Python range Function: Exclusive Stop and Descending Steps

Start with a small, explicit task

Make synthetic chart ticks at five-unit intervals. Produce ascending and descending versions, then check a step whose direction cannot reach the stop. No charting dependency is needed because the task is to verify the exact labels before displaying them.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_range.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

ticks = range(5, 20, 5)
ascending = list(ticks)
descending = list(range(15, 0, -5))
assert ascending == [5, 10, 15]
assert descending == [15, 10, 5]
assert list(range(5, 20, -5)) == []
assert len(ticks) == 3 and ticks[1] == 10
assert list(ticks) == ascending
try:
    range(0, 3, 0)
except ValueError:
    result = "zero step rejected"
else:
    raise AssertionError("expected invalid step")
print(ascending)
print(descending)
print(result)

Explain the result

The ascending sequence contains 5, 10 and 15 but excludes 20. The descending sequence starts at 15 and stops before zero. The negative step in range(5, 20, -5) moves away from the larger stop, so no value satisfies that sequence’s direction and the result is empty.

The example obtains length and an indexed item directly from ticks, then iterates it again. Unlike a consumed map iterator, this range can be iterated repeatedly. That does not mean you should materialize a huge range merely to run a loop: the list conversion here is for a deliberately tiny checked display.

Avoid these mistakes

A zero step is invalid rather than an infinite-repeat instruction. Ordinary range uses integers, not fractional increments; choose a different explicit representation when the task requires decimal steps. Check both direction and excluded stop instead of fixing an off-by-one error by blindly adding one.