How can I use list comprehension to get some data from another list?

问题

Hello I have the following list :

a = [{'Hello':5, 'id':[{'cat':'billy', 'dog': 'Paul'}, {'cat':'bill', 'dog': 'Pau'}]},
     {'Hello':1, 'id':[{'cat':'Harry', 'dog': 'Peter'}, {'cat':'Hary', 'dog': 'Pete'}]}]

and I would like to build the following list (using list comprehensions):

b = ['billy', 'bill', 'Hary', 'Harry']

I tried these without success:

[x for y in a for b in y['id'] for x in b]
[x for y in a for b in y['id'] for x in b['cat']]

回答1:

If you want to use a double loop:

[x['cat'] for y in a for x in y['id'] if type(x) is  dict]

you have to :

Get values in the a list as a list (for y in a)
get the 'id' in y (for x in y['id'])
Skip the string 'eat' by filtering for dictionary (if type(x) is dict)
Access the 'cat'

If you dict x, has multiple values, you can use

[x.values() for y in a for x in y['id'] if type(x) is  dict]]

回答2:

You want

b = [x['id'][0]['cat'] for x in a]

Each element of a is a dict that looks like

{
    'Hello': ...,
    'id': [
        {
            'cat': ...
        },
        ...
    ]
}

回答3:

You can use the following list-comprehension for this:

>>> a = [{'Hello':5, 'id':[{'cat':'billy', 'dog': 'Paul'}, {'cat':'bill', 'dog': 'Pau'}]},
     {'Hello':1, 'id':[{'cat':'Harry', 'dog': 'Peter'}, {'cat':'Hary', 'dog': 'Pete'}]}]

>>> [y['cat'] for x in a for y in x['id']]
['billy', 'bill', 'Harry', 'Hary']

回答4:

you can use build-in functions itemgetter and chain (from itertools module)

from itertools import chain
from operator import itemgetter

list(map(itemgetter('cat'), chain(*map(itemgetter('id'), a))))

output:

['billy', 'bill', 'Harry', 'Hary']

or you can use a list comprehension with 2 for loops inside, first to iterate over all the dict elements in the list and the second to iterate the innermost lists and grab the element from the key 'cat':

[i['cat'] for d in a for i in d['id']]

来源：https://stackoverflow.com/questions/59971046/how-can-i-use-list-comprehension-to-get-some-data-from-another-list

标签

python

list

dictionary

list-comprehension