delete all keys except one in dictionary

Question

I have a dictionary

lang = {'ar':'arabic', 'ur':'urdu','en':'english'}

What I want to do is to delete all the keys except one key. Suppose I want to save only en here. How can I do it ? (pythonic solution)
What I have tried:

In [18]: for k in lang:
   ....:     if k != 'en':
   ....:         del lang_name[k]
   ....

Which gave me the run time error:RuntimeError: dictionary changed size during iteration

Fabian · Accepted Answer · 2012-09-12 12:25:33Z

50

Why don't you just create a new one?

lang = {'en': lang['en']}

Edit: Benchmark between mine and jimifiki's solution:

$ python -m timeit "lang = {'ar':'arabic', 'ur':'urdu','en':'english'}; en_value = lang['en']; lang.clear(); lang['en'] = en_value"
1000000 loops, best of 3: 0.369 usec per loop

$ python -m timeit "lang = {'ar':'arabic', 'ur':'urdu','en':'english'}; lang = {'en': lang['en']}"
1000000 loops, best of 3: 0.319 usec per loop

Edit 2: jimifiki's pointed out in the comments that my solution keeps the original object unchanged.

edited Sep 12, 2012 at 12:25

answered Sep 12, 2012 at 11:53

Fabian

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8 Comments

jimifiki Over a year ago

Because he wants to modify the dictionary he access HERE as lang and elsewhere as something.attributeName. Maybe.

Fabian Over a year ago

@jimifiki something.attributeName?

jimifiki Over a year ago

let say he writes lang = someObject.attributeDict; lang = {'en':lang['en']}. Does someObject.attributeDict gets affected by these two lines?

Fabian Over a year ago

@jimifiki nope, and the OP did not state that he wants to do something like that.

Fabian Over a year ago

@Marko yep, it's built-in and you can use it from CLI and in Python files: docs.python.org/library/timeit.html

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jimifiki · Accepted Answer · 2012-09-12 12:10:51Z

36

This is quite fast:

En_Value = lang['en']
lang.clear() 
lang['en'] = En_Value

answered Sep 12, 2012 at 12:10

jimifiki

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Comments

thegrinner · Accepted Answer · 2012-09-12 11:55:14Z

11

Iterate over keys() instead:

for k in lang.keys():
    if k != 'en':
        del lang_name[k]

If you're using Python 3 I believe you need to use list(lang.keys()) instead.

answered Sep 12, 2012 at 11:55

thegrinner

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Comments

nehem · Accepted Answer · 2017-10-10 04:22:28Z

8

pop() it via for loop like this

[s.pop(k) for k in list(s.keys()) if k != 'en']

answered Oct 10, 2017 at 4:22

nehem

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1 Comment

Cristian Ciupitu Over a year ago

Calling pop so many times is going to be slow.

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