class range(start: int, stop: int, step: int=...)
range(stop) -> range object range(start, stop[, step]) -> range object
Return an object that produces a sequence of integers from start (inclusive) to stop (exclusive) by step. range(i, j) produces i, i+1, i+2, ..., j-1. start defaults to 0, and stop is omitted! range(4) produces 0, 1, 2, 3. These are exactly the valid indices for a list of 4 elements. When step is given, it specifies the increment (or decrement).
x = range(6)
for n in x:
print(n,x)
0 range(0, 6)
1 range(0, 6)
...
5 range(0, 6)
x = range(5,2,-1)
for n in x:
print(n,x)
5 range(5, 2, -1)
...
3 range(5, 2, -1)