[Patches] [ python-Patches-1629305 ] The Unicode "lazy strings" patches

Fri Jan 12 05:25:49 CET 2007

Patches item #1629305, was opened at 2007-01-06 09:37
Message generated for change (Comment added) made by lhastings
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Category: Core (C code)
Group: Python 3000
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Larry Hastings (lhastings)
Assigned to: Nobody/Anonymous (nobody)
Summary: The Unicode "lazy strings" patches

Initial Comment:
These are patches to add lazy processing to Unicode strings for Python 3000.  I plan to post separate patches for both "lazy concatenation" and "lazy slices", as I suspect "lazy concatenation" has a much higher chance of being accepted.

There is a long discussion about "lazy concatenation" here:
http://mail.python.org/pipermail/python-dev/2006-October/069224.html
And another long discussion about "lazy slices" here:
http://mail.python.org/pipermail/python-dev/2006-October/069506.html

Note that, unlike the 8-bit-character strings patches, I don't expect the "lazy slices" patch to be dependent on the "lazy concatenation" patch.  Unicode objects are stored differently, and already use a pointer to a separately-allocated buffer.   This was the big (and mildly controversial) change made by the 8-bit-character "lazy concatenation" patch, and "lazy slices" needed it too.  Since Unicode objects already look like that, the Unicode lazy patches should be independent.

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>Comment By: Larry Hastings (lhastings)
Date: 2007-01-12 04:25

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File Added: lch.py3k.unicode.lazy.concat.patch.53392.txt

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Comment By: Larry Hastings (lhastings)
Date: 2007-01-12 03:12

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Attached below you will find the full "lazy strings" patch, which has both
"lazy concatenation" and "lazy slices".  The diff is against the current
revision of the Py3k branch, #53392.  On my machine (Win32) rt.bat
produces identical output before and after the patch, for both debug and
release builds.

As I mentioned in a previous comment, you can read the description (and
ensuing conversation) about "lazy slices" here:
http://mail.python.org/pipermail/python-dev/2006-October/069506.html

One new feature of this version: I added a method on a Unicode string,
s.simplify(), which forces the string to "render" if it's one of my exotic
string subtypes (a lazy concatenation or lazy slice).  My goal is to
assuage fears about pathological memory-use cases where you have
long-lived tiny slices of gigantic strings.  If you realize you're having
that problem, simply add calls to .simplify() on the slices and the
problem should go away.

As for the semantics of .simplify(), it returns a reference to the string
s.  Honestly I wasn't sure whether it should return a new string or just
monkey with the existing string.  Really, rendering doesn't change the
string; it's the same string, with the exact same external behavior, just
with different bits floating around underneath.  For now it monkeys with
the existing string, as that seemed best.  (But I'd be happy to switch it
to returning a new string if it'd help.)

I had planned to make the "lazy slices" patch independent of the "lazy
concatenation" patch.  However, it wound up being a bigger pain that I
thought, and anyway I figure the likelyhood that "lazy slices" would be
accepted and "lazy concatenation" would not is effectively zero.  So I
didn't bother.  If there's genuine interest in "lazy slices" without "lazy
concatenation", I can produce such a thing.
File Added: lch.py3k.unicode.lazy.slice.and.concat.patch.53392.txt

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Comment By: Larry Hastings (lhastings)
Date: 2007-01-12 02:50

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File Added: lch.py3k.unicode.lazy.concat.patch.53392.txt

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Comment By: Larry Hastings (lhastings)
Date: 2007-01-12 02:42

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lemburg:

You're right, the possibility of PyUnicode_AS_UNICODE() returning NULL is
new behavior, and this could conceivably result in crashes.  To be clear:
NULL return values will only happen when allocation of the final "str"
buffer fails during lazy rendering.  This will only happen in
out-of-memory conditions; for right now, while the patch is under early
review, I suspect that's okay.

