Automatically delete class instance when one of its attributes becomes dead

Question

Set Up

Say I have a Snit:

class Snit(): pass

And a Snot, which contains weak references to up to, say,

Answer 1

Okay, so you have a Snot with a variable amount of Snits.

class Snot(object):

    def __init__(self, *snits):
        self.snits = [weakref.ref(s) for s in snits]

    def __eq__(self, other):
        if not isinstance(other, self.__class__) and other is not None:
            return NotImplemented
        # are all my snits still valid
        valid = all(s() for s in self.snits)
        if other is None:
            return not valid  # if valid is True, we are not equal to None
        else:
            # whatever it takes to see if this snot is the same as the other snot

Actually having the class instance disappear is going to take more work (such as having dict on the class to track them all, and then other data structures would just use weak-references -- but that could get ugly quick), so the next best thing will be having it become equal to None when any of its Snits goes away.

---

I see that snits and snots are both lists -- is order important? If order is not important you could use sets instead, and then it would be possible to have a performant solution where the the dead snot is actually removed from the data structure -- but it would add complexity: each Snot would have to keep track of which data struture it was in, and each Snit would have to keep a list of which Snots it was in, and the magic would have to live in __del__ which can lead to other problems...

Answer 2

Nothing is truly automatic. You'll need to either have a function that you run manually to check for dead Snits, or have a function that is part of Snot that is called whenever anything interesting happens to a Snot to check for, and remove, dead Snits.

For example:

class Snot:
    ...
    def __repr__(self):
        # check for and remove any dead Snits
        self._remove_dead_snits()
        return ...
    def _remove_dead_snits(self):
        if self.s1() is None:
             self.s1 = None
        ... # and so on and so forth

The fun part is adding that call to _remove_dead_snits for every interesting interaction with a Snot -- such as __getitem__, __iter__, and whatever else you may do with it.

---

Actually, thinking a bit more about this, if you only have the four possible Snits per each Snot you could use a SnitRef descriptor -- here's the code, with some changes to your original:

import weakref

class Snit(object):
    def __init__(self, value):
        self.value = value  # just for testing
    def __repr__(self):
        return 'Snit(%r)' % self.value

class SnitRef(object):   # 'object' not needed in Python 3
    def __get__(self, inst, cls=None):
        if inst is None:
            return self
        return self.ref()  # either None or the obj
    def __set__(self, inst, obj):
        self.ref = weakref.ref(obj)


class Snot(object):
    s0 = SnitRef()
    s1 = SnitRef()
    s2 = SnitRef()
    s3 = SnitRef()
    def __init__(self,s0=None,s1=None,s2=None,s3=None):
        self.s0 = s0
        self.s1 = s1
        self.s2 = s2
        self.s3 = s3

snits = [Snit(0), Snit(1), Snit(2), Snit(3)]
print snits
snot = Snot(*snits)
print(snot.s2)
snits.pop(2)
print snits
print(snot.s2)

and when run:

[Snit(0), Snit(1), Snit(2), Snit(3)]
Snit(2)
[Snit(0), Snit(1), Snit(3)]
None