update certain key-value pairs of a dict from another dict

[Peter Otten]

Tim Chase wrote:

> On 2016-11-11 11:17, Daiyue Weng wrote:
>> dict1 = {'A': 'a', 'B': 'b', 'C': 'c'}
>> dict2 = {'A': 'aa', 'B': 'bb', 'C': 'cc'}
>> 
>> I am wondering how to update dict1 using dict2 that
>> 
>> only keys 'A' and 'B' of dict1 are udpated. It will result in
>> 
>> dict1 = {'A': 'aa', 'B': 'bb', 'C': 'c'}
> 
> Use dict1's .update() method:
> 
>>>> dict1 = {'A': 'a', 'B': 'b', 'C': 'c'}
>>>> dict2 = {'A': 'aa', 'B': 'bb', 'C': 'cc'}
>>>> desired = {'A', 'B'}
>>>> dict1.update({k:v for k,v in dict2.items() if k in desired})
>>>> dict1
> {'C': 'c', 'B': 'bb', 'A': 'aa'}
> 
> 
> or do it manually
> 
>   for k in dict2:
>     if k in desired:
>       dict1[k] = dict2[k]

If desired is "small" compared to the dicts:

>>> for k in desired & dict1.keys() & dict2.keys():
...     dict1[k] = dict2[k]
... 
>>> dict1
{'A': 'aa', 'C': 'c', 'B': 'bb'}

The same using update(), with a generator expression that avoids the 
intermediate dict:

>>> dict1 = {'A': 'a', 'B': 'b', 'C': 'c'}
>>> dict1.update((k, dict2[k]) for k in desired & dict1.keys() & 
dict2.keys())
>>> dict1
{'A': 'aa', 'C': 'c', 'B': 'bb'}

As written this will add no new keys to dict1. If you want to allow new keys 
use 

desired & dict2.keys()

as the set of keys.