I need to ensure that there is only one instance of my python class on
my machine at a given time. (Not within an interpreter -- that would
just be a singleton -- but on the machine.) These instances are
created and destroyed, but there can be only one at a time.
So when my class is instantiated, I create a little lock file, and I
have a __del__ method that deletes the lock file. Unfortunately, there
seem to be some circumstances where my lock file is not getting
deleted. Then all the jobs that need that "special" class start
queueing up requests, and I get phone calls in the middle of the night.
Is there a better pattern to follow than using a __del__ method? I
just need to be absolutely, positively sure of two things:
1) There is only one instance of my special class on the machine at a
time.
2) If my special class is destroyed for any reason, I need to be able
to create another instance of the class. 14 1267
> So when my class is instantiated, I create a little lock file, and I have a __del__ method that deletes the lock file. Unfortunately, there seem to be some circumstances where my lock file is not getting deleted.
Maybe the interpreter died by the signal.. in that case the __del__
is not called.
You can try 'flock', instead of lock files.
import fcntl
class Test1(object):
def __init__(self):
self.lock=open('/var/tmp/test1', 'w')
fcntl.flock(self.lock.fileno(), fcntl.LOCK_EX)
print 'Lock aquired!'
def __del__(self):
fcntl.flock(self.lock.fileno(), fcntl.LOCK_UN)
self.lock.close()
In this case, if interpreter dies, the lock is released by OS.
If you try to create another instance in the same interpreter
or another, the call will block in __init__. You can change it to
raise an exception instead.
BranoZ
"Chris Curvey" <cc*****@gmail.com> writes: I need to ensure that there is only one instance of my python class on my machine at a given time.
I recommend modifying your requirements such that you ensure that
there is only one "active" instance of your class at any one time (or
something like that), and then use try:finally: blocks to ensure your
locks get removed.
Is there a better pattern to follow than using a __del__ method? I just need to be absolutely, positively sure of two things:
1) There is only one instance of my special class on the machine at a time. 2) If my special class is destroyed for any reason, I need to be able to create another instance of the class.
As another poster mentioned, you also need to work out what you're
going to do if your process gets killed in a way that doesn't allow
finally blocks to run (this doesn't have much to do with Python).
Cheers,
mwh
--
The above comment may be extremely inflamatory. For your
protection, it has been rot13'd twice.
-- the signature of "JWhitlock" on slashdot
On Mon, 15 Aug 2005, Chris Curvey wrote: Is there a better pattern to follow than using a __del__ method? I just need to be absolutely, positively sure of two things:
An old hack i've seen before is to create a server socket - ie, make a
socket and bind it to a port:
import socket
class SpecialClass:
def __init__(self):
self.sock = socket.socket()
self.sock.bind(("", 4242))
def __del__(self):
self.sock.close()
Something like that, anyway.
Only one socket can be bound to a given port at any time, so the second
instance of SpecialClass will get an exception from the bind call, and
will be stillborn. This is a bit of a crufty hack, though - you end up
with an open port on your machine for no good reason. If you're running on
unix, you could try using a unix-domain socket instead; i'm not sure what
the binding semantics of those are, though.
I think Brano's suggestion of using flock is a better solution.
tom
--
Gin makes a man mean; let's booze up and riot!
Tom Anderson wrote: Only one socket can be bound to a given port at any time, so the second instance of SpecialClass will get an exception from the bind call, and will be stillborn. This is a bit of a crufty hack, though - you end up with an open port on your machine for no good reason. If
If you bind with self.sock.bind(('localhost', 4242)) instead, at least
you don't have much of a security risk since the port won't be available
for connections from outside the same machine. Using '' instead of
'localhost' means bind to *all* interfaces, not just the loopback one.
-Peter
On Mon, 15 Aug 2005, Peter Hansen wrote: Tom Anderson wrote:
Only one socket can be bound to a given port at any time, so the second instance of SpecialClass will get an exception from the bind call, and will be stillborn. This is a bit of a crufty hack, though - you end up with an open port on your machine for no good reason. If If you bind with self.sock.bind(('localhost', 4242)) instead, at least you don't have much of a security risk since the port won't be available for connections from outside the same machine.
Excellent suggestion, thanks!
Using '' instead of 'localhost' means bind to *all* interfaces, not just the loopback one.
Doesn't '' mean 'bind to the *default* interface'?
tom
--
All we need now is a little energon and a lotta luck
Tom Anderson wrote: On Mon, 15 Aug 2005, Peter Hansen wrote: Using '' instead of 'localhost' means bind to *all* interfaces, not just the loopback one.
Doesn't '' mean 'bind to the *default* interface'?
What does "default" mean, and is that definition in conflict with what I
said?
The docs say it means INADDR_ANY. They don't say what that means, so
you'd have to read up on the C socket calls to learn more.
