Flattening a list of strings and lists to work on each item

Question

I am currently working with different input types -

My input could either look like this:


 [term1, term2, term3,...]

or

[[term1, term2], term3]

or

[[term1, term2], [term3, term4]]

I would like to find a way to flatten it to a format over which I could loop in order to use the terms.

My code at the moment looks like this:


for entry in initial_list:
    if type(entry) is str:
       do_something(entry)
    elif type(entry) is list:
       for term in entry:
          do_something(term)

The goal would be to perform the flattening in a more Pythonic way, using list comprehension or mapping, instead of explicit for loops.

Does this answer your question? [How to make a flat list out of a list of lists?](https://stackoverflow.com/questions/952914/how-to-make-a-flat-list-out-of-a-list-of-lists) — , Mar 10 '22 at 17:53
@SembeiNorimaki - That only works if every element of the list is another list. OP has mentioned that some of their items are strings. — not_speshal, Mar 10 '22 at 18:01
@Sembei Norimaki, Not quite. The answers there assume a uniform list of lists. — ikegami, Mar 10 '22 at 18:01
@ikegami in the linked question they propose using `merged = list(itertools.chain(*myList))` which works for me in the three cases the OP requested — , Mar 10 '22 at 18:05
@SembeiNorimaki in the case of 3 strings in the list, the method you mention splits them on a character by character basis — jinx, Mar 10 '22 at 23:06
Does this answer your question? [Flatten an irregular list of lists](https://stackoverflow.com/questions/2158395/flatten-an-irregular-list-of-lists) — MisterMiyagi, May 16 '22 at 09:50
This post is being discussed [on meta](https://meta.stackoverflow.com/questions/418099/opinionated-question-for-performance-and-readability-improvements). — MisterMiyagi, May 16 '22 at 09:50

score 2 · Answer 1 · answered Mar 10 '22 at 18:00

You're very close. It might however be better to use isinstance. Try:

result = list()
for entry in initial_list:
    if isinstance(entry, str):
        result.append(entry)
    elif isinstance(entry, list):
        for term in entry:
            result.append(term)

score 1 · Accepted Answer · answered Mar 10 '22 at 18:41

1

A more concise approach is:

for entry in initial_list:
    for term in ([entry] if isinstance(entry, str) else entry):
        do_something(term)

answered Mar 10 '22 at 18:41

Kodiologist

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Flattening a list of strings and lists to work on each item

2 Answers2