import pandas as pd

row_count = 0
amount_total = 0
region_parts = []

with pd.read_csv(
    "orders.csv",
    usecols=["region", "amount"],
    dtype={"region": "string", "amount": "int64"},
    chunksize=3,
) as reader:
    for chunk_number, chunk in enumerate(reader, start=1):
        chunk_amount = chunk["amount"].sum()
        row_count += len(chunk)
        amount_total += chunk_amount
        region_parts.append(chunk.groupby("region")["amount"].sum())
        print(f"chunk {chunk_number}: rows={len(chunk)} amount={chunk_amount}")

region_totals = pd.concat(region_parts).groupby(level=0).sum().sort_index()

print(f"rows processed={row_count}")
print(f"amount total={amount_total}")
print("region totals:")
print(region_totals.to_string())