Dynamically change __del__

[Nikolaus Rath]

Hi,

I'm trying to be very clever:

class tst(object):
def destroy(self):
print 'Cleaning up.'
self.__del__ = lambda: None
def __del__(self):
raise RuntimeError('Instance destroyed without running destroy! Hell may break loose!')

However, it doesn't work:

In [2]: t = tst()

In [3]: t = None
Exception RuntimeError: RuntimeError('Instance destroyed without running destroy! Hell may break loose!',) in <bound method tst.__del__ of <__main__.tst object at 0x978566c>> ignored

In [4]: t = tst()

In [5]: t.destroy()
Cleaning up.

In [6]: t = None
Exception RuntimeError: RuntimeError('Instance destroyed without running destroy! Hell may break loose!',) in <bound method tst.__del__ of <__main__.tst object at 0x978566c>> ignored

$ python -V
Python 2.6.4


Apparently Python calls the class attribute __del__ rather than the
instance's __del__ attribute. Is that a bug or a feature? Is there any
way to implement the desired functionality without introducing an
additional destroy_has_been_called attribute?


(I know that invocation of __del__ is unreliable, this is just an
additional safeguard to increase the likelihood of bugs to get noticed).



Best,

-Nikolaus

 

[Lie Ryan]

Hi,

I'm trying to be very clever:

Apparently Python calls the class attribute __del__ rather than the
instance's __del__ attribute. Is that a bug or a feature? Is there any
way to implement the desired functionality without introducing an
additional destroy_has_been_called attribute?


(I know that invocation of __del__ is unreliable, this is just an
additional safeguard to increase the likelihood of bugs to get noticed).

All Exception in __del__ is ignored for various reasons. It's safer if
you call destroy from inside __del__() so you cannot forget to remember
to call it manually.

def __del__(self):
self.destroy()

 

[Jerry Hill]

Apparently Python calls the class attribute __del__ rather than the
instance's __del__ attribute. Is that a bug or a feature? Is there any
way to implement the desired functionality without introducing an
additional destroy_has_been_called attribute?

It's a documented feature:
http://docs.python.org/reference/datamodel.html#new-style-special-lookup

I'm not aware of a way to get around it, so I think you'll need to
fall back to checking a flag in the class's __del__ method.

 

[Peter Otten]

Apparently Python calls the class attribute __del__ rather than the
instance's __del__ attribute. Is that a bug or a feature? Is there any
way to implement the desired functionality without introducing an
additional destroy_has_been_called attribute?

For newstyle classes __special__() methods are always looked up in the
class, never in the instance.

Your best bet is probably to manage the lifetime of the instance explicitly
with try...finally or a context manager:
.... def close(self):
.... print "Cleaning up."
........ print t
....
<__main__.T object at 0x7fc50c1e61d0>
Cleaning up.

Alternatively you can try and cook up something with weakref.ref and a
callback.

Peter

 

[Lie Ryan]

All Exception in __del__ is ignored for various reasons. It's safer if
you call destroy from inside __del__() so you cannot forget to remember
to call it manually.

def __del__(self):
self.destroy()

Never mind that, just realized you're asking for a different thing.