Least positive number in an array of integer numbers.

least_positive.py

def least_positive(nums):
    length = len(nums)
    is_valid = lambda e: 0 < e <= length

    def swap(i, j):
        nums[i], nums[j] = nums[j], nums[i]

    i = 0
    while i < length:
        e = nums[i]
        if is_valid(e) and nums[e - 1] != e:
            swap(i, e - 1)
        else:
            i += 1

    print nums

    for i, e in enumerate(nums):
        if not is_valid(e) or i != e - 1:
            return i + 1

    return length + 1

if __name__ == '__main__':
    print least_positive([0, 1, 3])
    print least_positive([-1, 2, 1, -3, 2, 4])
    print least_positive([0, 0, 3, 3, 3, 2, 1, 1])
    print least_positive([0, 0])
    print least_positive([])