delete all keys except one in dictionary

问题:

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

回答1:

This is quite fast:

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

回答2:

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.

回答3:

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.

回答4:

pop() it via for loop like this

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