Remove items from dictionary if the length of the item is 1 or less

Question

Is there a way to remove a key from a dictionary using it's index position (if it has one) instead of using the actual key (to avoid e.g. del d['key'], but use index position instead)?

If there is then don't bother reading the rest of this question as that's what I'm looking for too.

So, as an example for my case, I have the dictionary d which uses lists for the values:

d = {'acd': ['cad'], 'abd': ['bad', 'dab']}

I want to remove each key completely from such dictionary whose value's items have a length of less than 2 (so if there's only 1 item).

So, in this example, I would want to remove the key 'acd' because it's value's list only has 1 item ['cad']. 'abd' has 2 items ['bad', 'dab'], so I don't want to delete it - only if it contains 1 or less item. This dictionary is just an example - I am working with a much bigger version than this and I need it to remove all of the single item value keys.

I wrote this for testing but I'm not sure how to go about removing the keys I want - or determing what they are.

d = {'acd': ['cad'], 'abd': ['bad', 'dab']}

index_pos = 0

for i in d.values():

    #Testing and seeing stuff
    print("pos:", index_pos)
    print(i)
    print(len(i))

    if len(i) < 2:

        del d[???]
        #What do I do?

    index_pos += 1

I used index_pos because I thought it might be useful but I'm not sure.

I know I can delete an entry from the dictionary using

del d['key']

But how do I avoid using the key and e.g. use the index position instead, or how do I find out what the key is, so I can delete it?

You could iterate over `d.items()` so you'll have key-value pairs. — Filip Młynarski, Nov 24 '18 at 23:15

score 12 · Accepted Answer · answered Nov 24 '18 at 23:13

12

Just use a dictionary comprehension:

d = {'acd': ['cad'], 'abd': ['bad', 'dab']}
res = {k: v for k, v in d.items() if len(v) >= 2}

Yes, you are creating a new dictionary, but this in itself is not usually a problem. Any solution will take O(n) time.

You can iterate a copy of your dictionary while modifying your original one. However, you should find the dictionary comprehension more efficient. Don't, under any circumstances, remove or add keys while you iterate your original dictionary.

answered Nov 24 '18 at 23:13

jpp

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Thanks. Should I remove the old dictionary (as I don't need it) or does it make no difference to anything? Or...? – Matt Nov 24 '18 at 23:17
1

@Mandingo, It depends what you are looking to do. If you are assigning to the same variable and no longer need the old one, you don't need to do anything further. If you still need access to the original dictionary, just use a new variable name for the filtered one. – jpp Nov 24 '18 at 23:18
Also, why shouldn't I, under any circumstances, remove or add keys whilst I iterate over my original dictionary? – Matt Nov 24 '18 at 23:30
2

@Mandingo, The [docs](https://docs.python.org/3/library/stdtypes.html#dictionary-view-objects) say: `Iterating views while adding or deleting entries in the dictionary may raise a RuntimeError or fail to iterate over all entries.` More detail here: [Modifying a Python dict while iterating over it](https://stackoverflow.com/questions/6777485/modifying-a-python-dict-while-iterating-over-it). – jpp Nov 24 '18 at 23:31
1

Thanks for the help! – Matt Nov 24 '18 at 23:41

score 3 · Answer 2 · answered Nov 24 '18 at 23:20

If you doesn't want to create new dict here's how you could change your code. Here we iterate over copy of our dict and then delete keys of our original dict if length of its value is less than 2.

d = {'acd': ['cad'], 'abd': ['bad', 'dab']}

for key in d.copy():
    if len(d[key]) < 2:
        del d[key]

print(d)

Remove items from dictionary if the length of the item is 1 or less

2 Answers2