[FAQTS] Python Knowledge Base Update -- May 30th, 2000

Fiona Czuczman fiona at sitegnome.com
Tue May 30 14:02:40 CEST 2000

Hi Guys,

Below are the entries I've entered into http://python.faqts.com tonight.

cheers, Fiona Czuczman

## New Entries #################################################

How can I convert an integer to a string containing the bytes for that value?
Fiona Czuczman
David Goodger

What you want is the "struct" module:

    >>> import struct
    >>> struct.pack("h",300)
    >>> a = 300
    >>> b = struct.pack("h", a)
    >>> for c in b:
    ...     print ord(c)

See the Library Reference for details. Please note that the conversion
string limits the size of the integer (here, "h" is a short integer, two
bytes on my platform). If your value is greater than the converter 
allows for, you'll get only the least significant bytes:

    >>> struct.pack("h", 1000000)
    >>> struct.unpack("h", "B@")
    >>> struct.pack("i", 1000000)
    >>> struct.unpack("i", "\000\017B@")

"i" is for a regular integer, four bytes on my platform.

How can I present output from md5.digest like md5sum from GNU textutils?
Fiona Czuczman
Moshe Zadka


for c in digest:
        sys.stdout.write("%02X" % ord(c))

Can I create a rpm with only pyc files or do I need the py file also to make package distrubution?
Fiona Czuczman
Martin von Loewis

No, just the bytecode will do.

Can I take pyc file to different computer and will it run?
Fiona Czuczman
Martin von Loewis

Sure. The Python byte code format is the same on all machines.

I update a class with 'reload (module)'. How can I update the instances too?
Fiona Czuczman
Martin von Loewis

You'd have to find the instance one-by-one. If you manage to get hold
of them, you can set their __class__ attribute to the new class:

>>> class A:
...   def doit(self):print "old"
>>> a=A()
>>> class A:
...   def doit(self):print "new"
>>> a.doit()
>>> a.__class__=A
>>> a.doit()

Is the match() function any different really then having search() with a ^ at the begenning of the string?
Fiona Czuczman
Tim Peters

Yes, although the difference doesn't become clear until you exploit the
optional slicing arguments of compiled regexps:

>>> import re
>>> p = re.compile("^a")
>>> p.search("cba", 2)
>>> p = re.compile("a")
>>> p.match("cba", 2)
<re.MatchObject instance at ead590>

That is, while there is an "a" at index 2 of "cba", and "match" finds 
it, a caret does *not* match starting at index 2 (caret means 
start-of-string, and may or may not mean more than that depending on 
"multiline" mode -- using "match" instead cuts all that magical crap out 
of the equation).

Where can I find a Python script to download an entire web site?
Fiona Czuczman
Andrew Dalke, Simon

Try websucker, which is distributed with Python under Tools as part
of webchecker.  It isn't in Parnassus because it's part of Python

websucker.py, while an awesome utility, doesn't actually suck the entire
site.  For instance, it will pull the image files on a page if those
images are statically SRC-ed.  If there is an onmouseover directive, for
instance, which changes the image source to another file, it will not
grab the other file, even if it lies in the same directory as the first

Is there a standard way to read-write the e-mail with python?
Fiona Czuczman
Oleg Broytmann

Write with smtplib.py.

Read with mimetools.py

Download and read docs for extract_mime - it is a template for reading
e-mails from MTA (Mail Transfer Agent).

Master site:


Faster mirrors: 


## Edited Entries ##############################################

How can I embed/port Python to Vxworks?
Fiona Czuczman
Richard Wolff

What follows are some of the details.  This same information and the 
rest of the story can be found at

As usual, no warranty for fitness nor merchantability nor freedom from
un-wanted side-effects of creeping featurism.


Here's how I set up Python as an embedded task in VxWorks.  Note that
building the system for VxWorks is not going to be as nicely set up as
doing so for most systems.  We can't run configure for one thing.  So
there are a number of changes that you have to consider/make by hand.
Before getting to the details, a thank you to Paul Fernhout for some
helpful advice in a recent comp.lang.python posting.

Perhaps the best way to get started is to build Python on your 
workstation and use that as the start of the port.  This way, many of 
the files you need will be present.  For instance, the parser generator 
program will be built and run, so it isn't needed for the cross-compile. 
 You'll also have  config.h  file that might well serve as a useful 
starting point.

