How can I remove a dict item from a tuple of dicts?

Question

I have a tuple of dicts in form:

mylist = ({'a': 'a0',  't': 't0'}, {'a': 'a1',  't': 't1'}, {'a': 'a2',  't': 't2'}, ...)

How can I remove a dict in the for loop? I tried the following but at the end doc is still in the tuple:

for doc in mylist:
    if doc['a']=="a0":
        del doc

Are you trying to remove an item from a list as you are iterating through that list? That's usually a very bad idea. — Scott Hunter, Mar 07 '16 at 15:29
Tuples are immutable, you will have to create a new tuple that doesn't include the relevant items. — jonrsharpe, Mar 07 '16 at 15:29
Your code is basically doing `doc = mylist[0]; del doc`..., that's not going to affect the container in any way, you're simply removing the reference `doc` within the loop. — Ashwini Chaudhary, Mar 07 '16 at 15:32
@ScottHunter Actually they are not removing the item at all. — Ashwini Chaudhary, Mar 07 '16 at 15:33
@AshwiniChaudhary: But that is what the code is *meant* to do, however unsuccessfully. — Scott Hunter, Mar 07 '16 at 15:55

score 5 · Accepted Answer · answered Mar 07 '16 at 15:30

First of all, don't use the name list, you will shadow the built in list.

Tuples are immutable, so you have to build a new tuple.

>>> t = ({'a': 'a0',  't': 't0'}, {'a': 'a1',  't': 't1'}, {'a': 'a2',  't': 't2'})
>>> tuple(d for d in t if d['a'] != 'a0')
({'a': 'a1', 't': 't1'}, {'a': 'a2', 't': 't2'})

This assumes all your dictionaries actually have the key 'a'. If that does not have to be the case, feel free to add some error checking code or use the get method of the dictionary with a fallback value in the generator expression.

NixonInnes · Answer 2 · 2016-03-07T16:44:01.020

3

To start with; you have defined a Tuple, not a List. The primary difference between the two is that Tuples are immutable. This means if you want to modify your collection, you will need to use a List:

my_list = [{'a': 'a0',  't': 't0'}, {'a': 'a1',  't': 't1'}, {'a': 'a2',  't': 't2'}]

Note the use of square brackets, rather than round.

With that knowledge, you can either choose to:

Build another Tuple, omitting the values you wish to remove:

my_tuple = ({'a': 'a0',  't': 't0'}, {'a': 'a1',  't': 't1'}, {'a': 'a2',  't': 't2'})
my_new_tuple = tuple(item for item in my_tuple if item['a'] != 'a0')

Or, use a List, and remove the values you don't want:

my_list = [{'a': 'a0',  't': 't0'}, {'a': 'a1',  't': 't1'}, {'a': 'a2',  't': 't2'}]  
for item in my_list:
    if item['a'] == 'a0':
        my_list.remove(item)

edited Mar 07 '16 at 16:44

answered Mar 07 '16 at 16:04

NixonInnes

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@MSeifert You're right, my mistake! I've corrected it now. – NixonInnes Mar 07 '16 at 16:28
@timgeb You're right; i'll add another example without the nested loops. – NixonInnes Mar 07 '16 at 16:38
2

[`list.remove`](http://stackoverflow.com/questions/17299581/loop-forgets-to-remove-some-items) is a bad suggestion. – Ashwini Chaudhary Mar 07 '16 at 17:12
@AshwiniChaudhary Apparently so, it does have a relatively high level of computational complexity, but it serves as a relatively straight-forward example to checking a list. How would you do it differently? – NixonInnes Mar 08 '16 at 13:04

How can I remove a dict item from a tuple of dicts?

2 Answers2