Order Types

A market order says "execute immediately, at whatever the current best price is" — guaranteed to execute (assuming any liquidity exists), but the exact price isn't guaranteed. A limit order says "only execute at this price or better" — a limit BUY only fills at the limit price or lower; a limit SELL only fills at the limit price or higher. Guaranteed price, but NOT guaranteed to execute at all.

def would_limit_buy_execute(limit_price, market_price):
    return market_price <= limit_price

print(would_limit_buy_execute(100, 95))    # True -- market is cheaper than your limit
print(would_limit_buy_execute(100, 105))   # False -- market is more expensive, order waits

When a buy order and a sell order "cross" (the buyer's limit is at or above the seller's limit — they'd both be happy at some price in between), an exchange matches them and executes a trade:

def match_orders(buy_price, sell_price):
    if buy_price >= sell_price:
        return (buy_price + sell_price) / 2   # simplified matching price
    return None   # no overlap -- no trade happens

print(match_orders(101, 100))   # 100.5 -- buyer will pay up to 101, seller will take 100 or more
print(match_orders(99, 100))     # None -- buyer won't pay enough, no trade

Real exchanges use more nuanced matching rules (usually executing at the RESTING order's price, not a midpoint, and with strict time-priority among orders at the same price) — the midpoint here is a simplification to keep the core "do these orders overlap at all" logic clear.