How to get a raised exception from other thread

[dcrespo]

Hi all,

How can I get a raised exception from other thread that is in an
imported module?

For example:

---------------
programA.py
---------------

import programB

thread = programB.MakeTh read()
thread.start()

---------------
programB.py
---------------
import threading, time

class SomeException(E xception):
pass

class MakeThread(thre ading.Thread):
def __init__(self):
threading.Threa d.__init__(self )

def run(self):
i = 0
while 1:
print i
i += 1
time.sleep(1) #wait a second to continue
if i>10:
raise SomeException()


Thanks

Daniel

[Jeremy Moles]

Re: How to get a raised exception from other thread

On non-Windows system there are a ton of ways to do it--this is almost a
whole field unto itself. :) (D-BUS, fifos, sockets, shmfs, etc.) In
Windows, I wouldn't have a clue.

I guess this is a hard question to answer without a bit more
information. :)

On Fri, 2005-10-14 at 14:45 -0700, dcrespo wrote:[color=blue]
> Hi all,
>
> How can I get a raised exception from other thread that is in an
> imported module?
>
> For example:
>
> ---------------
> programA.py
> ---------------
>
> import programB
>
> thread = programB.MakeTh read()
> thread.start()
>
> ---------------
> programB.py
> ---------------
> import threading, time
>
> class SomeException(E xception):
> pass
>
> class MakeThread(thre ading.Thread):
> def __init__(self):
> threading.Threa d.__init__(self )
>
> def run(self):
> i = 0
> while 1:
> print i
> i += 1
> time.sleep(1) #wait a second to continue
> if i>10:
> raise SomeException()
>
>
> Thanks
>
> Daniel
>[/color]

[dcrespo]

Re: How to get a raised exception from other thread

Thanks for your answer, but I think that created thread in python
should create a thread either on windows and linux.

Can you give me Python example of how to do what I want to do? Thanks

[Peter Hansen]

Re: How to get a raised exception from other thread

dcrespo wrote:[color=blue]
> How can I get a raised exception from other thread that is in an
> imported module?[/color]

Define what "get" means for your purposes. It appears that you mean you
want to catch the exception, but in the thread which launched the other
thread in the first place. If that's true, please show where you would
expect to catch this exception, given that when you start the thread,
the main thread continues running and might even finish before the other
thread finishes.
[color=blue]
> thread = programB.MakeTh read()
> thread.start()[/color]
....
# later code which might have completed by the time the thread finishes
....

Are you looking, for example, for some kind of
thread.waitUnti lCompletionAndR eRaiseException s() method?

-Peter

[themightydoyle@gmail.com]

Re: How to get a raised exception from other thread

I also need an answer to this problem. I'm using windows. Throwing an
exception in thread B from thread A using a callback function.

The function runs, but the thread never actually catches the exception.
The try/except block is in the run() method (over-riden from the
Thread class)

[Peter Hansen]

Re: How to get a raised exception from other thread

themightydoyle@ gmail.com wrote:[color=blue]
> I also need an answer to this problem.[/color]

What _is_ the problem? We're still waiting for a clear description from
the OP as to what the problem really is. How would you describe what
_your_ problem is?
[color=blue]
> I'm using windows. Throwing an
> exception in thread B from thread A using a callback function.
>
> The function runs, but the thread never actually catches the exception.
> The try/except block is in the run() method (over-riden from the
> Thread class)[/color]

Rather than force us all to guess what you are doing wrong, maybe it
would be better if you posted a small amount of code that shows the problem.

Exceptions raised in threads can definitely be caught in the run()
method if the code is written correctly, so that is not in itself an issue.

-Peter

[themightydoyle@gmail.com]

Re: How to get a raised exception from other thread

Here's a dumbed down version of what i'm doing:

import time
import threading

class threader(thread ing.Thread):
def __init__(self):
threading.Threa d.__init__(self )
pass

def run(self):
try:
while 1:
time.sleep(5)
except SystemExit:
print "Got Exit Message in thread"

def killMe(self):
raise SystemExit



thread1 = threader()
thread2 = threader()

thread1.start()
thread2.start()

time.sleep(5)

try:
print "Killing thread 1"
thread1.killMe( )
print "killing thread 2"
thread2.killMe( )
except SystemExit:
print "Got exceptin in main thread"



The exception is not propogated to the threads I spawned, but instead
comes back in the main thread.

[themightydoyle@gmail.com]

Re: How to get a raised exception from other thread

Nevermind. I found a better solution. I used shared memory to create
a keep-alive flag. I then use the select function with a specified
timeout, and recheck the keep-alive flag after each timeout.

Thanx for all the input.

