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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Thu, 27 Mar 2014 01:32:36 -0600
Subject: Re: Time we switched to unicode? (was Explanation of this Python language feature?)
To: Python <python-list@python.org>
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Xref: csiph.com comp.lang.python:69174

On Tue, Mar 25, 2014 at 7:36 AM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> Yes, Python could have changed the meaning of {} to mean the empty set.
> But you know what? The empty set is not that important. Sets are not
> fundamental to Python. Python didn't even have sets until 2.3, and at
> first they were just a standard library module, not even built-in. Dicts,
> on the other hand, are fundamental to Python. They are used everywhere.
> Python is, in a very real sense, built on dicts, not sets. You can
> implement sets starting from dicts, but not the other way around: dicts
> are more fundamental than sets.

Challenge accepted!

The _lookup method in the following is based on a recipe from Raymond Hettinger.


from collections.abc import MutableMapping


class SetBasedDict(MutableMapping):

    def __init__(self, initial, **kwargs):
        self._contents = set()
        self.update(initial, **kwargs)

    def clear(self):
        self._contents.clear()

    def __iter__(self):
        for item in self._contents:
            yield item.key

    def __len__(self):
        return len(self._contents)

    def __getitem__(self, key):
        item = self._lookup(key)
        if item is None:
            raise KeyError(key)
        return item.value

    def __setitem__(self, key, value):
        item = self._lookup(key)
        if item is not None:
            item.value = value
        else:
            item = _DictItem(key, value)
            self._contents.add(item)

    def __delitem__(self, key):
        self._contents.remove(_DictItem(key, None))

    def _lookup(self, key):
        p = _DictSearchProxy(key)
        if p in self._contents:
            return p.match
        return None


class _DictItem:

    def __init__(self, key, value):
        self.key = key
        self.value = value

    def __hash__(self):
        return hash(self.key)

    def __eq__(self, other):
        if not isinstance(other, _DictItem):
            return NotImplemented
        return self.key == other.key


class _DictSearchProxy:

    def __init__(self, obj):
        self.obj = obj
        self.match = obj

    def __eq__(self, other):
        if not isinstance(other, _DictItem):
            return NotImplemented
        result = (self.obj == other.key)
        if result:
            self.match = other
        return result

    def __hash__(self):
        return hash(self.obj)