Cannot rename Python dict key values with pop - Bug?

Question

I cannot rename the Python dict using pop as described below. I've used suggestions from this post. Is this a Python bug? I'm using Python 3.5.2

>>> d = {'a': 'ABC', 'b': 'CDE', 'c': 'KRT'}
>>> for old_key in d.keys():
        d['NEW_KEY_' + old_key] = d.pop(old_key)
>>> d

{'NEW_KEY_NEW_KEY_NEW_KEY_b': 'CDE', 'NEW_KEY_NEW_KEY_a': 'ABC', 'c': 'KRT'}

@roganjosh presumabley the fact that `'NEW_KEY_NEW_KEY_NEW_KEY_b'` exists as does `'c'` — FHTMitchell, Aug 30 '18 at 10:42
@FHTMitchell You're probably right. It's still not buggy, it's just more fuel for the argument of not changing something while iterating over it :) — roganjosh, Aug 30 '18 at 10:44
For what it's worth, I'm unable to reproduce this in a Docker `python:3.5.2` container (it is reproable in `3.4.9`). — AKX, Aug 30 '18 at 10:45

score 2 · Accepted Answer · answered Aug 30 '18 at 10:43

2

In Python 3, you have to change for old_key in d.keys() into for old_key in list(d.keys()). That should work because you are iterating over a dynamic entity.

answered Aug 30 '18 at 10:43

honza_p

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This needs a bit more explanation IMO. How `dict.keys()` is a view and why you shouldn't mutate something you are iterating over. – FHTMitchell Aug 30 '18 at 10:44
You are welcome to update the answer. Ironically, the code posted in the question works for me already ;-) – honza_p Aug 30 '18 at 10:45

score 1 · Answer 2 · answered Aug 30 '18 at 10:44

1

It's mentioned in the documentation that changing a dictionary while iterating over it might fail to produce correct results. You'd better write something like

new_dict = {'NEW_KEY_' + key: value for key, value in old_dict.items()}

answered Aug 30 '18 at 10:44

blue_note

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This would be better, yes, unless the OP actually does need to modify the original dictionary in-place. – AKX Aug 30 '18 at 10:45
@AKX: in that case, he can just replace it. `old_dict = {...}` – blue_note Aug 30 '18 at 10:46
No, that just assigns a new value to the name `old_dict`. It's not the same as mutating the original dictionary. – AKX Aug 30 '18 at 10:46
Nice solution as well. Works perfectly. – HLopes Aug 30 '18 at 13:11

jpp · Answer 3 · 2018-08-30T10:53:32.250

1

You are changing your keys as you iterate over a view of them. That's not recommended.

Instead, you can take a copy of your keys and iterate over them. Since dictionary keys are unique, you can use list or set:

d = {'a': 'ABC', 'b': 'CDE', 'c': 'KRT'}

for old_key in list(d):
    d['NEW_KEY_' + old_key] = d.pop(old_key)

# {'NEW_KEY_a': 'ABC', 'NEW_KEY_b': 'CDE', 'NEW_KEY_c': 'KRT'}

You can, of course, use a dictionary comprehension, which you should find more efficient.

edited Aug 30 '18 at 10:53

answered Aug 30 '18 at 10:50

jpp

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@roganjosh, Don't see why not. Do you see a problem with this? The important thing is you aren't iterating over a view. – jpp Aug 30 '18 at 10:52
So the dictionary comprehension is the *recommended* method. But not everyone is comfortable with them. Where performance isn't a concern a regular `for` loop with `pop` is entirely fine. – jpp Aug 30 '18 at 10:54

Cannot rename Python dict key values with pop - Bug?

3 Answers3