[Dennis Lee Bieber]

Re: How to get a raised exception from other thread

On 15 Oct 2005 20:54:33 -0700, themightydoyle@ gmail.com declaimed the
following in comp.lang.pytho n:

[color=blue]
> try:
> print "Killing thread 1"
> thread1.killMe( )
> print "killing thread 2"
> thread2.killMe( )
> except SystemExit:
> print "Got exceptin in main thread"
>
>
>
> The exception is not propogated to the threads I spawned, but instead
> comes back in the main thread.[/color]

No surprise there... The function invocation is being made in the
CONTEXT of the main thread...
--[color=blue]
> =============== =============== =============== =============== == <
> wlfraed@ix.netc om.com | Wulfraed Dennis Lee Bieber KD6MOG <
> wulfraed@dm.net | Bestiaria Support Staff <
> =============== =============== =============== =============== == <
> Home Page: <http://www.dm.net/~wulfraed/> <
> Overflow Page: <http://wlfraed.home.ne tcom.com/> <[/color]

[Klaas]

Re: How to get a raised exception from other thread

In article <1129438147.182 266.196790@g44g 2000cwa.googleg roups.com>,
themightydoyle@ gmail.com wrote:
[color=blue]
> Nevermind. I found a better solution. I used shared memory to create
> a keep-alive flag. I then use the select function with a specified
> timeout, and recheck the keep-alive flag after each timeout.
>
> Thanx for all the input.[/color]

How about a threading.Event ?

-Mike

[Peter Hansen]

Re: How to get a raised exception from other thread

themightydoyle@ gmail.com wrote:[color=blue]
> Nevermind. I found a better solution. I used shared memory to create
> a keep-alive flag. I then use the select function with a specified
> timeout, and recheck the keep-alive flag after each timeout.[/color]

As Dennis points out, your original attempt was destined to fail because
you were calling the method from the main thread, not the one you wanted
to kill. Threads don't magically execute any methods that are attached
to the Thread instance just because they're attached. You have to
actually call those methods *from* the thread, which means from the
run() method or from any of the routines it calls (whether they are
methods on that Thread or not), but it must be done in the context of
the thread you want to raise exceptions in or it won't work.

More importantly, you've now described your use case (and I hope that of
the OP as well, since he hasn't replied yet): killing threads.

This is an oft-discussed topic here, and searching the archives will
probably give you lots more answers, but the short answer is you cannot
kill a thread in Python (except by exiting the entire process). Instead,
as you've discovered, you must ask it nicely to exit. The nearly
canonical approach to doing that is as follows:

class MyThread(thread ing.Thread):
def __init__(self, *args, **kwargs):
threading.Threa d.__init__(self , *args, **kwargs)
self._keepRunni ng = True

def stop(self, timeout=None):
self._keepRunni ng = False
# optional: wait for thread to exit
self.join(timeo ut=timeout)

def run(self):
while self._keepRunni ng:
# do stuff here

Now you can do things like this:

thread = MyThread()
thread.start()
# other stuff...
thread.stop() # block here until the thread exits

I hope that clarifies things a little more...

-Peter

[Lasse Vågsæther Karlsen]

Re: How to get a raised exception from other thread

Peter Hansen wrote:[color=blue]
> themightydoyle@ gmail.com wrote:
>[color=green]
>> Nevermind. I found a better solution. I used shared memory to create
>> a keep-alive flag. I then use the select function with a specified
>> timeout, and recheck the keep-alive flag after each timeout.[/color]
>
>
> As Dennis points out, your original attempt was destined to fail because
> you were calling the method from the main thread, not the one you wanted[/color]
<snip>

A similar problem exists in .NET but they "solved" it by creating an
Abort method that raises an exception in the thread itself, basically
doing what you wanted to do.

However, this leads to all sorts of problem, particular when that
exception is raised in a finally block in that thread, which could mean
that the finally block did not fully execute, leading to resource leaks
or similar problems.

They pondered a while about constructing non-interruptible blocks of
code for .NET 2.0 but I'm not entirely sure how that worked out.

In any case, the best way is to use some kind of signal that the thread
reacts to, either by having it specifically wait for a signallable
object (like an event) or just by checking a boolean variable or similar
for a magic value that means "now is a good time for you to terminate".

--
Lasse Vågsæther Karlsen

http://usinglvkblog.blogspot.com/

mailto:lasse@vk arlsen.no
PGP KeyID: 0x2A42A1C2

[Alex Martelli]

Re: How to get a raised exception from other thread

<themightydoyle @gmail.com> wrote:
[color=blue]
> Nevermind. I found a better solution. I used shared memory to create
> a keep-alive flag. I then use the select function with a specified
> timeout, and recheck the keep-alive flag after each timeout.[/color]

Definitely a better architecture. Anyway, one supported way for a
thread to raise an exception in a different thread is function
thread.interrup t_main(), which raises a KeyboardInterru pt in the *main*
thread (the one thread that's running at the beginning of your program).

