Consider a list L: list[int | None]. Find idx such that isinstance(L[idx], int) and all(i is None for i in L[idx + 1 :]) are both True.
My solution:
idx = [i for i, e in enumerate(L) if e is not None][-1]
For extra credit, idx should be 0 if all(i is None for i in L).
Your specification is incomplete. For cases without non-None elements, itâs impossible. (You cover that in the âextra creditâ version, but not in the main version.)
def ints(L):
return [i for i, e in [(0, 0)] + list(enumerate(L)) if e is not None][-1]
Returns 0 if L is empty or if it contains only Nones.
Does lint apply to golfing? e != None is shorter than is not None
Oh and this works
[(0, 0), *enumerate(L)]
True 49 [i for i, e in enumerate(L) if e is not None][-1]
True 40 max(i for i,x in enumerate(L)if x!=None)
True 39 len(L)+~[*map(type,L)][::-1].index(int)
True 32 ~[*map(type,L)][::-1].index(int)
Tester:
L = [None, 3, None, 1, None, 4, 0, None, None, None, None]
codes = '''\
[i for i, e in enumerate(L) if e is not None][-1]
max(i for i,x in enumerate(L)if x!=None)
len(L)+~[*map(type,L)][::-1].index(int)
~[*map(type,L)][::-1].index(int)
'''.splitlines()
for c in codes:
idx = eval(c)
correct = isinstance(L[idx], int) and all(i is None for i in L[idx + 1 :])
print(correct, len(c.strip()), c)
Note that this result does not distinguish between a case where only the element at index 0 is not None and a case where all elements are None.
What should be the result if the list is empty?
I donât think this works? I get -5 on your test. I guess the point is that this is still technically correct, just not the index you might expectâŠ
43 chars, 50 with bonus (was 44/51; removed a space)
''.join('ab'[l==None]for l in L).rfind('a') # -1 if L is all None
max(0,''.join('ab'[l==None]for l in L).rfind('a')) # 0 if L is all None
I donât think Iâm going to beat @Stefan2âs, though. That is some perl-level golf. 
Argh, it actually sometimes âfailsâ the test as written, for example for L = [3], since I compute -1 and then all(i is None for i in L[idx + 1 :]) is false. But I claim the test is wrong and the index is correct 
Tests all (solvable) cases with up to 10 elements of None, 0 and 1.
Using all(i is None for i in L[idx + 1 or len(L):]) (added or len(L)), to accept idx = -1 when thatâs where the last non-None is.
from itertools import product
codes = '''\
[i for i, e in enumerate(L) if e is not None][-1]
max(i for i,x in enumerate(L)if x!=None)
len(L)+~[*map(type,L)][::-1].index(int)
~[*map(type,L)][::-1].index(int)
'''.splitlines()
Ls = []
for n in range(11):
it = product((None, 0, 1), repeat=n)
next(it)
Ls += it
for c in codes:
cc = compile(c, '', 'eval')
correct = all(
isinstance(L[idx], int) and all(i is None for i in L[idx + 1 or len(L):])
for L in Ls
for idx in [eval(cc)]
)
print(correct, len(c.strip()), c)
I think thatâs already covered by the âextra creditsâ case. If L==[] then all(i is None for i in L) is True.
Traversing from the back should always win (regarding number of operations). This needs to be done in a function call so we can return immediately rather than finish entire traversal
def find_not_none(L: list[int | None]):
for i, val in enumerate(reversed(L)):
if val is not None:
return len(L) - 1 - i
return 0 # For your extra credit
idx = find_not_none(L)
Code golfing isnât about number of operations, though. Frequently the winners are rather the opposite, brute force solutions using huge numbers of operations even when few would suffice.
Ah I actually did not know what is code golfing. I assumed it is another version of LeetCode challenge 
Not really:
idx = next((len(L) - 1 - i for i, val in enumerate(reversed(L)) if val is not None), 0)
I think usually thatâs allowed.
Not counting the idx= (since I also didnât previously, and no real golfer would use such a long variable name anyway :-)).
while L.pop()==None:0
idx=len(L)
Although this would fail on [] or [None]
Yes, Iâm only doing the main version, not the "extra creditâ version.
The list must contain a non-None value, otherwise the task wouldâve specified what to do if thatâs not the case. And Alexanderâs original code, declared to be a solution, likewise âfailsâ on such unsolvable input. Put differently, as itâs not specified what to do in such cases, anything is allowed with them, so itâs not failing.