So far I've come up with four possible ways to resolve this problem, which
I will list here from least-likely to most-likely:

1. Redefine the API such that PyUnicode_AS_UNICODE() is allowed to return
NULL, and fix every place in the Python source tree that calls it to check
for a NULL return.  Document this with strong language for external C
module authors.
2. Change the length to 0 and return a constant empty string.  Suggest
that users of the Unicode API ask for the pointer *first* and the length
*second*.
3. Change the length to 0 and return a previously-allocated buffer of some
hopefully-big-enough-size (4096 bytes? 8192 bytes?), such that even if the
caller iterates over the buffer, odds are good they'll stop before they
hit the end.  Again, suggest that users of the Unicode API ask for the
pointer *first* and the length *second*.
4. The patch is not accepted.

Of course, I'm open to suggestions of other approaches.  (Not to mention
patches!)

Regarding your memory usage and "slice integers" comments, perhaps you'll
be interested in the full lazy patch, which I hope to post later today. 
"Lazy concatenation" is only one of the features of the full patch; the
other is "lazy slices".  For a full description of my "lazy slices"
implementation, see this posting (and the subsequent conversation) to
Python-Dev:
http://mail.python.org/pipermail/python-dev/2006-October/069506.html
And yes, lazy slices suffer from the same
possible-NULL-return-from-PyUnicode_AS_UNICODE() problem that lazy
concatenation does.

As for your final statement, I never claimed that this was a particularly
clean design. I merely claim it makes things faster and is (so far)
self-contained.  For the Unicode versions of my lazy strings patches, the
only files I touched were "Include/unicodeobject.h" and
"Objects/unicodeobject.c".  I freely admit my patch makes those files
*even fussier* to work on than they already are.  But if you don't touch
those files, you won't notice the difference*, and the patch makes some
Python string operations faster without making anything else slower.  At
the very least I suggest the patches are worthy of examination.

* Barring API changes to rectify the possible NULL return from
PyUnicode_AS_UNICODE() problem, that is.

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Comment By: M.-A. Lemburg (lemburg)
Date: 2007-01-10 20:59

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Larry, I probably wasn't clear enough:

PyUnicode_AS_UNICODE() returns a pointer to the underlying Py_UNICODE
buffer. No API using this macro checks for a NULL return value of the
macro since a Unicode object is guaranteed to have a non-NULL Py_UNICODE
buffer. As a result, a memory caused during the concatenation process
cannot be passed back up the call stack. The NULL return value would
result in a plain segfault in the calling API.

Regarding the tradeoff and trying such an approach: I've done such tests
myself (not with Unicode but with 8-bit strings) and it didn't pay off.
The memory consumption outweighs the performance you gain by using the 'x
+= y' approach. The ''.join(list) approach also doesn't really help if
you're after performance (for much the same reasons). 

In mxTextTools I used slice integers pointing into the original parsed
string to work around these problems, which works great and avoids
creating short strings altogether (so you gain speed and memory).

A patch I would find a lot more useful is one to create a Unicode
alternative to cStringIO - for strings, this is by far the most performant
way of creating a larger string from lots of small pieces. To complement
this, a smart slice type might also be an attractive target; one that
breaks up a larger string into slices and provides operations on these,
including joining them to form a new string.

I'm not convinced that murking with the underlying object type and doing
"subtyping" on-the-fly is a clean design.

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Comment By: Larry Hastings (lhastings)
Date: 2007-01-10 20:30

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Much of what I do in Python is text processing.  My largest Python project
to date was an IDL which spewed out loads of text; I've also written an
HTML formatter or two.  I seem to do an awful lot of string concatenation
in Python, and I'd like it to be fast.  I'm not alone in this, as there
have been several patches to Python in recent years to speed up string
concatenation.

Perhaps you aren't familiar with my original justification for the patch. 
I've always hated the "".join() idiom for string concatenation, as it
violates the "There should be one--and preferably only one--obvious way to
do it" principle (and arguably others).  With lazy concatenation, the
obvious way (using +) becomes competitive with "".join(), thus dispensing
with the need for this inobvious and distracting idiom.

For a more thorough dissection of the (original) patch, including its
implementation and lots of discussion from other people, please see the
original thread on c.l.p:
http://groups.google.com/group/comp.lang.python/browse_frm/thread/b8a8f20bc3c81bcf
Please ignore the benchmarks there, as they were quite flawed.

