# A Stock object represents purchases of shares of a stock.
class Stock:
    # Initializes a new Stock with no shares purchased.
    def __init__(self, symbol):
        self._symbol = symbol     # stock symbol, e.g. "YHOO"
        self._total_shares = 0    # total shares purchased
        self._total_cost = 0.0    # total cost for all shares
    
    # Read-only properties to access Stock's state.
    @property
    def symbol(self):
        return self._symbol
    
    @property
    def total_cost(self):
        return self._total_cost
    
    @property
    def total_shares(self):
        return self._total_shares

    # Returns the total profit or loss earned on this stock,
    # based on the given price per share.
    # pre: current_price >= 0.0
    def profit(self, current_price):
        if current_price < 0.0:
            raise ValueError("Current price cannot be negative")
        return self._total_shares * current_price - self._total_cost

    # Records purchase of the given shares at the given price.
    # pre: shares >= 0 && price_per_share >= 0.0
    def purchase(self, shares, price_per_share):
        if shares < 0 or price_per_share < 0.0:
            raise ValueError("Shares/price cannot be negative")
        self._total_shares += shares
        self._total_cost += shares * price_per_share