__del__ pattern?

[Chris Curvey]

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.

[BranoZ]

> 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(sel f.lock.fileno() , fcntl.LOCK_EX)
print 'Lock aquired!'

def __del__(self):
fcntl.flock(sel f.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

[Michael Hudson]

"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

[Tom Anderson]

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!

[Peter Hansen]

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

[Tom Anderson]

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

[Peter Hansen]

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

[bryanjugglercryptographer]

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

[bryanjugglercryptographer]

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

[BranoZ]

br************* **********@yaho o.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