One problem. Many right answers.

import csv, sys

t = {}
for r in csv.reader(open(sys.argv[1])):
    t[r[0]] = t.get(r[0], 0) + float(r[1])
print(t)

"""Single-pass aggregation in integer cents, built for throughput on a hot path.

No per-row objects, no Decimal, no validation overhead: split, parse to int cents, accumulate.
Money stays exact because it never touches float.
"""
from collections import defaultdict


def total_cents_per_account(rows):
    totals = defaultdict(int)
    for acct, amount, _ts in rows:
        neg = amount[:1] == "-"
        whole, _, frac = (amount[1:] if neg else amount).partition(".")
        cents = int(whole) * 100 + int((frac + "00")[:2])
        totals[acct] += -cents if neg else cents
    return totals

"""Decimal-exact ledger aggregation with schema validation, audit logging, and a reconciliation
invariant. Built for a regulated core where correctness and traceability outrank speed."""
from __future__ import annotations

import logging
from dataclasses import dataclass
from decimal import Decimal, InvalidOperation
from typing import Iterable

logger = logging.getLogger("ledger.aggregate")


@dataclass(frozen=True)
class Transaction:
    account_id: str
    amount: Decimal
    timestamp: str


class TransactionError(ValueError):
    """A row failed validation; carries enough context for the audit trail."""


def parse_row(row: list[str]) -> Transaction:
    if len(row) != 3:
        raise TransactionError(f"expected 3 fields, got {len(row)}")
    account_id, raw_amount, timestamp = (field.strip() for field in row)
    if not account_id:
        raise TransactionError("missing account id")
    try:
        amount = Decimal(raw_amount)
    except InvalidOperation as exc:
        raise TransactionError(f"invalid amount {raw_amount!r}") from exc
    return Transaction(account_id, amount, timestamp)


def totals_per_account(rows: Iterable[list[str]]) -> dict[str, Decimal]:
    """Sum amounts per account using exact decimal arithmetic. Invalid rows are rejected and
    logged, never silently dropped, and the result is reconciled against a running grand total."""
    totals: dict[str, Decimal] = {}
    grand = Decimal("0")
    rejected = 0
    for row in rows:
        try:
            tx = parse_row(row)
        except TransactionError:
            rejected += 1
            logger.warning("rejected transaction row", exc_info=True)
            continue
        totals[tx.account_id] = totals.get(tx.account_id, Decimal("0")) + tx.amount
        grand += tx.amount
    if sum(totals.values(), Decimal("0")) != grand:
        raise AssertionError("reconciliation failed: per-account totals do not sum to grand total")
    logger.info("aggregated %d accounts (%d rows rejected)", len(totals), rejected)
    return totals

"""Aggregation that treats its input as hostile: every resource is bounded and every field is
strictly validated before use. Built for a security-critical boundary facing untrusted data."""
from decimal import Decimal, InvalidOperation

MAX_ROWS = 1_000_000        # refuse unbounded input (DoS bound)
MAX_FIELD_LEN = 64          # bound per-field length (memory + log-injection guard)
MAX_ACCOUNTS = 100_000      # bound distinct-account cardinality


def aggregate(rows):
    totals: dict[str, Decimal] = {}
    for i, row in enumerate(rows):
        if i >= MAX_ROWS:
            raise ValueError("row limit exceeded")
        if len(row) != 3:
            raise ValueError("malformed row: wrong field count")
        account_id, raw_amount, _timestamp = row
        if len(account_id) > MAX_FIELD_LEN or len(raw_amount) > MAX_FIELD_LEN:
            raise ValueError("field exceeds maximum length")
        if not account_id.isalnum():            # strict allow-list, never a blocklist
            raise ValueError("account id is not alphanumeric")
        try:
            amount = Decimal(raw_amount)
        except InvalidOperation:
            raise ValueError("amount is not a valid decimal")
        if not amount.is_finite():
            raise ValueError("amount is not finite")
        if account_id not in totals and len(totals) >= MAX_ACCOUNTS:
            raise ValueError("account cardinality limit exceeded")
        totals[account_id] = totals.get(account_id, Decimal("0")) + amount
    return totals

"""
Summing transactions, explained.

Goal: given a list of transactions, find the total amount for each account.
A transaction is a pair (account_id, amount). We keep a running total for each
account in a dictionary, adding each amount to the matching account as we go.
"""
from collections import defaultdict


def total_per_account(transactions):
    """Return the total amount for each account.

    Args:
        transactions: an iterable of (account_id, amount) pairs, for example
            [("alice", 10.0), ("bob", 12.5), ("alice", 20.0)]

    Returns:
        A dictionary mapping each account id to the sum of its amounts, for example
            {"alice": 30.0, "bob": 12.5}
    """
    totals = defaultdict(float)
    for account_id, amount in transactions:
        # defaultdict(float) starts each new account at 0.0, so we can just add.
        totals[account_id] += amount
    return dict(totals)