The files from the distribution that you need to modify are in Include,
Modules, Objects, Parser, and Python.  At the top level, you'll need to
change Makefile and config.h.  The diff file (pydiff) will make a lot
of changes for you, but you should probably take a look at what's going
on first.  There may well be better ways of doing things.  By the way,
the diff is against Python 1.5.2 and is for VxWorks 5.2 (pre-Tornado).

You'll need (or want) to do the following:
1) Edit the top level Makefile to reflect the correct definitions for
CC, RANLIB, LD, and AR.  Make the same edits in the Makefiles in
Objects, Parser, and Python directories.  Make the same edits in
Makefile.pre in Modules.

2) Edit config.h

3) Get a version of getopt.c and put it in the top level directory, 
along with a suitable getopt.h, which can go in the Include directory.  
Or put getopt.c somewhere else and modify Makefiles suitably.

4) Edit Setup (and Setup.local, Setup.thread).

5) If you include modules that I didn't, you'll probably want to edit
that module's ".c" file.  If you have a different version of VxWorks,
look for the VXWORKS define and modify as needed.

6) Modify Modules/python.c as you will.  You probably will want to
modify the arg0 value.

7) Remove the buildno file if you want.

8) In Modules, run makesetup to build the Makefile and config.c.  If you
have used both Setup and Setup.local, run   makesetup Setup Setup.local

9) At the top level, run make.  The output will be "python.o" in this

10) Load that into your VxWorks system.  On my system, Python took too
much stack space to run under the shell, so I spawn the task, which is
what "sppy" ("spawn python") does.  If Python comes alive, but you get
lots of xxx:yyy: No such file or directory    and  xxx:yyy: File exists,
don't worry.  Those messages are from 'stat' as it probes the file
system on xxx looking for library files.  I don't know how to stop
this; it's both annoying and harmless.

10a) You should 'cd "python-top-level-directory"' before you load the
python.o file, or you should be sure that you have the Lib directory
in a standard place (or you have modified the Makefiles to reflect
suitable prefix and exec-prefix directories).  Or you might set
the PYTHONHOME environment variable in Modules/python.c before Python
goes through its initialization steps.

11) When the >>> prompt appears, it's likely that a carriage return will
bring back the shell prompt.  That's because the shell runs at much
higher priority than Python and it grabs all the characters.  It needs
to be demoted, which is what the "demoteShell" code in Modules/python.c
does (with a vengeance...there's probably no need to suspend the shell).
So run that routine from the shell, and then run Python as you will.

12) To exit, the usual command sequence (or sys.exit) will get you
back to the shell prompt, at which time, you should raise the shell
priority back to 1 (use "restoreShell").

13) You can make things more interesting by having Python code that
will allow you to run VxWorks routines.  You need a slight extension of
Python, to be found in my "Modules/vxmodule.c".  Modify Setup (or
Setup.local, rerun makesetup, and then the top level make).

14) A simple VxWorks class makes use of the vxmodule code.  See
VxWorks.py Thus, i = VxWorks.Vxw("i","") builds an 'i' such that 'i()'
gives the same task information table from the Python prompt as it does
from the VxWorks shell.  Or,  routeAdd = VxWorks.Vxw("routeAdd", "ss")
builds a routeAdd function that can be used to add a route to the
VxWorks routing table  (routeAdd(", "gateway")), and so forth.

15) If you have the VxWorks module in place, you can then define
        rs = VxWorks.Vxw("restoreShell","")
and when you run rs(), you have, for interactive use, effectively
suspended Python, as the shell will get all the input characters.
"demoteShell" and 'rs' thus allows you to switch back and forth between
Python and the VxWorks shell.

Richard Wolff
May, 2000

P.S.  I did this work because I want to run various routines in the
VxWorks environment under the control of a "scripting" language.  There
are three approaches here...embed Python in VxWorks, which is what this
note is about, connect Python running on a workstation to the VxWorks
shell via telnet, or connect Python on the workstation to a process on
the VxWorks box via a socket connection.  Which way to go is nice
question.  The other question one ought to answer is, "why aren't you
using rt-linux?".

P.P.S.  All this code is released under the same terms and conditions
as the Python license.

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