Or some helpful soul will clarify for the class... :-)
-Peter
Tom Anderson wrote: On Mon, 15 Aug 2005, Chris Curvey wrote:
Is there a better pattern to follow than using a __del__ method? I just need to be absolutely, positively sure of two things:
An old hack i've seen before is to create a server socket - ie, make a socket and bind it to a port:
import socket
class SpecialClass: def __init__(self): self.sock = socket.socket() self.sock.bind(("", 4242)) def __del__(self): self.sock.close()
Something like that, anyway.
Only one socket can be bound to a given port at any time, so the second instance of SpecialClass will get an exception from the bind call, and will be stillborn. This is a bit of a crufty hack, though - you end up with an open port on your machine for no good reason.
Much worse, it's a bug. That pattern is for programs that need to
respond at a well-known port. In this case it doesn't work; the
fact that *someone* has a certain socket open does not mean that
this particular program is running.
--
--Bryan
Chris Curvey wrote: I need to ensure that there is only one instance of my python class on my machine at a given time. (Not within an interpreter -- that would just be a singleton -- but on the machine.) These instances are created and destroyed, but there can be only one at a time.
So when my class is instantiated, I create a little lock file, and I have a __del__ method that deletes the lock file. Unfortunately, there seem to be some circumstances where my lock file is not getting deleted. Then all the jobs that need that "special" class start queueing up requests, and I get phone calls in the middle of the night.
For a reasonably portable solution, leave the lock file open.
On most systems, you cannot delete an open file, and if the
program terminates, normally or abnormally, the file will be
closed.
When the program starts, it looks for the lock file, and if
it's there, tries to delete it; if the delete fails, another
instance is probably running. It then tries to create the
lock file, leaving it open; if the create fails, you probably
lost a race with another instance. When exiting cleanly, the
program closes the file and deletes it.
If the program crashes without cleaning up, the file will still
be there, but a new instance can delete it, assuming
permissions are right.
There are neater solutions that are Unix-only or Windows-only.
See BranzoZ's post for a Unix method.
--
--Bryan br***********************@yahoo.com wrote: For a reasonably portable solution, leave the lock file open. On most systems, you cannot delete an open file,..
On most UNIXes, you can delete an open file.
Even flock-ed. This is BTW also an hack around flock.
1. Process A opens file /var/tmp/test1, and flocks descriptor.
2. Process H unlinks /var/tmp/test1
3. Process B opens file /var/tmp/test1, and flocks _another_
descriptor
4. Processes A and B are running simultaneously
Do you need protection agains H ?
Use file that is writeable by A and B in a directory that is
writeable only by root.
BranoZ br***********************@yahoo.com writes: Chris Curvey wrote: I need to ensure that there is only one instance of my python class on my machine at a given time. (Not within an interpreter -- that would just be a singleton -- but on the machine.) These instances are created and destroyed, but there can be only one at a time.
So when my class is instantiated, I create a little lock file, and I have a __del__ method that deletes the lock file. Unfortunately, there seem to be some circumstances where my lock file is not getting deleted. Then all the jobs that need that "special" class start queueing up requests, and I get phone calls in the middle of the night.
For a reasonably portable solution, leave the lock file open. On most systems, you cannot delete an open file,
Uh, you can on unix -- what else did you have in mind for "most
systems"?
Cheers,
mwh
--
Well, yes. I don't think I'd put something like "penchant for anal
play" and "able to wield a buttplug" in a CV unless it was relevant
to the gig being applied for... -- Matt McLeod, asr
> So when my class is instantiated, I create a little lock file, and I have a __del__ method that deletes the lock file.
Is there a better pattern to follow than using a __del__ method? I just need to be absolutely, positively sure of two things:
1) There is only one instance of my special class on the machine at a time. 2) If my special class is destroyed for any reason, I need to be able to create another instance of the class.
Just some ideas
1) You could open a socket listening to a port
* Not sure what happens if the interpreter dies
* There cold be conflicts with other programs
2) Update the lockfile every xxx. If the lockfile
is older than e.g. 2*xxx disregard it.
3) Write the process id into the lockfile and check
if a process with this id is alive.
* I don't know if / how this can be done in python
Leonhard
BranoZ wrote: br***********************@yahoo.com wrote:
For a reasonably portable solution, leave the lock file open. On most systems, you cannot delete an open file,.. On most UNIXes, you can delete an open file. Even flock-ed. This is BTW also an hack around flock.
Yes, sorry; my bad.
Use file that is writeable by A and B in a directory that is writeable only by root.
Is that portable? What's the sequence the program should try?
--
--Bryan
Bryan Olson wrote: > Use file that is writeable by A and B in a directory that is > writeable only by root. Is that portable?
I have the feeling that you are asking if it works on Windows.
No idea! I have only user experience with Windows.
On UNIX it is as portable as 'flock', which means all modern
Unices (be careful about NFS).
What's the sequence the program should try?
1.
open a file, which name was previously agreed on
(like /var/tmp/<prog-name>-<user-name>)
If it fails, report error and exit. System error or
somebody has created unaccessible file by the same name.
2.
Try to aquire a flock on the descriptor from step 1.
