Can I Use A List Comprehension On A List Of Dictionaries If A Key Is Missing?

I want to count the number of times specific values appear in a list of dictionaries. However, I know that some of these dictionaries will not have the key. I do not know which tho

Solution 1:

You can explicitly check if key2 is in a dictionary:

Counter(r['key2'] for r in list_of_dicts if 'key2' in r)

Solution 2:

get lets you specify a default value to return if the key isn't present. thus:

In [186]: Counter(r.get('key2',None) for r in list_of_dicts)
Out[186]: Counter({'testing': 3, None: 3})

Where the None entry tells us how many dictionaries were missing this value. It might be nice to know that. If you don't care, it probably doesn't matter much whether you use this or the if clause.