<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def aggregate_routes( feed: "Feed", by: str = "route_short_name", route_id_prefix: str = "route_" ) -> "Feed": """ Aggregate routes by route short name, say, and assign new route IDs. Parameters feed : "Feed" by : string A column of ``feed.routes`` route_id_prefix : string Prefix to use when creating new route IDs Returns ------- "Feed" The result is built from the given "Feed" as follows. Group ``feed.routes`` by the ``by`` column, and for each group 1. Choose the first route in the group 2. Assign a new route ID based on the given ``route_id_prefix`` string and a running count, e.g. ``'route_013'`` 3. Assign all the trips associated with routes in the group to that first route 4. Update the route IDs in the other "Feed" tables """

    if by not in feed.routes.columns:
        raise ValueError(f"Column {by} not in feed.routes")

    feed = feed.copy()

    # Create new route IDs
    routes = feed.routes
    n = routes.groupby(by).ngroups
    k = int(math.log10(n)) + 1  # Number of digits for padding IDs
    nrid_by_orid = dict()
    i = 1
    for col, group in routes.groupby(by):
        nrid = f"route_{i:0{k}d}"
        d = {orid: nrid for orid in group["route_id"].values}
        nrid_by_orid.update(d)
        i += 1

    routes["route_id"] = routes["route_id"].map(lambda x: nrid_by_orid[x])
    routes = routes.groupby(by).first().reset_index()
    feed.routes = routes

    # Update route IDs of trips
    trips = feed.trips
    trips["route_id"] = trips["route_id"].map(lambda x: nrid_by_orid[x])
    feed.trips = trips

    # Update route IDs of transfers
    if feed.transfers is not None:
        transfers = feed.transfers
        transfers["route_id"] = transfers["route_id"].map(
            lambda x: nrid_by_orid[x]
        )
        feed.transfers = transfers

    return feed