Hallöchen!
When my __del__ methods are called because the program is being
terminated, I experience difficulties in calling functions that I
need for a clean shutdown of my instances. So far, there has been
only one of these functions, and a class-local alias solved the
problem. However, now there are many of them.
Is there a way to detect whether the program is being terminated?
(In this case, I wouldn't care and return from __del__ immediately.)
Or do you know a cleaner solution? For example, if I have
import vpp43
would it help to say
def __init__(self):
__vpp43 = vpp43
...
to guarantee that I can access all the routines in vpp43 in the
__del__ method?
Tschö,
Torsten.
--
Torsten Bronger, aquisgrana, europa vetus
Jul 19 '05
13  1412 
Peter Hansen <pe***@engcorp. com> writes on Fri, 17 Jun 2005 08:43:26 -0400: ...
And I don't recall the last time I saw a __del__ in third-party code I
was examining.
What's your use case for del?
I had to use one a few days ago:
To call the "unlink" method of a "minidom" object when
its "container" is destroyed.
It would have been possible to let the cyclic garbage
collector find and eliminate the cyclic "minidom" objects.
But, DOMs may be large and I create lots of them in
a short time (each in its own container)...
Dieter
Torsten Bronger wrote: Torsten Bronger wrote: keithley = GpibInstrument( 14)
keithley.write( "*IDN?")
print keithley.read()
[on using atexit] However, this doesn't close sessions while the program is running.
If the programmer has the above code in a function which is called
repeatedly, he may run into trouble. IIRC my current VISA DLL has
only 256 session slots.
Hmm... it sounds to me as though the design of this DLL is such that one
is expected to hold a session open for the duration of the application.
So instead of creating new GpibInstrument objects as needed, then just
sort of dropping them, the programmers would have to create the required
instrument objects at the start of the program and reuse them when needed.
But that's as far as my thoughts take me on this, without working
directly with you. I guess you'll have to explore the __del__ approach
through to its conclusion before you'll be confident it won't work (or
perhaps you'll find a solution after all!).
-Peter
"Torsten Bronger" <br*****@physik .rwth-aachen.de> wrote in message
news:87******** ****@wilson.rwt h-aachen.de... Torsten Bronger wrote:
keithley = GpibInstrument( 14)
keithley.write( "*IDN?")
print keithley.read()
A keithley.close( ) would be a wart in my opinion; instead I want
to hide the whole session thing from the programmer. Besides, I
haven't yet given up the hope that the issues with __del__ can be
tackled.
However, this doesn't close sessions while the program is running.
If the programmer has the above code in a function which is called
repeatedly, he may run into trouble. IIRC my current VISA DLL has
only 256 session slots.
Deleting during a run is a different issue from deleting during program
shutdown, which is where you started us. In CPython, 'del keithley' will,
if keithley is the last reference to the object, dependably delete and
object, calling __del__ on the way. For other implementations , deleting
the last reference makes the object eligible to be deleted 'sometime' (if
and when garbage collection happens). So an explicit close is the only
dependable cross-implementation method that I know of.
Terry J. Reedy
Hallöchen!
"Terry Reedy" <tj*****@udel.e du> writes: "Torsten Bronger" <br*****@physik .rwth-aachen.de> wrote:
[...]
However, this doesn't close sessions while the program is
running. If the programmer has the above code in a function
which is called repeatedly, he may run into trouble. IIRC my
current VISA DLL has only 256 session slots.
Deleting during a run is a different issue from deleting during
program shutdown, which is where you started us.
Well, I talked about shutdown in the OP because only *then* I had
experienced problems. Cleaning up is important in every phase of
the program, though.
[...] So an explicit close is the only dependable
cross-implementation method that I know of.
I agree, but I could live with the little danger of a Python
implementation that frees so slowly that the DLL runs out of
handles. One could add a close() method for emergency use.
However, this __del__ thingy is so damn tempting ... ;-)
Tschö,
Torsten.
--
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