weakref list in python

Question

I'm in need of a list of weak references that deletes items when they die. Currently the only way I have of doing this is to keep flushing the list (removing dead references manually).

I'm aware there's a WeakKeyDictionary and a WeakValueDictionary, but I'm really after a WeakList, is there a way of doing this?

Here's an example:

import weakref

class A(object):
    def __init__(self):
       pass

class B(object):
    def __init__(self):
        self._references = []

    def addReference(self, obj):
        self._references.append(weakref.ref(obj))

    def flush(self):
        toRemove = []

        for ref in self._references:
            if ref() is None:
                toRemove.append(ref)

        for item in toRemove:
            self._references.remove(item)

b = B()

a1 = A()
b.addReference(a1)
a2 = A()
b.addReference(a2)

del a1
b.flush()
del a2
b.flush()

Answer 1

You can use WeakSet from the very same weakref module (it's actually defined elsewhere by the way, but it's imported there).

>>> from weakref import WeakSet
>>> s = WeakSet()
>>> class Obj(object): pass # can't weakref simple objects
>>> a = Obj()
>>> s.add(a)
>>> print len(s)
1
>>> del a
>>> print len(s)
0

Answer 2

You could implement it yourself, similarly to how you have done, but with a list subclass that calls flush() before attempting to access an item.

Obviously you don't want to do this on every access, but you can optimize this by setting a callback on the weak reference to mark the list dirty when something dies. Then you only need to flush the list when something has died since the last access.

Here's a list class implemented using this method. (Note that it's not tested much, and some methods aren't implemented very efficiently (eg. those which just convert to a real list and call the method on that), but it should be a reasonable starting point:

import weakref

class WeakList(list):
    def __init__(self, seq=()):
        list.__init__(self)
        self._refs = []
        self._dirty=False
        for x in seq: self.append(x)

    def _mark_dirty(self, wref):
        self._dirty = True

    def flush(self):
        self._refs = [x for x in self._refs if x() is not None]
        self._dirty=False

    def __getitem__(self, idx):
        if self._dirty: self.flush()
        return self._refs[idx]()

    def __iter__(self):
        for ref in self._refs:
            obj = ref()
            if obj is not None: yield obj

    def __repr__(self):
        return "WeakList(%r)" % list(self)

    def __len__(self):
        if self._dirty: self.flush()
        return len(self._refs)

    def __setitem__(self, idx, obj):
        if isinstance(idx, slice):
            self._refs[idx] = [weakref.ref(obj, self._mark_dirty) for x in obj]
        else:
            self._refs[idx] = weakref.ref(obj, self._mark_dirty)
        
    def __delitem__(self, idx):
        del self._refs[idx]

    def append(self, obj):
        self._refs.append(weakref.ref(obj, self._mark_dirty))

    def count(self, obj):
        return list(self).count(obj)

    def extend(self, items):
        for x in items: self.append(x)
        
    def index(self, obj):
        return list(self).index(obj)
    
    def insert(self, idx, obj):
        self._refs.insert(idx, weakref.ref(obj, self._mark_dirty))
        
    def pop(self, idx):
        if self._dirty: self.flush()
        obj=self._refs[idx]()
        del self._refs[idx]
        return obj

    def remove(self, obj):
        if self._dirty: self.flush() # Ensure all valid.
        for i, x in enumerate(self):
            if x == obj:
                del self[i]
        
    def reverse(self):
        self._refs.reverse()

    def sort(self, cmp=None, key=None, reverse=False):
        if self._dirty: self.flush()
        if key is not None:
            key = lambda x,key=key: key(x())
        else:
            key = apply
        self._refs.sort(cmp=cmp, key=key, reverse=reverse)

    def __add__(self, other):
        l = WeakList(self)
        l.extend(other)
        return l

    def __iadd__(self, other):
        self.extend(other)
        return self
        
    def __contains__(self, obj):
        return obj in list(self)

    def __mul__(self, n):
        return WeakList(list(self)*n)
        
    def __imul__(self, n):
        self._refs *= n
        return self

Answer 3

As I needed a weakref list like you, I've made one and publish it on pypi.

now you can do:

pip install weakreflist

then:

from weakreflist import WeakList

Answer 4

Why can't you just do it like this:

import weakref

class WeakList(list):
    def append(self, item):
        list.append(self, weakref.ref(item, self.remove))

And then do similar for __iadd__, extend etc. Works for me.

Answer 5

Use a callback function passed to second argument of a weakref.

This code should function:

import weakref

class WeakRefList(list):

    def add_reference(self, obj):
        self._references.append(weakref.proxy(obj, self.remove))

Answer 6

How do you plan on using B? The only thing I ever do with the weakref list I built is iterate over it, so its implementation is simple:

import weakref

class WeakRefList(object):
    "weakref psuedo list"
    def __init__(yo):
        yo._items = list()
    def __iter__(yo):
        yo._items = [s for s in yo._items if s() is not None]
        return (s() for s in yo._items if s() is not None)
    def __len__(yo):
        yo._items = [s for s in yo._items if s() is not None]
        return len(yo._items)
    def append(yo, new_item):
        yo._items.append(weakref.ref(new_item))
        yo._items = [s for s in yo._items if s() is not None]