import math
orders=[(100,"paid",0),(80,"refunded",20),(50,"cancelled",0),(None,"paid",0)]
total=0; count=0; missing=0
for amount,status,refund in orders:
    if amount is None:
        missing += 1
        continue
    if status not in {"paid","refunded"}:
        continue
    if not 0 <= refund <= amount:
        raise ValueError("invalid refund")
    total += amount-refund; count += 1
average=total/count if count else None
assert (total,count,missing,average)==(160,2,1,80)
assert math.isclose(0.1+0.2,0.3)
assert -3//2 == -2
print("total/count/missing/average",total,count,missing,average)
print("float exact equality",0.1+0.2==0.3)