遍历字典列表

Question

Would love some help on the following simple question. How can I iterate through a list of dictionaries? I would only need the keys of each dictionary. EG:

list_ = [{'negative': 'sad'}, {'negative': 'missed'},
         {'positive': 'already :D'},{'negative': 'cry'}, 
         {'negative': 'cheating'}, {'negative': 'worry'},
         {'positive': 'Chilling'},]

I would need to append the keys in the following list tweet_list

What is wrong with the following code? Apologies for the basic question, but it's mainly the data structure of the list_ that is causing me troubles.

EG:

for key in list_():
    append.tweet_list(key[0])
    print(tweet_list)

Answer 1

Note the different options in Python 2 and 3:

Code

[d.keys()[0] for d in list_]                               # python 2

[next(iter(d.viewkeys())) for d in list_]                  # python 2

[next(iter(d.keys())) for d in list_]                      # python 2/3

[k for d in list_ for k in d]                              # python 2/3

The middle options leverage dictionary views as opposed to lists. The last option is most Pythonic (suggested by @Yaroslav Surzhikov).

---

Timings

Confirmed in Python 3 (wherever possible) via %timeit -n 100000 :

1. 6.58 µs: [list(d.keys())[0] for d in list_]
2. 3.23 µs: [d.keys()[0] for d in list_]
3. 3.97 µs: [next(iter(d.viewkeys())) for d in list_]
4. 4.52 µs: [next(iter(d.keys())) for d in list_]
5. 1.75 µs: [key for d in list_ for key in d]

Contribution by @Yaroslav Surzhikov

Answer 2

You want this list comprehension:

>>> [list(d.keys())[0] for d in list_]
['negative', 'negative', 'positive', 'negative', 'negative', 'negative', 'positive']