[LWN Logo]
Date: Thu, 18 Jan 2001 22:18:01 -0500
From: "A.M. Kuchling" <akuchlin@mems-exchange.org>
To: lwn@lwn.net, editors@linuxtoday.com
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 / <python-list@python.org>.  These summaries are
archived at <URL:http://www.amk.ca/python/dev/>.

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 late).

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 release.

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

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)

   17 seconds for compiling with _MT (threadsafe libc)
    6 seconds to do the work <wink>
    5 seconds for "other stuff", best guess mostly a poor
          platform malloc/realloc
    2 seconds for not optimizing the loop
   30 total"

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

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.


Ka-Ping Yee announced pydoc, a tool for browsing the python
documentation: "At the shell prompt, 'pydoc <name>' displays
documentation on <name>, very much like 'man'."  ?!ng posted a sample
transcript of pydoc in action:

It's been checked into the sandbox area of the CVS tree:

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

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__.  

Other things

Christian Tismer released a set of Stackless Python patches for 2.0.
Note that a bugfix release was made on January 15th.

Michel Pelletier has written a draft PEP for adding interfaces to
Python.  No PEP number has been allocated yet.

Related Links

Python-dev archives:

Python project page on SourceForge:

Python Enhancement Proposals (PEPs):