Aahz wrote:
> In article <gt8v37$kib$0...@news.t-online.com>,
> Peter Otten <__pete...@web.de> wrote:
>>Aahz wrote:
>>> In article <gt1kb7$jqg$0...@news.t-online.com>,
>>> Peter Otten <__pete...@web.de> wrote:
>>>>
>>>>Here's a trick to find the actual element. I think Raymond Hettinger
>>>>posted an implementation of this idea recently, but I can't find it at
>>>>the moment.
>>>
>>> Your code is inverted from Raymond's:
>>
>>I can't see the inversion.
>
> You were wrapping the objects inserted into the set; Raymond's trick
> involved only wrapping the comparison object. It's therefore much more
> convenient.
I think you are misreading my code. I took the items (of class X) as they
were specified by the OP.
The reason I changed their __eq__() method is not that I did not understand
Raymond's trick, but rather a quirk in the set's item lookup:
>>> class A(object):
... def __init__(self, value):
... self.value = value
... def __hash__(self):
... return hash(self.value)
... def __eq__(self, other):
... return self.value == other.value
...
>>> item = A("a")
>>> container = map(A, "abc")
>>> from get_equivalent import get_equivalent
TestResults(failed=0, attempted=0)
get_equivalent() is Raymond's implementation from the recipe. Let's try it:
>>> get_equivalent(container, item)
<__main__.A object at 0x2091e50>
>>> _ is not item
True
Works. Now the same with a set:
>>> container = set(container)
>>> print get_equivalent(container, item)
None
Oops.
wanted in some_list
performs wanted.__eq__(candidate)
where candidate is an item in the list.
wanted in some_set
tries
candidate.__eq__(wanted)
first. You must ensure that this fails in an orderly manner for
wanted.__eq__(candidate)
to be tried at all:
>>> def __eq__(self, other):
... if not isinstance(other, A):
... return NotImplemented
... return self.value == other.value
...
>>> A.__eq__ = __eq__
>>> print get_equivalent(container, item)
<__main__.A object at 0x2091e50>
>>> _ is not item
True
Peter
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