How do I delete elements in dictionary in a for loop

Question

I want to delete some items in a dictionary in a for loop, then I follow the codes from this How to delete items from a dictionary while iterating over it?

Looping over .keys:

b = {'a':1, 'b' : 2}
for k in b.keys():
    if b[k] == 1:
        del b[k]

Results in:

Traceback (most recent call last):
  File "<input>", line 1, in <module>
RuntimeError: dictionary changed size during iteration
b
{'b': 2}

Similarly, looping over .items():

b = {'a':1, 'b' : 2}
for k, v in b.items():
    if v == 1:
        del b[k]

Traceback (most recent call last):
  File "<input>", line 1, in <module>
RuntimeError: dictionary changed size during iteration
b
{'b': 2}

After the loop, it returns the correct result, but RuntimeError is also raised, how can I fix it?

Dimitris Fasarakis Hilliard · Accepted Answer · 2016-12-06T11:13:24.583

3

You alter d with del d[k] so iterating over it's .keys or .values makes no difference, you alter the underlying iterator and so it complains.

You could either make a list of the .keys, with list(b), or .items, with list(b.items()), essentially copying them to a new iterator that doesn't relate to the dictionary you're altering:

# list(b) would suffice instead of .keys
for k in list(b.keys()):
    if b[k] == 1:
        del b[k]

or, use a dict comprehension:

d = {k:b[k] for k in b if b[k] != 1}

to filter out unwanted elements.

edited Dec 06 '16 at 11:13

answered Dec 06 '16 at 10:28

Dimitris Fasarakis Hilliard

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`list` is unnecessary too. for k in v would have worked the same. – Pierre Barre Dec 06 '16 at 10:47
What `v` are you referring to? @user312016 – Dimitris Fasarakis Hilliard Dec 06 '16 at 11:13
Oh, I missread your question, my example was bad, b instead of v: for k in b. – Pierre Barre Dec 06 '16 at 11:22
@user312016 `for k in b` doesn't do the trick here, you're still iterating over an iterator (implicitly `d.keys()`) that's connected to `b`, the `RuntimeError` still gets raised. – Dimitris Fasarakis Hilliard Dec 06 '16 at 11:25
What I meant is for k in dict(b) would probably be more readable (assuming the dict is not too large, in which case that may impact performance). – Pierre Barre Dec 06 '16 at 11:27

Pierre Barre · Answer 2 · 2016-12-06T10:32:49.357

You can't remove elements in a dict while iterating over it.

The best way would be to use a dict comprehension:

b = {'a':1, 'b' : 2}
b = {k: v for k, v in b.items() if v != 1}

You could also make a copy of your dict while iterating over it:

b = {'a':1, 'b' : 2}
for k, v in dict(b).items():
    if v == 1:
        del b[k]

Here, you are iterating over a copy of b while modifying the original b, this is why it works: you are never removing elements from the copy.

How do I delete elements in dictionary in a for loop

2 Answers2