How to join strings in an object list

Question

', '.join(strings) can be used to concatenate all strings in a list.

Now I have a list storing objects, and I'd like to join their names together. e.g.:

>>> a=[{'name': 'John', 'age': '21'},{'name': 'Mark', 'age': '25'}]
>>> a[0]['name'] + ', ' + a[1]['name']
'John, Mark'

Is there a simple way like join() above to do this?

`', '.join([x['name'] for x in a])` I doubt there's anything simpler. — vaultah, Jul 21 '14 at 05:31
@vaultah It would be simpler in some sense to remove the square brackets in your answer, and just use `','.join(x['name'] for x in a)`. — Ray Toal, Jul 21 '14 at 05:35
@RayToal brackets make the performance difference in case of `join`. — vaultah, Jul 21 '14 at 05:37
Thanks @vaultah, you should post it as an answer, even you might think it is simple :) Can you elaborate why it has performance difference? — Deqing, Jul 21 '14 at 05:39
Indeed the lists vs. generators will differ in performance. I was playing on "simpler" in terms of the number of characters in the solution and probably should have had a smiley. :) — Ray Toal, Jul 21 '14 at 05:42
@Deqing Please check my answer now, I included a link to another answer where the reason for the performance difference is discussed. — thefourtheye, Jul 21 '14 at 05:52

score 3 · Accepted Answer · edited May 23 '17 at 11:50

You have list of dictionaries not list of objects. We can join them together like this

print ", ".join(d["name"] for d in a)

d["name"] for d in a is called a generator expression, str.join will get the values from the generator expression one by one and join all of them with , in the middle.

But, if the list of dictionaries is too big then we can create a list of strings ourselves and pass it to the str.join like this

print ", ".join([d["name"] for d in a])

Here, [d["name"] for d in a] is called list comprehension, which iterates through a and creates a new list with all the names and we pass that list to be joined with ,.

I just noticed the discussion in the comments section. Please check this answer to know, why List Comprehension is better than GenExp in string joining.

How to join strings in an object list

1 Answers1