-----------------------------
On Fri, Mar 20, 2015 8:05 PM CET Alan Gauld wrote:
>On 20/03/15 09:37, Peter Otten wrote:
>
>> def close_relay(e=None,v=None,t=None):
>> try:
>> if not relay_closed()
>> really_close_relay()
>> except:
>> really_close_relay()
>
>The purpose of the if clause is to ensure that
>if the function is called many times you only
>close the relay once (I surmised that more than
>once could be harmful?)
>
>> import sys, atexit
>> atexit.register(close_relay)
>> sys.excepthook = close_relay
>
>atexit should be overkill, but it might be needed
>if for some reason the interpreter dies while
>performing the usual cleanup.
>
>excepthook replaces the usual exception mechanism
>with a clean up. This is needed for cases where an
>exception occurs before getting to the finally. We
>want to close the relay ASAP, not waiting till
>the interpreter decides to call the finally.
>
>> try:
>> main program here
>> finally:
>> close_relay()
>
>This is the happy path where everything shuts down as expected.
>
>> That reeks of cargo cult. Are there actual scenarios for each of the three
>> mechanisms where it is the only one that works?
>
>In real-time you never trust anything.
>Always cover your back.
>
>> I would expect that
>>
>> try:
>> main program here
>> finally:
>> close_relay()
>>
>> provides the same level of confidence,
>
>Only if the interpreter is behaving as normal.
>The hooks are to try (we hope) to catch cases where
>the interpreter has broken its normal flow.
>
>So the scenarios are:
>
>1) an unexpected exception occurs - close the relay ASAP.
> - Use excepthook
>
>2) The interpreter gets sent a kill or similar unexpected
>termination - use atexit because finally may not get
>called. (I'm not sure, so belt n' braces here)
>(BTW Does anyone know what the interpreter does when
>suspending - Ctrl-Z in Unix land?)
No experience with it, but I would first check the 'signal' module
import signal
import sys
def signal_term_handler(signal, frame):
print 'got SIGTERM'
sys.exit(0)
signal.signal(signal.SIGTERM, signal_term_handler)
https://nattster.wordpress.com/2013/06/05/catch-kill-signal-in-python/
>
>3) Normal program exit. Use the finally clause.
>
>But its only ever going to be a best endeavour, that's
>why Python is not suitable for true real-time/critical apps...
>But I'd never trust any environment to its usual behaviour
>if there is a possibility of something being broken.
>In this case the relay and its battery pack.
>
>> the program closes normally or the main code raises an exception, but not if
>> the process is killed.
>
>What's not clear in the Python documentation is how Python responds
>to a kill(or suspend). I'd hope the atexit got called even in a kill.
>I would not expect the finally to be executed.
>
>Of course, if its a seg fault you are probably stuffed either way...
>
>
>--
>Alan G
>Author of the Learn to Program web site
>http://www.alan-g.me.uk/
>http://www.amazon.com/author/alan_gauld
>Follow my photo-blog on Flickr at:
>http://www.flickr.com/photos/alangauldphotos
>
>
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