There's also a supported, documented function to raise any given
exception in any existing thread, but it's deliberately NOT directly
exposed to Python code -- you need a few lines of C-coded extension (or
pyrex, ctypes, etc, etc) to get at the functionality. This small but
non-null amount of "attrition" was deliberately put there to avoid
casual overuse of a facility intended only to help in very peculiar
cases (essentially in debuggers &c, where the thread's code may be buggy
and fail to check a keep-alive flag correctly...!-).


Alex

[Antoon Pardon]

Re: How to get a raised exception from other thread

Op 2005-10-14, dcrespo schreef <dcrespo@gmail. com>:[color=blue]
> Hi all,
>
> How can I get a raised exception from other thread that is in an
> imported module?
>
> For example:[/color]

You could try the following class, it needs ctypes and if the exception
is raised while in a C-extention, the exception will be delayed until
the extention is left. Some delay is unavoidable. You may also need
to fiddle with the length of the sleep at the end of this script
or the values in the xrange calls in the Continue threads

---------------------------- except.py --------------------------------

import os
import ctypes
from time import sleep
from random import randint


class TimeOut(Excepti on):
pass


class Alarm(Exception ):
pass

import threading

class Xthread(threadi ng.Thread):


def start(self):
self.__original _run = self.run
self.run = self.__run
threading.Threa d.start(self)


def __run(self):
self.__thrd_id = threading._get_ ident()
try:
self.__original _run()
finally:
self.run = self.__original _run


def raize(self, excpt):

Nr = ctypes.pythonap i.PyThreadState _SetAsyncExc(se lf.__thrd_id, ctypes.py_objec t(excpt))
while Nr > 1:
ctypes.pythonap i.PyThreadState _SetAsyncExc(se lf.__thrd_id, None)
sleep(0.1)
Nr = ctypes.pythonap i.PyThreadState _SetAsyncExc(se lf.__thrd_id, ctypes.py_objec t(excpt))

def alarm(self, tm):

alrm = threading.Timer (tm, self.raize, (TimeOut,))
alrm.start()
return alrm

class Continue(Xthrea d):

def run(self):

self.id = os.getpid()
print self.id, "Begin"
i = 0
try:
for _ in xrange(randint( 0,20)):
for e in xrange(4 * 100000):
i = i + e
print self.id, "Finished"
except Alarm:
print self.id, "Interupted "


lst = [Continue() for _ in xrange(10)]

for T in lst:
T.start()

try:
sleep(15)
finally:
for T in lst:
T.raize(Alarm)

[Antoon Pardon]

Re: How to get a raised exception from other thread

Op 2005-10-17, Alex Martelli schreef <aleaxit@yahoo. com>:[color=blue]
><themightydoyl e@gmail.com> wrote:
>[color=green]
>> Nevermind. I found a better solution. I used shared memory to create
>> a keep-alive flag. I then use the select function with a specified
>> timeout, and recheck the keep-alive flag after each timeout.[/color]
>
> Definitely a better architecture. Anyway, one supported way for a
> thread to raise an exception in a different thread is function
> thread.interrup t_main(), which raises a KeyboardInterru pt in the *main*
> thread (the one thread that's running at the beginning of your program).
>
> There's also a supported, documented function to raise any given
> exception in any existing thread, but it's deliberately NOT directly
> exposed to Python code -- you need a few lines of C-coded extension (or
> pyrex, ctypes, etc, etc) to get at the functionality. This small but
> non-null amount of "attrition" was deliberately put there to avoid
> casual overuse of a facility intended only to help in very peculiar
> cases (essentially in debuggers &c, where the thread's code may be buggy
> and fail to check a keep-alive flag correctly...!-).[/color]

I find this rather arrogant. It is not up to the developers of python
to decide how the users of python will use the language. If someone
has use of specific functionality he shouldn't be stopped because his
use is outside the intentions of the developers.

Just suppose you are writing a chess program. You let the program
"think" while it is the users move, but this thinking has to be
interrupted and some cleanup has to be made after the users move
has made a lot of this thinking obsolete by his move. Such code
could be mixture of loops and recursion that makes use of a simple
flag to end it all, unpractical. Use of an exception raised from
somewhere else would IMO in this case be an acceptable choice.

Why should the coder of this software have to go through this
deliberate set up attrition, to get at this functionality, just
because it wasn't intented to be used in such a way by the
developers?

As far as I know, pyrex and ctypes weren't intended to get
at the Python/C api. But they didn't create extra hurdles
for those who could use it that way.

--
Antoon Pardon