[Bug Report] get_volume_at_price crashes on every populated level (calls a non-existent OrderTree.get_price)

[OPTIONPOOL]

Found while running OrderBook through an open-source matching-engine benchmark, the Matching Engine Performance Challenge — it cross-checks engines against the byte-identical consensus of other open source engines. The matching itself is correct, but auditing the book through the benchmark needs a way to read resting depth, and the engine's public depth accessor crashes the moment it's pointed at a real level — querying it took this one-line fix.

OrderBook.get_volume_at_price(side, price) raises AttributeError for any price that actually has volume. After the price_exists guard passes, both branches call get_price on the side's OrderTree (orderbook/orderbook.py:184 for bids, :189 for asks):

        if side == 'bid':
            volume = 0
            if self.bids.price_exists(price):
                volume = self.bids.get_price(price).volume   # <-- no such method
            return volume

Cause. OrderTree (orderbook/ordertree.py) has no get_price method. The accessor for a price level is get_price_list (ordertree.py:23), which returns the OrderList at that price; get_price simply doesn't exist on the class. So self.bids.get_price(price) blows up with AttributeError: 'OrderTree' object has no attribute 'get_price' before .volume is ever read.

This only fires on a populated level. get_volume_at_price reaches the bad call solely through the price_exists(price) guard — when no order rests at that price the guard is false, the function returns the pre-set volume = 0, and the typo is never exercised. That's why the function looks fine until you query a price that has resting size, which is exactly what a depth read does.

Repro. Rest a single bid, then ask for its volume:

from orderbook import OrderBook
ob = OrderBook()
ob.process_order({'type':'limit','side':'bid','quantity':5,'price':100,'trade_id':1}, False, False)
ob.get_volume_at_price('bid', 100)
# AttributeError: 'OrderTree' object has no attribute 'get_price'

(get_volume_at_price('bid', 100) against an empty book returns 0 and never crashes, which is what hides the bug.)

Fix. Call the method that exists at both sites (orderbook.py:184 and :189):

                volume = self.bids.get_price_list(price).volume   # was get_price(...)

get_price_list(price) returns the OrderList resting at that price (ordertree.py:23-24), and OrderList.volume (orderbook/orderlist.py:15) is the per-level share volume kept in sync on every append/remove — i.e. exactly the number get_volume_at_price is meant to return. With this change the call returns the resting depth (e.g. 8 after two bids of 5 and 3 at the same price) instead of raising; it only fixes which method name is invoked, not what the book holds.

Happy to share the failing workload.