from abc import ABC, abstractmethod

class Strategy(ABC):
    """
    Abstract base class for all strategies.
    """

    @abstractmethod
    def generate_signals(self, data):
        """
        Generates trading signals for each stock.

        :param data: A pandas DataFrame of historical price data.
        :return: A pandas Series with the same index as the input data,
                 containing the trading signals (e.g., 'buy', 'hold', 'sell').
        """
        pass