custom dict that allows delete during iteration

As you note, you can store the items to delete somewhere and defer the deletion of them until later. The problem then becomes when to purge them and how to make sure that the purge method eventually gets called. The answer to this is a context manager which is also a subclass of dict.

class dd_dict(dict):    # the dd is for "deferred delete"
    _deletes = None
    def __delitem__(self, key):
        if key not in self:
            raise KeyError(str(key))
        dict.__delitem__(self, key) if self._deletes is None else self._deletes.add(key)
    def __enter__(self):
        self._deletes = set()
    def __exit__(self, type, value, tb):
        for key in self._deletes:
            try:
                dict.__delitem__(self, key)
            except KeyError:
                pass
        self._deletes = None

Usage:

# make the dict and do whatever to it
ddd = dd_dict(a=1, b=2, c=3)

# now iterate over it, deferring deletes
with ddd:
    for k, v in ddd.iteritems():
        if k is "a":
            del ddd[k]
            print ddd     # shows that "a" is still there

print ddd                 # shows that "a" has been deleted

If you’re not in a with block, of course, deletes are immediate; as this is a dict subclass, it works just like a regular dict outside of a context manager.

You could also implement this as a wrapper class for a dictionary:

class deferring_delete(object):
    def __init__(self, d):
        self._dict = d
    def __enter__(self):
        self._deletes = set()
        return self
    def __exit__(self, type, value, tb):
        for key in self._deletes:
            try:
                del self._dict[key]
            except KeyError:
                pass
        del self._deletes
    def __delitem__(self, key):
        if key not in self._dict:
            raise KeyError(str(key))
        self._deletes.add(key)

d = dict(a=1, b=2, c=3)

with deferring_delete(d) as dd:
    for k, v in d.iteritems():
        if k is "a":
            del dd[k]    # delete through wrapper

print d

It’s even possible to make the wrapper class fully functional as a dictionary, if you want, though that’s a fair bit more code.

Performance-wise, this is admittedly not such a win, but I like it from a programmer-friendliness standpoint. The second method should be very slightly faster since it’s not testing a flag on each delete.