Hrvoje Niksic wrote:
I've stumbled upon an oddity using sets. It's trivial to test if a
value is in the set, but it appears to be impossible to retrieve a
stored value,
Set elements, by definition, do not have keys or position by which to
grab. When they do, use a dict or list.
other than by iterating over the whole set. Let me
describe a concrete use case.
Imagine a set of objects identified by some piece of information, such
as a "key" slot (guaranteed to be constant for any particular element).
The object could look like this:
class Element(object):
def __init__(self, key):
self.key = key
def __eq__(self, other):
return self.key == other
def __hash__(self):
return hash(self.key)
# ...
Now imagine a set "s" of such objects. I can add them to the set:
>>> s = set()
>>> s.add(Element('foo'))
>>> s.add(Element('bar'))
I can test membership using the keys:
>>> 'foo' in s
True
>>> 'blah' in s
False
But I can't seem to find a way to retrieve the element corresponding to
'foo', at least not without iterating over the entire set. Is this an
oversight or an intentional feature? Or am I just missing an obvious
way to do this?
Use a dict, like you did.
I know I can work around this by changing the set of elements to a dict
that maps key -> element, but this feels unsatisfactory.
Sorry, that is the right way.
> It's
redundant, as the element already contains all the necessary
information,
Records in a database have all the information of the record, but we
still put out fields for indexes.
tjr
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