I was just testing some algorithms to flatten a list, so I created 3 lists inside a list, and then tried to flatten it. I never touch the original list, the variables are named different, but when I try to see the original list, it has been modified, any idea why this is happening?
In [63]: xxx = [['R', 'L', 'D'], ['U', 'O', 'E'], ['C', 'S', 'O']]
In [64]: def flat_ind(lst):
...: one = lst[0]
...: for l in lst[1:]:
...: one += l
...: return one
...:
In [65]: flat = flat_ind(xxx)
In [66]: flat
Out[66]: ['R', 'L', 'D', 'U', 'O', 'E', 'C', 'S', 'O']
In [67]: xxx
Out[67]:
[['R', 'L', 'D', 'U', 'O', 'E', 'C', 'S', 'O'],
['U', 'O', 'E'],
['C', 'S', 'O']]
I understand that one is still pointing to the original lst and that is the reason it is modifying it, but still, I though that, since this was inside a function, it would not happen, more importantly
how do I make this not happen?
Thanks!
"I understand that one is still pointing to the original lst and that is the reason it is modifying it, but still, I though that, since this was inside a function, it would not happen,"
That doesn't make any sense. It doesn't matter where you mutate an object, it will still be mutated.
In any case, the mutation occurs because of this:
one += l
which is an in-place modification. You could use
one = on + l
instead, but that would be highly inefficient. As others have pointed out, you could just copy that first list,
one = lst[0][:]
But the idiomatic way to flatten a regularly nested list like this is to simply:
flat = [x for sub in xxx for x in sub]
Or,
from itertools import chain
flat = list(chain.from_iterable(xxx))