Re: How to kill a thread?

On Jun 10, 2:03 am, Rhamphoryncus <rha...@xxxxxxxxx> wrote:
On Jun 9, 2:52 pm, Fuzzyman <fuzzy...@xxxxxxxxx> wrote:



On Jun 9, 9:20 pm, Rhamphoryncus <rha...@xxxxxxxxx> wrote:

On Jun 9, 5:33 am, Antoon Pardon <apar...@xxxxxxxxxxxxxxx> wrote:

On 2008-06-07, Rhamphoryncus <rha...@xxxxxxxxx> wrote:

On Jun 6, 12:44 pm, The Pythonista <n...@xxxxxxxxx> wrote:
It's always been my understanding that you can't forcibly kill a thread
in Python (at least not in a portable way).  The best you can do is
politely ask it to die, IIRC.

Inherently, the best you can do in most languages is ask them politely
to die.  Otherwise you'll leave locks and various other datastructures
in an inconvenient state, which is too complex to handle correctly..
The exception is certain functional languages, which aren't capable of
having threads and complex state in the same sense.

Well it would of course depend on what is considered asking politely?

If one thread could cause an exception being thrown in an other thread,
would this be considered a polite way to ask? Would it be considered
an acceptable way?

The exception must not be raised until a point explicitly designed as
safe is hit.  Otherwise, any function that manipulates data you'll
still use will potentially be buggered.  Consider sys.stdout: codecs,
buffering, lots to go wrong.

Java and .NET both have ways of killing threads. They both work by
raising a 'ThreadAbort' (or similar) exception in the target thread.
In early implementations they both suffered from a similar problem.
You could protect locks (etc) by having a finally block that would
release all resources as needed - but what happens if the thread abort
exception is raised *inside* the finally block?

Java responded by deprecating thread aborting. .NET responded by
ensuring that a thread abort exception would never be raised inside a
finally block - if that happened the exception would only be raised
once the code has left the finally block.

Aborting threads in .NET can be extremely useful. Politely asking a
thread to die is no good if the task the thread is executing is
extremely coarse grained - it may not be able to respond to the
request for some time. If your code is structured correctly
(performing a long running background calculation for example) then
you may *know* that you can kill it without problems, but Python has
no native API to do this.

So how does .NET deal with the sys.stdout corruption?  Does it?


That has never been an issue for us.

If you've carefully written your code to use only safe primitives and
local state (discarded if interrupted) then yes, it could be
interruptible.  At this point you could specially mark that block of
code as safe, leaving the vast majority of other code unsafe by
default.  Then again, since you're going to the trouble of carefully
designing and auditing your code you could just make it cancellable
like blocking I/O should be - just by polling a flag at key points
(and you're CPU-bound anyway, so it's not expensive.)

The only place I know of that you *need* arbitrary interruption is
hitting CTRL-C in the interactive interpreter.  At this point it's a
debugging tool though, so the risk of weirdness is acceptable.

We use background threads for long running calculations that we know
are safe to abort. Resources that need protecting we do with finally
blocks which the thread abort honours.

The calculation is 'coarse grained' (it can call into .NET APIs that
can take a relatively long time to return) - so polling for exit
wouldn't work anyway. We also run user code and can't expect their
code to poll for exit conditions.

This system works fine for us. We had fun syncing results back when
the calculation terminates (race conditions if a new one needs to
start just as the old one ends) - but that is the fun of threads in
the first place and nothing to do with how we terminate calculations
early.

Michael Foord
http://www.ironpythoninaction.com/
.

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