Python set().issubset() not working as expected

python, python-2.7, set

Solution

Your objects are being hashed by their `id` (you didn't override `__hash__`). Of course they're not subsets since `xx` and `yy` contain unique objects.

In order to do this, you need to come up with some sort of `__hash__` function. `__hash__` should always return the same value for an object which is why it's usually understood that you don't mutate a hashable object. For instance, one choice could be:

class T(object):
    #<snip> ...

    def __hash__(self):
        return hash(self.value)

    #... </snip>

with the understanding that `self.value` can't change for the life of the object. (Note, I don't claim that is a good choice. The actual hash you use is really dependent on your actual application)

Now the why -- The magic behind sets (and dicts) and the amazing performance is that they rely on hashs. Basically, every hashable object is turned into a "unique" (in a perfect world) number. Python takes that "unique" number and turns it into an array index that it can use to get a handle on the object (the magic here is a bit difficult to explain, but it's not important for this discussion). So, rather than looking for an object by comparing it to all the other objects in the "array" (usually called a table) -- an expensive operation, it knows exactly where to look for the object based on the hash value (cheap). By default, user defined objects are hashed by their `id` (memory address). When you create the set `xx`, python looks at each of the objects `id`s and puts them in a based on their ids. Now when you do `xx.issubset(yy)`, python looks at all of the ids in `xx` and checks to see if they are all in `yy`. But none of them are in `yy` since they're all unique objects (and therefore have unique hash values).

But you say, "why did `xx-(yy-xx) == xx` work?" Good question. Lets pull this apart.

First, we have `(yy - xx)`. This returns an empty set because again, no elements in `xx` are also in `yy` (they all hash to different values since they all have unique `id`s). So you're really doing

xx - set([]) == xx

which should be pretty obvious why that is `True`.

Problem

I'm trying to use `set().issubset()` for comparison of sequences. As you can imagine, it's not working as expected ;) In advance: sorry for the long code-blob. ``` class T(object): def __init__(self, value, attributes = None): self.value = value self.attributes = Attributes(attributes) def __eq__(self, other): if not isinstance(other, T): return False if self.value == other.value and self.attributes == other.attributes: return True else: return False def __ne__(self, other): if not isinstance(other, T): return True if self.value != other.value or self.attributes != other.attributes: return True else: return False class Attributes(dict): def __init__(self, attributes): super(dict, self) self.update(attributes or dict()) def __eq__(self, other): if self.items() == other.items(): return True else: return False def __ne__(self, other): if not self.items() == other.items(): return True else: return False def __cmp__(self, other): return self.items().__cmp__(other.items()) x = [T("I", {'pos': 0}), T("am", {'pos': 1}), T("a", {'pos': 2}), T("test", {'pos': 3})] y = [T("a", {'pos': 2}), T("test", {'pos': 3})] xx = set(x) yy = set(y) assert y[0] == x[2], "__eq__ did fail, really?" #works assert y[1] == x[3], "__eq__ did fail, really?" #works assert xx-(yy-xx) == xx, "set subtract not working" #works, but is nonsense. see accepted answer assert not xx.issubset(yy), "i am doing it wrong..." #works assert yy.issubset(xx), "issubset not working :(" #FAILS! ``` Running above code fails the last assertion: ``` $ python Python 2.7.2 (v2.7.2:8527427914a2, Jun 11 2011, 15:22:34) [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> import issubsettest Traceback (most recent call last): File "<stdin>", line 1, in <module> File "issubsettest.py", line 52, in <module> assert yy.issubset(xx), "issubset not working :(" AssertionError: issubset not working :( >>> ``` What am I missing here?

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