Python's __del__ has some nasty issues when dealing with objects, list, garbage collection, etc.

issues_with_deleting_objects.py

# Hypothetical class for saving the Universe.
class Particle:
    def __init__(self, name):
        self.name = name
        print self.name, 'created'

    def __del__(self):
        print self.name, 'deleted'

# Create new particle objects.
alpha = Particle('alpha')
beta = Particle('beta')

# Insert them into a list.
particles = [alpha, beta]

# After some conditions have been met
# alpha and/or beta must be deleted from the list
# Let's assume we want to delete alpha for now.
# But, surprisingly, the statement below will not execute the alpha.__del__()
del particles[0]

print 'alpha should now be deleted (but the alpha.__del__() is not triggered).\n' \
      'the "alpha deleted" message should be above this line!'

# The alpha.__del__() only executes if ALL other variables referencing alpha have been deleted.
# Hence, we should delete the original/first alpha variable also.
# Generally, x.__del__ only executes if ALL references related to x are deleted.
# See also: http://www.electricmonk.nl/log/2008/07/07/python-destructor-and-garbage-collection-notes/
# After the line below, the alpha.__del__() will now be executed (finally!).
del alpha

print 'End of Program.\n' \
      'Scope of all variables should end at this part.\n' \
      'The beta variable will be out of the program scope at this moment and will be now deleted automatically.'

# Moral of the story: if you need to force execute x.__del__(), you need to delete all object references to x.
# For example, in the example above you NEED to execute these two statements:
# del particles[0]
# del alpha
# Hence, if you only used an object once, you only need to delete the only reference and
# it will execute the x.__del__() immediately.
# particles = [Particle('alpha'), Particle('beta')]
# del particles[0] will automatically execute the Particle('alpha').__del__()
# because there are no other objects related to it.
# Other related reference:
# http://stackoverflow.com/questions/8907905/python-del-myclass-doesnt-call-object-del