Recursivity - Problem

[perfringo]

Some observations:

- function can return three different datatypes: None, int and str. Whatever usage follows, it must be very defensive (None if empty list, int if list with one integer, string as expected result)

- for me personally it's more natural to return only one datatype. In case of empty list it should be empty string

- it works only when elements are strings, otherwise TypeError will be raised

It's of course upon specific needs but nevertheless I would have solved this problem differently. I would have separated flattening the list and manipulating list elements.

I would have created stream of elements of flattened list:

def flat_stream(lst):
    for item in lst:
        if isinstance(item, (list, tuple)):
            yield from flat_stream(item)
        else:
            yield item

This returns generator object which is memory efficient. It preserves all elements in their original type (which is often needed).

Now I can manipulate this stream whatever way I want:

>>> ls = ["a",["b","6"],"e",["5",["g","h"]],"i",["k"]]
>>> ''.join(el for el in flat_stream(ls))
'ab6e5ghik'
>>> list(flat_stream(ls))
['a', 'b', '6', 'e', '5', 'g', 'h', 'i', 'k']

If there are different types of elements and I want to manage them differently (data often needs verifying, cleansing etc) then I can do it:

>>> ls = ["a",("b","6"),"e",[5,["g","h"]],"i",["k"]]                 # there is int and tuple in list
>>> ''.join(str(el) for el in flat_stream(ls))                       # convert all elements to str
'ab6e5ghik'
>>> ''.join(el for el in flat_stream(ls) if isinstance(el, str))     # ignore int
'ab6eghik'
>>> [el for el in flat_stream(ls) if not isinstance(el, str)]        # ignore strings
[5]