uses either 16-bit code units or 21-bit code points in its string datatype, and that, thanks to PEP 393, CPython 3.3 and further will always use 21-bit code points (but Jython and IronPython may forever use their platform's native 16-bit code unit representing string type). And then we add APIs that can be used everywhere to look for code points (even if the string contains code points), graphemes, or larger constructs. I'd like those APIs to be designed using a garbage-in-garbage-out principle, where if the input conforms to some Unicode requirement, the output does too, but if the input doesn't, the output does what makes most sense. Validation is then limited to codecs, and optional calls.

Clear enough. I disagree that that will be enough for constructing large-scale Unicode-conformant applications. Somebody is going to have to produce batteries for those applications, and I think they should be included in Python. I agree that it's proper that I and those who think the same way take responsibility for writing and implementing a PEP.

If you index or slice a string, or create a string from chr() of a surrogate or from some other value that the Unicode standard considers an illegal code point, you better know what you are doing.

I think that's like asking a toddler to know that the stove is hot. The consequences for the toddler of her ignorance are much greater, but the informational requirement is equally stringent. Of course application writers are adults who could be asked to learn, but economically I think it make a lot more sense to include those batteries. IMHO YMMV, obviously.

I want chr(i) to be valid for all values of i in range(2**21),

I quite agree (ie, for str). Thus I perceive a need for another type.

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