How to use a timer in Python?

[Nico Grubert]

Hi there,

on a Linux machine running Python 2.3.5. I want to create a file
'newfile' in a directory '/tmp' only if there is no file 'transfer.lock'
in '/temp'.
A cronjob creates a file 'transfer.lock' in '/temp' directory every 15
minutes while the cronjob is doing something. This job takes around 30
seconds. During these 30 seconds the 'transfer.lock' file is present in
the '/temp' directory and I must not create 'newfile'. After the cronjob
has been finished, the 'transfer.lock' file is deleted from '/temp' and
I can create 'newfile'.
How can I use a timer that waits e.g. 10 seconds if 'transfer.lock' is
present and then checks again if 'transfer.lock' is still there?

I want to do something like this:

import os
if 'transfer.lock' in os.listdir('/temp'):
# ...wait 10 seconds and then check again if
# 'transfer.lock' is in os.listdir('/temp')
else:
# create 'newfile'
Nico

[Sybren Stuvel]

Nico Grubert enlightened us with:

How can I use a timer that waits e.g. 10 seconds if 'transfer.lock'
is present and then checks again if 'transfer.lock' is still there?

Make a while loop in which you sleep. Or use the threading module if
you want to go multi-threading.

Sybren
--
The problem with the world is stupidity. Not saying there should be a
capital punishment for stupidity, but why don't we just take the
safety labels off of everything and let the problem solve itself?
Frank Zappa

[Wolfram Kraus]

Nico Grubert wrote:

Hi there,

on a Linux machine running Python 2.3.5. I want to create a file
'newfile' in a directory '/tmp' only if there is no file 'transfer.lock'
in '/temp'.
A cronjob creates a file 'transfer.lock' in '/temp' directory every 15
minutes while the cronjob is doing something. This job takes around 30
seconds. During these 30 seconds the 'transfer.lock' file is present in
the '/temp' directory and I must not create 'newfile'. After the cronjob
has been finished, the 'transfer.lock' file is deleted from '/temp' and
I can create 'newfile'.
How can I use a timer that waits e.g. 10 seconds if 'transfer.lock' is
present and then checks again if 'transfer.lock' is still there?

I want to do something like this:

import os
if 'transfer.lock' in os.listdir('/temp'):
# ...wait 10 seconds and then check again if
# 'transfer.lock' is in os.listdir('/temp')
else:
# create 'newfile'
Nico

import time
time.sleep(10)

For more information: help(time.sleep) ;-)

HTH,
Wolfram

[Nick Craig-Wood]

Nico Grubert <ni*********@gmail.com> wrote:

on a Linux machine running Python 2.3.5. I want to create a file
'newfile' in a directory '/tmp' only if there is no file 'transfer.lock'
in '/temp'.
A cronjob creates a file 'transfer.lock' in '/temp' directory every 15
minutes while the cronjob is doing something. This job takes around 30
seconds. During these 30 seconds the 'transfer.lock' file is present in
the '/temp' directory and I must not create 'newfile'. After the cronjob
has been finished, the 'transfer.lock' file is deleted from '/temp' and
I can create 'newfile'.

That all sounds very race-y to me! The cron-job and the other process
need to take the same lock, otherwise the cron-job will start 1ms
after the other process checks for transfer.lock and before it has a
chance to create newfile and there will be trouble.

Using files as locks isn't brilliant because the operations "read to
see if the lock is there" and "create the file isn't" aren't atomic.
Ie someone can get in there after you read the directory but before
you create the file.

However creating a directory is atomic, so you can take the lock by
os.mkdir("/tmp/lock"). If that succeeded you got the lock, if it
failed (threw OSError) then you didn't. If it failed then just
time.sleep(1) and try again. This kind of locking works cross
platform too. You can use it in shell too, eg "mkdir /tmp/lock ||
exit 1" in your cronjob.

You could wrap the locking up into a module of course, and I bet
someone already did.

--
Nick Craig-Wood <ni**@craig-wood.com> -- http://www.craig-wood.com/nick