Filter a dict of dict

Question

I new in Python and I am not sure it is a good idea to use dict of dict but here is my question. I have a dict of dict and I want to filter by the key of the inside dict:

Answer 1

field = 'id1'
dict( (k,{field: d[field]}) for k,d in a.items() if field in d)

Answer 2

You can do with a dictionary comprehension:

def query(data, query):
    return {key : {query : data[key][query]} 
            for key in data if query in data[key]}

You have to look at each entry of the dictionary, which can cost a lot of time if you have many entries or do this a lot. A database with a index can speed this up.

Answer 3

According to the precision you gave to Duncan, here is another filtering on a list using dictionary comprehension:

>>> my_list = ['id1', 'id2']
>>> {k1 : {k2: v2 for (k2, v2) in a[k1].iteritems() if k2 in my_list} for k1 in a}
{'key3': {'id1': [4, 5, 6]}, 'key2': {}, 'key1': {'id2': [0, 1, 2], 'id1': [0, 1, 2]}}

EDIT: you can also remove empty values with another dict compreehension, but that "begins" to be difficult to read... :-)

>>> {k3: v3 for k3, v3 in {k1 : {k2: v2 for (k2, v2) in a[k1].iteritems() if k2 in my_list} for k1 in a}.iteritems() if v3}
{'key3': {'id1': [4, 5, 6]}, 'key1': {'id2': [0, 1, 2], 'id1': [0, 1, 2]}}

Answer 4

Try this:

>>> { k: v['id1'] for k,v in a.items() if 'id1' in v }
{'key3': [4, 5, 6], 'key1': [0, 1, 2]}

For Python 2.x you might prefer to use iteritems() instead of items() and you'll still need a pretty recent python (2.7 I think) for a dictionary comprehension: for older pythons use:

dict((k, v['id1']) for k,v in a.iteritems() if 'id1' in v )

If you want to extract multiple values then I think you are best to just write the loops out in full:

def query(data, wanted):
    result = {}
    for k, v in data.items():
        v2 = { k2:v[k2] for k2 in wanted if k2 in v }
        if v2:
            result[k] = v2
    return result

giving:

>>> query(a, ('id1', 'id2'))
{'key3': {'id1': [4, 5, 6]}, 'key1': {'id2': [0, 1, 2], 'id1': [0, 1, 2]}}