是否可以使用列表推导从dicts的词典创建所有成员dict键的列表？

Question

Say I have a dict of dicts:

foo = {
    'category': {
        'key1': 'bar',
        'key2': 'bar',
        'key3': 'bar',
    },
    'category2': {
        'key4': 'bar',
        'key5': 'bar',
    },
}

To get a single list of all keys in the member-dicts, I have a function as follows:

def _make_list():
    baz = list()
    for key,val in foo.items():
        baz += list(val.keys())
    return baz

The generated list looks like: ['key1', 'key2', 'key3', 'key4', 'key5', ] .

This is simple enough, and it works, but I wonder: is there a way to accomplish this with a one-liner list comprehension ? The keys of the member dicts will always be unique.

Answer 1

Here's one way to do it:

>>> [k for d in foo.values() for k in d]
['key1', 'key2', 'key3', 'key4', 'key5']

Answer 2

An approach would be using itertools.chain() :

import itertools

[k for k in itertools.chain(*(d.keys() for d in foo.values()))]

If what you want is just a one line of code, and not necessarily a list comprehension, you can also try (mentioned by @Duncan):

list(itertools.chain(*foo.values()))

Output:

>>> [k for k in itertools.chain(*(d.keys() for d in foo.values()))]
['key3', 'key2', 'key1', 'key5', 'key4']
>>>
>>> list(itertools.chain(*foo.values()))
['key3', 'key2', 'key1', 'key5', 'key4']