Unbound names in __del__

Michael Hoffman cam.ac.uk at mh391.invalid
Fri Jun 17 09:40:18 CEST 2005


Torsten Bronger wrote:

> 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.

__del__ is messy and I avoid it whenever possible. Make sure you read 
all the caveats and warnings on these pages:

http://www.python.org/doc/ref/customization.html
http://www.python.org/doc/lib/module-gc.html

Most importantly, "It is not guaranteed that __del__() methods are 
called for objects that still exist when the interpreter exits."

> 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?

A similar idiom is found in the standard library (e.g. tempfile.py):

         # Cache the unlinker so we don't get spurious errors at
         # shutdown when the module-level "os" is None'd out.  Note
         # that this must be referenced as self.unlink, because the
         # name TemporaryFileWrapper may also get None'd out before
         # __del__ is called.
         unlink = _os.unlink

         def close(self):
             if not self.close_called:
                 self.close_called = True
                 self.file.close()
                 self.unlink(self.name)

         def __del__(self):
             self.close()
-- 
Michael Hoffman



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