defaultdict equivalent for lists

Question

Is there\How would you build an equivalent of python's very useful collections.defaultdict?

Imagined usage of such a container:

>>> a = collections.defaultlist(0)
>>> a[2]=7
>>> a[4]='x'
>>> a
[0,0,7,0,'x']

UPDATE: I've added a follow up question to add even more functionality to this construct

@Josh Lee: Did you read the tags at all? No JavaScript at all-- this is a Python question. — Platinum Azure, Jan 03 '12 at 22:31
@CatPlusPlus - Half because I need it for something specific, half out of curiosity :) — Jonathan Livni, Jan 03 '12 at 22:34
@CatPlusPlus - Due to a series of annoying constraints, which I should definitely remove by a future refactoring effort, I _currently_ need _exactly_ this — Jonathan Livni, Jan 03 '12 at 22:40
constraints which should be refactored out (I know guys, please don't give me a rough time about it): This object travels around the system where many modules (consumers) depend on it having at least a tuple behavior, giving semantic meaning to items' index and to the list's length. Producers of this object don't know in advance its length and iterate over various sources while building it. Basically I'm trying to encapsulate a reoccuring list building logic which I could rewrite several times, but why? especially after I've discovered the incredible ease of using `defaultdict`... — Jonathan Livni, Jan 04 '12 at 13:36
@wim - I think Finn's answer gives an intuitive approach to assignment to `-1` index by keeping it simple - simple access index `-1` without any special behavior — Jonathan Livni, Jan 04 '12 at 13:46

score 12 · Accepted Answer · edited Jan 03 '12 at 23:55

12

I think this would be a bit confusing to use; however, here's my first thought on how to do it:

class defaultlist(list):
    def __init__(self, fx):
        self._fx = fx

    def __setitem__(self, index, value):
        while len(self) <= index:
            self.append(self._fx())
        list.__setitem__(self, index, value)

This takes a callable (I think that's how defaultdict works) for the default value.

When I run:

a = defaultlist(int)
print a
a[2] = 7
a[4] = 'x'
print a

I get back:

[]
[0, 0, 7, 0, 'x']

edited Jan 03 '12 at 23:55

Ethan Furman

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answered Jan 03 '12 at 23:08

Finn

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The question seems to want the default to be a value, rather than a callable (so in that respect, different than `defaultdict`). It would be trivial to adapt yours to fit the question (append `self._fx` rather than `self._fx()`). – John Y Jan 03 '12 at 23:15
Now try `a[4]='x'` `a[2]=7` and see the bug in this code. Easy to fix though. – Mark Ransom Jan 03 '12 at 23:32
It wasn't my fix - thank Ethan. – Mark Ransom Jan 04 '12 at 01:20
@MarkRansom: finding the bug is usually the harder part. – Ethan Furman Jan 04 '12 at 02:40
You're right, `defaultdict` does take only callables, therefor so should `defaultlist`. You could always use `lambda:0` instead of `0` – Jonathan Livni Jan 04 '12 at 13:28
@Jonathan: If `0` is what you want, just use `int`. – Ethan Furman Jan 04 '12 at 17:58
To mirror `defaultdict` properly, it should also handle `__getitem__` e.g. if `x = defaultlist(list)` I want to do `x[3].append(2)` or `x[5] += [0, 1]` – Arthur Tacca Dec 07 '21 at 12:55

Gregg Lind · Answer 2 · 2012-01-06T16:08:58.010

4

If all you need is indexed access and not slicing / append, etc, then just use a defaultdict.

(if you really want perl / js semantics on this, you could subclass list __get__ and __set__)

edited Jan 06 '12 at 16:08

answered Jan 03 '12 at 22:41

Gregg Lind

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score 2 · Answer 3 · answered Nov 11 '15 at 07:22

A slightly enhanced version from answer by @Finn.

class defaultlist(list):
    """List returning default value when accessing uninitialized index.

    Original implementation: http://stackoverflow.com/a/8719940/315168
    """

    def __init__(self, fx):
        self._fx = fx

    def __setitem__(self, index, value):
        while len(self) <= index:
            self.append(self._fx())
        list.__setitem__(self, index, value)

    def __getitem__(self, index):
        """Allows self.dlist[0] style access before value is initialized."""
        while len(self) <= index:
            self.append(self._fx())
        return list.__getitem__(self, index)

score 2 · Answer 4 · answered Jan 04 '12 at 00:14

My proposal:

def xtend(f):
    def wrap(self, index, *args):
        if len(self) <= index:
            self.extend([self._gen()] * (index - len(self) + 1))
        return f(self, index, *args)
    return wrap

class defaultlist(list):
    def __init__(self, gen, lst = []):
        list.__init__(self, lst)
        self._gen = gen

    __setitem__ = xtend(list.__setitem__)
    __getitem__ = xtend(list.__getitem__)

Results:

>>> a = defaultlist(int, [1, 2, 3])
>>> a[10] = 'x'
>>> a[2] = 7
>>> print a
[1, 2, 7, 0, 0, 0, 0, 0, 0, 0, 'x']

score 0 · Answer 5 · edited Nov 13 '12 at 11:21

0

Perhaps the easiest way is using a dict:

>>> a = {}
>>> a[2] = 7
>>> a[4] = 'x'
>>> [a[i] if i in a else 0 for i in xrange(max(a) + 1)]
[0, 0, 7, 0, 'x']

edited Nov 13 '12 at 11:21

answered Jan 04 '12 at 00:40

fransua

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defaultdict equivalent for lists

5 Answers5

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