On Tue, Mar 25, 2014 at 7:36 AM, Steven D'Aprano
<steve+comp.lang.pyt...@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)
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