Ian Wienand: Python and --prefix
Something interesting I discovered about Python and
--prefix that I can't see a lot of documentation on...
When you build Python you can use the standard --prefix
flag to configure to home the installation as you require.
You might expect that this would hard-code the location to look for
the support libraries to the value you gave; however in reality it
doesn't quite work like that.
Python will only look in the directory specified by prefix
after it first searches relative to the path of the executing
binary. Specifically, it looks at argv[0] and works through
a few steps is argv[0] a symlink? then dereference
it. Does argv[0] have any slashes in it? if not, then
search the $PATH for the binary. After this, it starts
searching for dirname(argv[0])/lib/pythonX.Y/os.py, then
dirname(argv[0])/../lib and so on, until it reaches the root.
Only after these searches fail does the interpreter then fall back to
the hard-coded path specified in the --prefix when
configured.
What is the practical implications? It means you can move around a
python installation tree and have it all "just work", which is nice.
In my situation, I noticed this because we have a completely
self-encapsulated build toolchain, but we wish to ship the same
interpreter on the thing that we're building (during the build, we run
the interpreter to create .pyc files for distribution, and we
need to be sure that when we did this we didn't accidentally pick up
any of the build hosts python; only the toolchain python).
The PYTHONHOME environment variable does override this
behaviour; if it is set then the search stops there. Another
interesting thing is that sys.prefix is therefore not
the value passed in by --prefix during configure, but the
value of the current dynamically determined prefix value.
If you run an strace, you can see this in operation.
Firstly it dereferences symlinks. Then it looks for
Modules/Setup to see if it is running out of the build tree.
Then it starts looking for os.py, walking its way upwards.
One interesting thing that may either be a bug or a feature, I haven't
decided, is that if you set the prefix to / then the
interpreter will not go back to the root and then look in
/lib. This is probably pretty obscure usage though!
All this is implemented in Modules/getpath.c
which has a nice big comment at the top explaining the rules in
detail.
readlink("/usr/bin/python", "python2.7", 4096) = 9
readlink("/usr/bin/python2.7", 0xbf8b014c, 4096) = -1 EINVAL (Invalid argument)
stat64("/usr/bin/Modules/Setup", 0xbf8af0a0) = -1 ENOENT (No such file or directory)
stat64("/usr/bin/lib/python2.7/os.py", 0xbf8af090) = -1 ENOENT (No such file or directory)
stat64("/usr/bin/lib/python2.7/os.pyc", 0xbf8af090) = -1 ENOENT (No such file or directory)
stat64("/usr/lib/python2.7/os.py", st_mode=S_IFREG 0644, st_size=26300, ... ) = 0
stat64("/usr/bin/Modules/Setup", 0xbf8af0a0) = -1 ENOENT (No such file or directory)
stat64("/usr/bin/lib/python2.7/lib-dynload", 0xbf8af0a0) = -1 ENOENT (No such file or directory)
stat64("/usr/lib/python2.7/lib-dynload", st_mode=S_IFDIR 0755, st_size=4096, ... ) = 0
Part of our 
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The following announce is lazily copied from 
As I 
I've just finally finished reading Simon Reynold's dense "Rip it Up and
Start Again": a history of post-punk music from 1978-1984.
I often think of music journalism in the terms that Zappa famously phrased as
(roughly) "people who can't write, writing for people who can't read". This
book sometimes suffers from that syndrome, of attempting to describe audio in
terms of other senses, which sometimes wanders deep into synesthesia territory,
but for the most part the book is coherent and enjoyable.
It's also very "dense": you leave each page with another half-dozen songs or
bands of music genres to go and investigate. I'm surprised that there isn't a
commercially-released anthology counterpart to the book. There are, however,
several playlists, many of which are locked away on Spotify, but via 
Yesterday we did held 
Quick howto for everyone having Asus UX31e aka Zenbook and wants the most stable vanilla kernel with long-time battery life and working touchpad made by Sentelic.
Wow, a lot happens in 3 months. I'm sure I've missed something...
I think the biggest achievement would be toilet training. Zoe's been using
the toilet for some time now, but still wearing diapers, and just after
Christmas (when she was sick and vomited on the carpet) we figured we had
nothing to lose, so we ditched the diapers. I can count on the fingers of
one hand the number of accidents she's had since then. I'm really happy to
have this out of the way so early. Using a toilet away from home is still
more of a challenge, but she seems to have very good bladder control, and
fairly predictable bowel movements.
Speaking of sick, she was the sickest she's ever been around Christmas time.
It coincided with her top canines finally coming through. She had a cold,
which progressed into an ear infection, and she had this dreadful persistent
dry cough, which then progressed into a dreadful productive wet cough. You
knew she was really sick because she was extremely cuddly and lethargic.
Since around that time, her sleep has gone completely to hell. We've had
some really bad nights where she'll wake up after a couple of hours of going
down for bed, or for a while, she was reliably waking up at 4am and taking a
good couple of hours to settle back down. Lately she's been sleeping through
again, but waking up at 6am instead of the usual 7-7:30am. We bought a clock
that changes colour at programmable times, so we're trying to train her that
if she wakes up and the clock isn't green yet, she should try and go back to
sleep. Not a huge amount of success there yet.
Christmas itself was good, but fairly quiet because Zoe was sick.
Sarah managed to get a 
I am currently working on a 
My 