Python delete items from dictionary who's value fit a function?

Question

Naivelly I would use:

for k, v in dictionary.items():
    if foo(v):
        del dictionary[k]

However this will cause an iterator exception as I edit the dictionary size while iterating over it. So how could I do this? - and especially, without making a copy of the dictionary, the copy approach:

copydict = dict(dictionary)
for k, v in copydict.items():
    if foo(v):
        del dictionary[k]

I wish to see how python would do such in place editing without actually using explicit iterators.

As an aside... `copydict = dictionary.copy()` is preferred... (it has its own `.copy` for a reason) — Jon Clements, Oct 25 '14 at 00:12

Padraic Cunningham · Accepted Answer · 2014-10-25T00:34:23.417

3

Find all the keys to remove then delete after:

d = {1: 2, 3: 4, 5: 6}
remove = [k for k  in d if foo(d[k])]
for k in remove:
    del d[k]


In [38]: %%timeit
   ....: d = {1: 2, 3: 4, 5: 6}
   ....: matching_items = ((k, v) for k, v in list(d.items()) if v > 2)
   ....: d.clear()
   ....: d.update(matching_items)
   ....: 
100000 loops, best of 3: 2.85 µs per loop

In [40]: %%timeit
        d = {1: 2, 3: 4, 5: 6}
        remove =[k for k  in d if d[k]> 2]
        for k in remove:
           del d[k]
   ....: 
1000000 loops, best of 3: 873 ns per loop

edited Oct 25 '14 at 00:34

answered Oct 25 '14 at 00:08

Padraic Cunningham

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I suspected mine was a bad idea, but you'd already nicked the good idea... was experimenting... thanks for the timeit's - have a +1 :) – Jon Clements Oct 25 '14 at 01:08

score 1 · Answer 2 · edited May 23 '17 at 12:11

1

Here is an answer from a previous Stack Overflow question: Modifying a Python dict while iterating over it

dictionary.items() returns a view which can't be modified But if you iterate over list(dictionary.items()) you can modify the dictionary without the risk of iterator exception.

edited May 23 '17 at 12:11

Community

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answered Oct 25 '14 at 00:07

cshortt

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All links in that answer lead to python 2.x.. In python 3 iteritems deprecated, and items() was changed to act like iteritems (from what I understand). So is this advice still true for 3.3? – paul23 Oct 25 '14 at 00:10
1

items is an iterator in python 3 – Padraic Cunningham Oct 25 '14 at 00:11
@PadraicCunningham Don't you mean this answer is incorrect then for python 3? – paul23 Oct 25 '14 at 00:15
you are right: here is the python 3 docs: https://docs.python.org/3/library/stdtypes.html#dict-views – cshortt Oct 25 '14 at 00:15
@paul23,yes you cannot iterate over dict.items in python 3 and delete a key – Padraic Cunningham Oct 25 '14 at 00:17
it works if you cast `dict.items()` to a list as such: `list(dict.items())` -- changed the answer code. – cshortt Oct 25 '14 at 00:23
still won't work, you cannot iterate over and change the dict – Padraic Cunningham Oct 25 '14 at 00:33

score 0 · Answer 3 · edited Oct 25 '14 at 00:30

0

for k, v in dictionary.viewkeys():
     if f(dictionary[k]):
         del dictionary[k]

edited Oct 25 '14 at 00:30

answered Oct 25 '14 at 00:04

flyingfoxlee

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viewitems doesn't allocate memory for an actual copy? – paul23 Oct 25 '14 at 00:07
In fact only keys are needed, so viewkeys is suffice. – flyingfoxlee Oct 25 '14 at 00:12
again I guess this is for python 2.x? And not working with python's 3.x's new view objects? – paul23 Oct 25 '14 at 00:20
@paul23 you just use `.keys()` in 3.x – Jon Clements Oct 25 '14 at 00:23

Python delete items from dictionary who's value fit a function?

3 Answers3