[Tutor] flatten a python list

[Mats Wichmann]

On 5/30/21 9:58 PM, Manprit Singh wrote:
> Dear sir,
> 
> consider a problem to get a flat list for below given list :
> lst = [[2, 6], [5, 8], [6, 0]]
> the flatten list will be :
> ans = [2, 6, 5, 8, 6, 0]
> 
> I have seen in several texts around the internet and even in the textbooks,
> the approach followed is to use nested for loop.
> ans = []
> for ele in lst:
>    for num in ele:
>      ans.append(num)
> 
> instead of using this for loop if i write :
> ans = []
> for ele in lst:
>    ans.extend(ele)
> 
> Although the answer is correct, I need your comments on this approach.
> 
> Just for the sake of healthy discussion, i am putting this question
> Although i can do it in a very efficient manner with list comprehension as
> given below:
> ans = [num for ele in lst for num in ele]

How about this for grins, should handle the "arbitrary layout" question 
Alan raised.

def flatten(lst):
     for item in lst:
         if not isinstance(item, list):
             yield item
         else:
             yield from flatten(item)

lst = [[2, 6], [5, 8], [6, 0]]
ans = list(flatten(lst))
print(ans)
[2, 6, 5, 8, 6, 0]
# you can leave out the list() if an iterator is ok for your use

This has some flaws - but the other approaches do, too.  e.g. what if 
some item is a tuple, or a string is passed...  In a lot of cases this 
probably isn't what you want:

print(list(flatten("a string")))
['a', ' ', 's', 't', 'r', 'i', 'n', 'g']

in other words, as a general purpose approach you'd want to do some data 
validation.