Python Modulo: Assign a Cyclic Roster with Signed Offsets

Start with a small, explicit task

Assign positions in a three-person fictional roster for offsets before and after the starting point. The roster repeats indefinitely as a mathematical model, but the script checks only a fixed finite set. It does not schedule real staff or determine fair workload distribution.

Run the complete example

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

roster = ["Ari", "Bo", "Cy"]
offsets = [-2, -1, 0, 1, 2, 3, 4]
indexes = [offset % len(roster) for offset in offsets]
assigned = [roster[index] for index in indexes]
assert indexes == [1, 2, 0, 1, 2, 0, 1]
assert assigned == ["Bo", "Cy", "Ari", "Bo", "Cy", "Ari", "Bo"]
assert -1 % 3 == 2 and 1 % -3 == -2
assert -1 == (-1 // 3) * 3 + (-1 % 3)
print(indexes)
print(assigned)
print(-1 % 3, 1 % -3)

Explain the result

Taking each offset modulo three gives only 0, 1 or 2, so every resulting index selects a roster member. Offset three wraps to the same position as zero, while offset -1 selects the preceding position, 2. This allows a cyclic rule without building repeated copies of the roster.

The sign comparison shows why assumptions from another language can be misleading: -1 % 3 is 2 here, while 1 % -3 is -2. The remainder is consistent with the floor-division quotient, as the final identity assertion checks. Use the positive roster length as the divisor for this indexing task.

Avoid these mistakes

An empty roster has length zero and cannot be used as the divisor; decide how that input should be rejected before assigning work. Also distinguish remainder from percentage formatting: the operator here is arithmetic, and the inputs are deliberately integers.