Python function with double condition

Question

I'd like to create a function that returns the elements of a list on odd positions or the negative elements of a list.

My solution works with the first assertion, but the second generates an AssertionError, because returns [-1, -2, 1] instead of [-1, -2]. Any suggestions?

def solution(input):
  output = []
  for item in input: 
    if item < 0:
        output.append(item)
    elif not item % 2 == 0:
        output.append(item)
  return output

assert solution([0,1,2,3,4,5]) == [1,3,5]
assert solution([1,-1,2,-2]) == [-1,-2]

Answer 1

You want the numbers on odd positions, but your % check is checking the actual values in the list rather than their positions.

Try using enumerate to get the index alongside the value as you iterate through the list:

def solution(input):
  output = []
  for ix, item in enumerate(input): 
    if item < 0 or ix % 2 != 0:
        output.append(item)
  return output

Answer 2

Also for completeness purpose you may want to consider adding this to your already existing code:

    if any(i < 0 for i in output):
            return [i for i in output if i < 0]

, since it tests if a negative exists and return only those if so. The answer by HumphreyTriscuit is, however, the better solution from my point of view.

Answer 3

One line to define solution function:

def solution(input):
    return [input[pos] for pos in range(len(input)) if not pos %2 == 0 or input[pos] < 0]

print solution([0,1,2,3,4,5,7])
print solution([1,-1,2,-2, -3])