Why is the python destructor being called?

python

Solution

The output in question is replicated only in interactive shell.

In interactive session, additional variable `_` exists, which refer to last value.

Using sys.getrefcount to inspect reference count:

>>> import sys
>>> class SmartPhone:
...     def __del__(self):
...        print "destroyed"
...
>>> y = SmartPhone()
>>> sys.getrefcount(y) # not printed, _ does not reference SmartPhone object yet.
2
>>> y
<__main__.SmartPhone instance at 0x000000000263B588>
>>> sys.getrefcount(y) # y printed, _ reference SmartPhone object.
3

`2`, `3` in above output should be `1`, `2`. They are printed that way, because getrefcount() increase reference count temporarily as mentioned in getrefcount documentation.

I changed SmartPhone as follow to make it easy to inspect what's going on.

>>> class SmartPhone(object):
...     def __init__(self, name):
...         self.name = name
...     def __repr__(self):
...         return super(SmartPhone, self).__repr__() + ' name=' + self.name
...     def __del__(self):
...        print "destroyed", self
...
>>> y = SmartPhone('first')
>>> del y # deleted immediately, because only "y" reference it.
destroyed <__main__.SmartPhone object at 0x00000000024FEFD0> name=first
>>> y = SmartPhone('second')
>>> y # at this time, _ reference to second y (y's reference count is now 2)
<__main__.SmartPhone object at 0x00000000024FEFD0> name=second
>>> y
<__main__.SmartPhone object at 0x00000000024FEFD0> name=second
>>> y
<__main__.SmartPhone object at 0x00000000024FEFD0> name=second
>>> del y # not deleted immediately, because _ reference it.
>>> y = SmartPhone('third') # _ still reference the second y, because nothing is printed.
>>> y # second y is deleted, because _ now reference the third y. (no reference to the second y)
destroyed <__main__.SmartPhone object at 0x00000000024FEFD0> name=second
<__main__.SmartPhone object at 0x000000000264A470> name=third

Problem

Original source