[Python-Dev] PEP 393 Summer of Code Project

[Guido van Rossum]

On Tue, Aug 30, 2011 at 11:03 PM, Stephen J. Turnbull
<stephen at xemacs.org> wrote:
[me]
>  > That sounds like a contradiction -- it wouldn't be a UTF-16 array if
>  > you couldn't tell that it was using UTF-16.
>
> 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, 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)?

This is why I find the issue of Python, the language (and stdlib), as
a whole "conforming to the Unicode standard" such a troublesome
concept -- I think it is something that an application may claim, but
the language should make much more modest claims, such as "the regular
expression syntax supports features X, Y and Z from the Unicode
recommendation XXX, or "the UTF-8 codec will never emit a sequence of
bytes that is invalid according Unicode specification YYY". (As long
as the Unicode references are also versioned or dated.)

I'm fine with saying "it is hard to write Unicode-conforming
application code for reason ZZZ" and proposing a fix (e.g. PEP 393
fixes a specific complaint about code units being inferior to code
points for most types of processing). I'm not fine with saying "the
string datatype should conform to the Unicode standard".

> 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.

> And I
> understand those requirements to mean that operations on UTF-16
> streams should produce UTF-16 streams, or raise an error.  Without
> that closure property for basic operations on str, I think it's a bad
> idea to say that the representation of text in a str in a pre-PEP-393
> "narrow" build is UTF-16.  For many users and app developers, it
> creates expectations that are not fulfilled.

Ok, I dig this, to some extent. However saying it is UCS-2 is equally
bad. I guess this is why Java and .NET just say their string types
contain arrays of "16-bit characters", with essentially no semantics
attached to the word "character" besides "16-bit unsigned integer".

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).
We could even go further and have .lower() and friends look at
graphemes (multi-code-point characters) if the Unicode std has a
useful definition of e.g. lowercasing graphemes that differed from
lowercasing code points.

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.)

> It's true that common usage is that an array of code units that
> usually conforms to UTF-16 may be called "UTF-16" without the closure
> properties.  I just disagree with that usage, because there are two
> camps that interpret "UTF-16" differently.  One side says, "we have an
> array representation in UTF-16 that can handle all Unicode code points
> efficiently, and if you think you need more, think again", while the
> other says "it's too painful to have to check every result for valid
> UTF-16, and we need a UTF-16 type that supports the usual array
> operations on *characters* via the usual operators; if you think
> otherwise, think again."

I think we should just document how it behaves and not get hung up on
what it is called. Mentioning UTF-16 is still useful because it
indicates that some operations may act properly on surrogate pairs.
(Also because of course character properties for BMP characters are
respected, etc.)

> Note that despite the (presumed) resolution of the UTF-16 issue for
> CPython by PEP 393, at some point a very similar discussion will take
> place over "characters" anyway, because users and app developers are
> going to want a type that handles composition sequences and/or
> grapheme clusters for them, as well as comparison that respects
> canonical equivalence, even if it is inefficient compared to str.
> That's why I insisted on use of "array of code points" to describe the
> PEP 393 str type, rather than "array of characters".

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

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.

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 want
chr(i) to be valid for all values of i in range(2**21), so it can be
used to create a lone surrogate, or (on systems with 16-bit
"characters") a surrogate pair. And also ord(chr(i)) == i for all i in
range(2**21). I'm not sure about ord() on a 2-character string
containing a surrogate pair on systems where strings contain 21-bit
code points; I think it should be an error there, just as ord() on
other strings of length != 1. But on systems with 16-bit "characters",
ord() of strings of length 2 containing a valid surrogate pair should
work.

-- 
--Guido van Rossum (python.org/~guido)