How to extract items from list and use them as names of new sets of list?

Question

Suppose I have a list called "Collection":

Collection = ['a','b','c','d']

From this list, I would like to create 4 lists:

a_list = []
b_list = []
c_list = []
d_list = []

Is there a way to do something like the followings to create the above 4 lists?

for item in Collection:
    item_list = []

@juanpa.arrivillaga: thank you for your quick response.. could you please elaborate more? — Starz, Apr 25 '17 at 16:45
Use a `dict`, don't dynamically name variables. You'll end up using the global name-space dictionary, but then, you might as well just use your own dictionary. — juanpa.arrivillaga, Apr 25 '17 at 16:45
There are ways to do what you want, but I would urge you just to use a dictionary if you want to relate a string to a value. — juanpa.arrivillaga, Apr 25 '17 at 16:47

Remolten · Accepted Answer · 2017-04-25T16:59:32.950

3

Generally, it is considered a bad idea to make variable names dynamic, which is why there is no easy way to do it in Python (and other languages as well).

The closest thing you can get is by using dictionaries:

c = ['a', 'b', 'c', 'd'] 
dict_with_new_lists = {item: [] for item in c}
# Now contains: {'a': [], 'c': [], 'b': [], 'd': []}

Now to access your lists:

some_variable = 'a'
print(dict_with_new_lists[some_variable])
# etc...

edited Apr 25 '17 at 16:59

answered Apr 25 '17 at 16:48

Remolten

2,614
2
25
29

defaultdict is better ! – anati Apr 25 '17 at 16:51
1

@anati only if you want the `defaultdict` behavior. – juanpa.arrivillaga Apr 25 '17 at 16:53
@juanpa.arrivillaga I don't believe it is applicable to write create dictionary manually like in answer, suppose the list is large. – anati Apr 25 '17 at 16:56
@anati it is totally possibly to do it programmatically. – juanpa.arrivillaga Apr 25 '17 at 16:57
Remolten and @juanpa.arrivillaga: thank you for your help.. i won't do it dynamically then. My problem is that the number of items in `c = ['a','b','c','d',...]` is determined by the users and vary from one user to another. I am still not sure how to generalize the code.. – Starz Apr 25 '17 at 16:57
@Lam use 'defaultdict' – anati Apr 25 '17 at 16:58
I have edited my answer to dynamically create the dictionary. – Remolten Apr 25 '17 at 17:00
got it! thank you Remolten and @juanpa.arrivillaga: i want to put a check mark and thumbs up for your answer and comment, but my reputation is too low.. – Starz Apr 25 '17 at 17:04
You can still mark an answer as your solution by clicking the checkmark. This works regardless of your reputation. Voting up is the privilege that is unlocked later. – Remolten Apr 25 '17 at 17:06
@Remolten What is the pythonic way: dict comprehension or defualtdict ? – anati Apr 25 '17 at 17:06
Ahh ok.. Done.. I just can't check mark for both.. – Starz Apr 25 '17 at 17:11

anati · Answer 2 · 2017-04-25T17:01:17.743

2

I think the best approach is define default dictionary:

from collections import defaultdict

collection_dict = defaultdict(list)

Example:

from collections import defaultdict

collection_dict = defaultdict(list)

Collection = ['a','b','c','d']
for elt in Collection:
    collection_dict[elt].append('sample')
print(collection_dict)

> defaultdict(<class 'list'>, {'d': ['sample'], 'a': ['sample'], 'c':
> ['sample'], 'b': ['sample']})

edited Apr 25 '17 at 17:01

answered Apr 25 '17 at 16:51

anati

264
2
13

1

got it! thank you @anati: i want to put a check mark and thumbs up for your answer and comment, but my reputation is too low.. – Starz Apr 25 '17 at 17:03

How to extract items from list and use them as names of new sets of list?

2 Answers2