custom dict that allows delete during iteration

Question

UPDATED based on Lennart Regebro\'s answer

Suppose you iterate through a dictionary, and sometimes need to delete an element. The following is very efficient:

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Answer 1

Naive implementation for Python 2.x and 3.x:

import sys
from collections import deque


def _protect_from_delete(func):
    def wrapper(self, *args, **kwargs):
        try:
            self._iterating += 1
            for item in func(self, *args, **kwargs):
                yield item
        finally:
            self._iterating -= 1
            self._delete_pending()
    return wrapper

class DeletableDict(dict):
    def __init__(self, *args, **kwargs):
        super(DeletableDict, self).__init__(*args, **kwargs)
        self._keys_to_delete = deque()
        self._iterating = 0

    if sys.version_info[0] != 3:
        iterkeys = _protect_from_delete(dict.iterkeys)
        itervalues = _protect_from_delete(dict.itervalues)
        iteritems = _protect_from_delete(dict.iteritems)
    else:
        keys = _protect_from_delete(dict.keys)
        values = _protect_from_delete(dict.values)
        items = _protect_from_delete(dict.items)  
    __iter__ = _protect_from_delete(dict.__iter__)

    def __delitem__(self, key):
        if not self._iterating:
            return super(DeletableDict, self).__delitem__(key)
        self._keys_to_delete.append(key)

    def _delete_pending(self):
        for key in self._keys_to_delete:
            super(DeletableDict, self).__delitem__(key)
        self._keys_to_delete.clear()

if __name__ == '__main__':
    dct = DeletableDict((i, i*2) for i in range(15))
    if sys.version_info[0] != 3:
        for k, v in dct.iteritems():
            if k < 5:
                del dct[k]
        print(dct)
        for k in dct.iterkeys():
            if k > 8:
                del dct[k]
        print(dct)
        for k in dct:
            if k < 8:
                del dct[k]
        print(dct)
    else:
        for k, v in dct.items():
            if k < 5:
                del dct[k]
        print(dct)

When iterating over keys, items or values it sets flag self._iterating. In __delitem__ it checks for ability to delete item, and stores keys in temporary queue. At the end of iterations it deletes all pending keys.

It's very naive implementation, and I wouldn't recommend to use it in production code.

EDIT

Added support for Python 3 and improvements from @jsbueno comments.

Python 3 run on Ideone.com