If it fails, some running process already has the lock, exit
3.
lock will be released and lockfile closed automaticaly by OS
on process exit.
import sys, fcntl
try:
lockfile=open('/var/tmp/test1', 'w')
fcntl.flock(lockfile.fileno(),
fcntl.LOCK_EX | fcntl.LOCK_NB)
except IOError:
print sys.exc_info()[1]
sys.exit(-1)
You can flock any open file, no matter if it is read/write/append.
BranoZ
On Tue, 16 Aug 2005 08:03:58 -0400, Peter Hansen <pe***@engcorp.com> wrote: Tom Anderson wrote: On Mon, 15 Aug 2005, Peter Hansen wrote: Using '' instead of 'localhost' means bind to *all* interfaces, not just the loopback one.
Doesn't '' mean 'bind to the *default* interface'?
What does "default" mean, and is that definition in conflict with what I said?
The docs say it means INADDR_ANY. They don't say what that means, so you'd have to read up on the C socket calls to learn more.
Or some helpful soul will clarify for the class... :-)
INADDR_ANY means "every network interface you can find". This includes the
local loopback and all physical and logical network interfaces.
/Jorgen
--
// Jorgen Grahn <jgrahn@ Ph'nglui mglw'nafh Cthulhu
\X/ algonet.se> R'lyeh wgah'nagl fhtagn! This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
by: seth |
last post by:
Last week I encountered an AttributeError in my unit tests that I
wasn'table to catch with an "except AttributeError" statement.
The problem stemmed from a class that raised an error inside...
|
by: Kepes Krisztian |
last post by:
Hi !
I very wonder, when I get exp. in java with GC.
I'm Delphi programmer, so I get used to destructorin objects.
In Java the final method is not same, but is like to destructor (I has...
|
by: Peter Abel |
last post by:
I have an application, which is an instance of a class
with a deeply nested object hierarchy. Among others one
method will be executed as a thread, which can be stopped.
Everything works fine...
|
by: Emmanuel |
last post by:
Hi,
I run across this problem, and couldn't find any solution (python 2.2.2)
:
Code :
===========
from __future__ import generators
>>> class titi:
|
by: schwerdy |
last post by:
Hello developers!
I'm using Python 2.3.4 under debian Sarge and want to write a small
logger class. My source code reads:
#***************************************************
import sys, time...
|
by: Mike C. Fletcher |
last post by:
I'm looking at rewriting parts of Twisted and TwistedSNMP to eliminate
__del__ methods (and the memory leaks they create). Looking at the docs
for 2.3's weakref.ref, there's no mention of whether...
|
by: Baoqiu Cui |
last post by:
Today I was playing with a small Python program using Python 2.4
on Cygwin (up-to-date version, on Windows XP), but ran into a
strange error on the following small program (named bug.py):
...
|
by: Erwan Adam |
last post by:
Hello all,
Can someone reproduce this bug ... I use :
python
Python 2.4.3 (#2, Sep 18 2006, 21:07:35)
on linux2
Type "help", "copyright", "credits" or "license" for more information.
...
|
by: George Sakkis |
last post by:
I'm baffled with a situation that involves:
1) an instance of some class that defines __del__,
2) a thread which is created, started and referenced by that instance,
and
3) a weakref proxy to the...
|
by: ryjfgjl |
last post by:
In our work, we often receive Excel tables with data in the same format. If we want to analyze these data, it can be difficult to analyze them because the data is spread across multiple Excel files...
|
by: BarryA |
last post by:
What are the essential steps and strategies outlined in the Data Structures and Algorithms (DSA) roadmap for aspiring data scientists? How can individuals effectively utilize this roadmap to progress...
|
by: nemocccc |
last post by:
hello, everyone, I want to develop a software for my android phone for daily needs, any suggestions?
|
by: Hystou |
last post by:
There are some requirements for setting up RAID:
1. The motherboard and BIOS support RAID configuration.
2. The motherboard has 2 or more available SATA protocol SSD/HDD slots (including MSATA, M.2...
|
by: marktang |
last post by:
ONU (Optical Network Unit) is one of the key components for providing high-speed Internet services. Its primary function is to act as an endpoint device located at the user's premises. However,...
|
by: Hystou |
last post by:
Most computers default to English, but sometimes we require a different language, especially when relocating. Forgot to request a specific language before your computer shipped? No problem! You can...
|
by: Oralloy |
last post by:
Hello folks,
I am unable to find appropriate documentation on the type promotion of bit-fields when using the generalised comparison operator "<=>".
The problem is that using the GNU compilers,...
|
by: Hystou |
last post by:
Overview:
Windows 11 and 10 have less user interface control over operating system update behaviour than previous versions of Windows. In Windows 11 and 10, there is no way to turn off the Windows...
|
by: tracyyun |
last post by:
Dear forum friends,
With the development of smart home technology, a variety of wireless communication protocols have appeared on the market, such as Zigbee, Z-Wave, Wi-Fi, Bluetooth, etc. Each...
| |