data = [-2000, 1000, 2000, 3500, 3800, 4500, 4600, 5000, 6000]
lst = [min(data)]
for i in sorted(data[1:]):
    if i - lst[-1] > 999:
        lst.append(i)
print(lst)
Filtering a list. Get elements of list only with a certain distance between items?