Date: Thu, 18 Jan 2001 22:18:01 -0500
From: A.M. Kuchling firstname.lastname@example.org
Subject: python-dev summary, Jan. 1-15
Python-dev summary, January 1-15, 2001
To comment on material in this python-dev summary, you can
simply post to comp.lang.python / . These summaries are archived
Heading for 2.1alpha1
The pace of checkins has been steadily increasing in preparation
for the release of the first alpha of Python 2.1, still scheduled
for Friday, January 19 (but don't be surprised if it's a few days
The following PEPs had their implementations committed: PEP 208,
"Reworking the Coercion Model"; PEP 230, "Warning Framework"; PEP
232, "Function Attributes". PEP 222, "Web Library Enhancements",
has been abandoned, or at least deferred to some future Python
Speeding up file.readline()
Improving the speed of line-by-line file access generated the
most discussion during this period.
New users wanting to compare Python and Perl often try the
benchmark of writing simple loops in both languages to read all the
lines in the file. Python 2.0 comes out slower in this test, but no
one had ever looked into the cause. As a datapoint, Tim Peters
measured a "while 1: line = file.readline()" loop at 30 seconds on
Windows, while a Perl loop took 6 seconds, for a factor of 7
difference; the numbers are somewhat better on most Unix machines,
but Python is still consistently slower.
This benchmark came up for discussion in comp.lang.python. The
python-dev discussion started with a patch submitted by Jeff Epler
that added an xreadlines() function, analogous to xrange(), so you
could write "for line in file.xreadlines(): ...". "The desire is to
iterate over file contents in a way that satisfies the following
criteria: * Uses the 'for' syntax, because this clearly captures
the underlying operation. (files can be viewed as sequences of
lines when appropriate) * Consumes small amounts of memory even
when the file contents are large. * Has the lowest overhead that
can reasonably be attained."
Many false paths were followed in the resulting lengthy threads.
I don't propose to summarize every single red herring, but will
instead jump right to the conclusions.
The loop inside Python's readline() function is a
straightforward while loop using getc(). In multithreaded
environments, getc() has to lock the file object, and this adds
additional overhead. On Windows the overhead is startling: "It
looks like we're paying (on Win98SE) approximately:
Perl is fast because it accesses the internals of C's FILE objects,
accessing the stream's buffers directly. The problem is that this
is nonportable and, as Tim Peters discovered, complicated: "20
years ago I may have thought this was fun. I thought debugging
large systems of m4 macros was fun then, and I'm not sure this is
either better or worse than that -- well, it's worse, because I
understood m4's implementation."
Windows has very high locking overhead for some reason, but this
overhead is still present on Unix platforms. The Single Unix
Specification includes a getc_unlocked() function that does no
locking and saves this overhead; the flockfile() and funlockfile()
functions can be used to lock the stream outside of an inner loop.
Most Unix platforms seem to gain an improvement of a factor of 4 or
so, but Mark Favas reported an extreme value: Tru64, the simple
while loop went from 322 seconds to 10 seconds, a factor of 32.
Windows doesn't have a getc_unlocked(), so a different solution
needs to be used there. Tim came up with a scheme using fgets();
fgets on its own isn't suitable for reading lines, because there's
no way to detect embedded nulls, but you can fill a buffer with
non-null bytes, read a line into the buffer, and then search from
the left for a newline. "Surprise? Despite all the memsets, memchrs
(looking for a newline), and one-at-a-time backward searches
(looking for a null byte), it's a huge win on Windows." The simple
while loop went from 30 seconds to 13 seconds.
Some improvements were also made to the fileinput module, and
Jeff Epler's xreadlines module was added, with the right glue to
add an .xreadlines() method to file objects that automatically
imports the module. 'for line in file.xreadlines()' usually takes
about half the time of the simple while loop.
Therefore, Python 2.1 will have significantly faster file I/O
for the common task of processing a file line-by-line.
Try it out! The more favorable reaction to pydoc and the more
testing it gets, the better the chance of it slipping into 2.1.
Exporting names from a module
Patch #102808 adds a mechanism to export names from a module.
GvR's description: "... if there's a variable __exports__ in the
module, it is a list of identifiers, and any access from outside
the module to names not in the list is disallowed. This affects
access using the getattr and setattr protocols (which raise
AttributeError for disallowed names), as well as "from M import v"
(which raises ImportError)."
Reactions were mixed. Some people liked the idea, and thought it
would make some additional optimizations possible. To quote Neil
Schemenauer, "It should allow some attribute access inside of
modules to become faster (like LOAD_FAST for locals). I think that
optimization could be implemented without too much difficulty."
Other people thought that applying it only to modules would cause
people to assume that it would also work for classes, but the patch
didn't implement that.
The final decision was to reject the patch. However, one part of
the patch was added; 'from Module import *' will now look for a
variable named __all__ containing a list of strings, and will only
import the symbols listed in __all__.
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