Python conditions and loops: distinguish no value from an empty value

Operation contract

The import accepts integer amounts greater than zero. It skips None, zero and negative adjustments using explicit checks, then accumulates the accepted values. A truth check alone could collapse an absent amount and a valid domain value of zero; the program instead names the rejection policy. It also uses for-else to report that no accepted amount exceeded the review threshold.

Failure and ownership boundary

Loop else runs when the loop ends without break. It does not mean the condition inside the loop was false on every iteration. Modifying the input collection while walking it can skip or invalidate work; build a separate result or define an iterator-local edit contract. Python lists: slicing copies the outer sequence, not nested objects and Python input exercise: accept an explicit integer grammar extend this boundary.

Working program

amounts = [None, 0, 125, -10, 75]
accepted = []
for amount in amounts:
    if amount is None or amount <= 0:
        continue
    accepted.append(amount)
print(sum(accepted))
for amount in accepted:
    if amount > 500:
        print("manual review")
        break
else:
    print("no review threshold reached")

Output

200
no review threshold reached

Costs and limits

For n inputs the filtering takes O(n) time and retains O(k) accepted values. A running total alone could use bounded storage, but this fixture keeps values for the second inspection.

Common Mistakes

• Do not equate None with every false-valued domain input.
• Loop else describes break behavior.

Connected lessons

Python lists: slicing copies the outer sequence, not nested objects, Python input exercise: accept an explicit integer grammar, Python generators: lazy iteration does not make retained output free.

Follow the related contract

Python break, continue and loop else: encode the stopping condition, Python range and enumerate: indexes are not stable record identities.