Unwanted behaviour from dict.fromkeys

python

I'd like to initialise a dictionary of sets (in Python 2.6) using dict.fromkeys, but the resulting structure behaves strangely. More specifically:

>>>> x = {}.fromkeys(range(10), set([]))
>>>> x
{0: set([]), 1: set([]), 2: set([]), 3: set([]), 4: set([]), 5: set([]), 6: set([]), 7: set([]), 8: set([]), 9: set([])}
>>>> x[5].add(3)
>>>> x
{0: set([3]), 1: set([3]), 2: set([3]), 3: set([3]), 4: set([3]), 5: set([3]), 6: set([3]), 7: set([3]), 8: set([3]), 9: set([3])}

I obviously don't want to add 3 to all sets, only to the set that corresponds to x[5]. Of course, I can avoid the problem by initialising x without fromkeys, but I'd like to understand what I'm missing here.

The second argument to dict.fromkeys is just a value. You've created a dictionary that has the same set as the value for every key. Presumably you understand the way this works:

>>> a = set()
>>> b = a
>>> b.add(1)
>>> b
set([1])
>>> a
set([1])

you're seeing the same behavior there; in your case, x[0], x[1], x[2] (etc) are all different ways to access the exact same set object.

This is a bit easier to see with objects whose string representation includes their memory address, where you can see that they're identical:

>>> dict.fromkeys(range(2), object())
{0: <object object at 0x1001da080>,
 1: <object object at 0x1001da080>}

You can do this with a generator expression:

x = dict( (i,set()) for i in range(10) )

In Python 3, you can use a dictionary comprehension:

x = { i : set() for i in range(10) }

In both cases, the expression set() is evaluated for each element, instead of being evaluated once and copied to each element.

Because of this from the dictobject.c:

while (_PyDict_Next(seq, &pos, &key, &oldvalue, &hash))
{
            Py_INCREF(key);
            Py_INCREF(value);
            if (insertdict(mp, key, hash, value))
                return NULL;
}

The value is your "set([])", it is evaluated only once then their result object reference count is incremented and added to the dictionary, it doesn't evaluates it every time it adds into the dict.

Unwanted behaviour from dict.fromkeys

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