And, no, I haven't seen a lot of code manipulating Unicode strings yet,
but then I'm not a Python shaker-and-mover.  Obviously I expect to see a
whole lot more when Py3k is adopted.

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Comment By: Josiah Carlson (josiahcarlson)
Date: 2007-01-10 18:24

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>From what I understand, the point of the lazy strings patch is to make
certain operations faster.  What operations?  Generally speaking, looped
concatenation (x += y), and other looping operations that have
traditionally been slow; O(n^2).

While this error is still common among new users of Python, generally
users only get bit once.  They ask about it on python-list and are told: z
= []; z.append(y); x = ''.join(z) .

Then again, the only place where I've seen the iterative building up of
*text* is really in document reformatting (like textwrap).  Basically all
other use-cases (that I have seen) generally involve the manipulation of
binary data.  Larry, out of curiosity, have you found code out there that
currently loops and concatenates unicode?

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Comment By: Larry Hastings (lhastings)
Date: 2007-01-09 01:26

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Continuing the comedy of errors, concat patch #2 was actually the same as
#1, it didn't have the fix for detecting a NULL return of PyMem_NEW(). 
Fixed in concat patch #3.  (Deleting concat patch #2.)
File Added: lch.py3k.unicode.lazy.concat.patch.3.txt

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Comment By: Larry Hastings (lhastings)
Date: 2007-01-09 01:10

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Revised the lazy concatenation patch to add (doh!) a check for when
PyMem_NEW() fails in PyUnicode_AsUnicode().
File Added: lch.py3k.unicode.lazy.concat.patch.2.txt

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Comment By: Larry Hastings (lhastings)
Date: 2007-01-08 18:50

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jcarlson:
The first time someone calls PyUnicode_AsUnicode() on a concatenation
object, it renders the string, and that's an O(something) operation.  In
general this rendering is O(i), aka linear time, though linear related to
*what* depends.  (It iterates over the m concatenated strings, and each of
the n characters in those strings, and whether n or m is more important
depends on their values.)  After rendering, the object behaves like any
other Unicode string, including O(1) for array element lookup.

If you're referring to GvR's statement "I mention performance because s[i]
should remain an O(1) operation.", here:
http://mail.python.org/pipermail/python-3000/2006-December/005281.html
I suspect this refers to the UCS-2 vs. UTF-16 debate.

lemberg:
Your criticisms are fair; lazy evaluation is a tradeoff.  In general my
response to theories about how it will affect performance is "I invite you
to try it and see".

As for causing memory errors, the only problem I see is not checking for a
NULL return from PyMem_NEW() in PyUnicode_AsUnicode().  But that's a bug,
not a flaw in my approach, and I'll fix that bug today.  I don't see how
"[my] approach can cause memory errors" in any sort of larger sense.

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Comment By: M.-A. Lemburg (lemburg)
Date: 2007-01-08 10:59

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While I don't think the added complexity in the implementation is worth
it, given that there are other ways of achieving the same kind of
performance (e.g. list of Unicode strings), some comments:

 * you add a long field to every Unicode object - so every single object
in the system pays 4-8 bytes for the small performance advantage

 * Unicode objects are often references using PyUnicode_AS_UNICODE(); this
operation doesn't allow passing back errors, yet your lazy evaluation
approach can cause memory errors - how are you going to deal with them ? 
(currently you don't even test for them)

 * the lazy approach keeps all partial Unicode objects alive until they
finally get concatenated; if you have lots of those (e.g. if you use x +=
y in a loop), then you pay the complete Python object overhead for every
single partial Unicode object in the list of strings - given that most
such operations use short strings, you are likely creating a memory
overhead far greater than the the total length of all the strings

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Comment By: Josiah Carlson (josiahcarlson)
Date: 2007-01-07 05:08

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What are the performance characteristics of each operation?  I presume
that a + b for unicode strings a and b is O(1) time (if I understand your
implementation correctly).  But according to my reading, (a + b + c +
...)[i] is O(number of concatenations performed).  Is this correct?

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