Splitting a python dict from a certain key

Question

I'm trying to split a dict from a certain point in the dict. It seemed like doing a simple items_dict[3:] would work but it did not work.

items_dict = {
    "Cannon barrels":10,
    "Cannon furnace":12,
    "Candle":36,
    "Bronze arrowheads":39,
    "Iron arrowheads":40,
    "Steel arrowheads":41,
    "Mithril arrowheads":42,
    "Adamant arrowheads":4
}
print items_dict[3:] # Nope, this won't work
print items_dict["Candle"] # This will of course, but only returns the Candle's number

I only figured out how to slice a dictionary by keys that start with a certain string, but I just want to know how to slice a dictionary similar to a list.

Looks like maybe you should rethink your datastructures. I don't think it's a good idea to use an OrderedDict here. — John La Rooy, May 20 '13 at 02:54

score 4 · Answer 1 · answered May 20 '13 at 02:49

4

Dictionaries don't have order, so you can't split it from a certain point. Looking at the dictionary you have there, you can't know ahead of time what the first element will be.

answered May 20 '13 at 02:49

mgilson

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score 2 · Accepted Answer · answered May 20 '13 at 02:52

2

If you want to split after n keys - no guarantee over the order.

n=3
d1 = {key: value for i, (key, value) in enumerate(d.items()) if i < n}
d2 = {key: value for i, (key, value) in enumerate(d.items()) if i >= n}

answered May 20 '13 at 02:52

karthikr

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`viewitems` is python2.7. `items` will work on python2 or python3. – mgilson May 20 '13 at 02:53
You could also do something like: `{k:d[k] for k in islice(d,...)}` – mgilson May 20 '13 at 02:54
Using an OrderedDict as suggested in an answer below and in the comment above, this worked just as needed! – Hairr May 20 '13 at 03:12
@mgilson, islice is much better, but with an `iter(d)` so you save looping over stuff multiple times. – John La Rooy May 20 '13 at 03:36
@gnibbler -- Sorry, you're going to need to walk me through this one a bit more. How does using `islice` cause you to loop multiple times? – mgilson May 20 '13 at 12:54
@mgilson, if you do `islice(d, n)` followed by `islice(d, n, None)` you will be iterating over the first `n` of `d` twice. – John La Rooy May 20 '13 at 12:59
@gnibbler -- Oh, I got ya. You mean to split the list into 2 parts. In that case I suppose it becomes `iterd = iter(d); {k:d[k] for k in islice(iterd,n)}; {k:d[k] for k in iterd}` – mgilson May 20 '13 at 13:03

score 2 · Answer 3 · answered May 20 '13 at 03:06

2

If you want a dictionary where the keys are stored in order, use collections.OrderedDict

http://docs.python.org/2/library/collections.html#collections.OrderedDict

answered May 20 '13 at 03:06

xuanji

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score 1 · Answer 4 · answered May 20 '13 at 02:51

1

items = [
    ("Cannon barrels",10),
    ("Cannon furnace",12),
    ("Candle",36),
    ....
   ]

items_dict = dict(items)

items_3_dict = dict(items[3:])

doesnt exactly answer your question (see @mgilson answer) , but provides a path forward

answered May 20 '13 at 02:51

Joran Beasley

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HennyH · Answer 5 · 2013-05-20T03:21:37.920

1

ditems = items_dict.items()
d1, d2 = dict(ditems[:3]), dict(ditems[3:])

print(d1)
print(d2)
{'Iron arrowheads': 40, 'Adamant arrowheads': 4, 'Mithril arrowheads': 42}
{'Candle': 36, 'Cannon barrels': 10, 'Steel arrowheads': 41, 'Cannon furnace': 12, 'Bronze arrowheads': 39}

Or creating a function to split an iterable about an n-th value

from itertools import islice

def split(iterable,point): 
    return islice(iterable,None,point), islice(iterable,point,None)

d1, d2 = (dict(segment) for segment in split(items_dict.items(),3))

This will split it about the third entry.

edited May 20 '13 at 03:21

answered May 20 '13 at 03:02

HennyH

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no it wont ... a dict has no concept of "third entry" ... it will create a dict with 3 of the items and another dict with the rest ... but there is no concept of "third entry" – Joran Beasley May 20 '13 at 03:10
@JoranBeasley (yes it's unorderded) but it still has N (k,v) pairs which I meant by entry, so even though the 3rd pair is indeterminate it still has a third pair. – HennyH May 20 '13 at 03:19

score 0 · Answer 6 · answered May 28 '13 at 09:42

You could make a custom class (example only - slicing only works for [1:4] syntax, and dict[missing_key] returns None instead of throwing an exception):

>>> class SliceDict(collections.OrderedDict):
...     def __getitem__(self, val):
...         if isinstance(val, slice):
...             return {key: value for i, (key, value) in enumerate(d.items()) if val.start < i < val.stop}
...         else:
...             return self.get(val)

Now we can add your items dict:

>>> d = SliceDict(items_dict)
SliceDict([('Adamant arrowheads', 4), ('Mithril arrowheads', 42), ('Iron arrowheads', 40), ('Candle', 36), ('Cannon barrels', 10), ('Steel arrowheads', 41), ('Cannon furnace', 12), ('Bronze arrowheads', 39)])
>>> d[1:5]
{'Candle': 36, 'Cannon barrels': 10, 'Iron arrowheads': 40}
>>> d['Candle']
36

Splitting a python dict from a certain key

6 Answers6