finding object using IS instead of ==

[Andrew Dalke]

Mark Hahn:
> If I have a large collection (list, tuple, or dict) of objects, can I
locate
> one quickly that matches a given object using the IS equivalence operator
> instead of the value == operator?

If you write your own code then, yes, you can do it for list.  Here's
some untested code.

def is_index(data, x):
    for term in data:
      if term is x:
        return term
    raise ValueError("not in data")

This would work for dicts if you have data == dict.iterkeys(), but
that won't take advantages of the hash lookup properties.

I don't know any way you can leverage 'is' tests over '==' for
dict's "__in__" searching.  I tried

    >>> class CmpIs:
...    def __init__(self, obj):
...       self.obj = obj
...    def __hash__(self):
...       return hash(self.obj)
...    def __cmp__(self, other):
...       x = cmp(id(self), id(other))
...       if x == 0:
...          self.match = other
...       return x
...

but that doesn't work because the == test inside of the
dict's linear search of the hash chain appears to do

  if term_in_dict == object_to_find

giving __cmp__ precedence to the object already in the
dictionary and so never getting to my overridden __cmp__.

                    Andrew
                    dalke at dalkescientific.com