Spyder中遇到 AstockTrading() takes no arguments

这是我的代码：

class AstockTrading(object):
    def _init_(self, strategy_name, period):
        self._strategy_name = strategy_name
        self._period = period

    def strategy(self):
        self._period = None

# instance
a1 = AstockTrading('ma', 20)
a1._period

这是控制台：

a1 = AstockTrading('ma', 20)
Traceback (most recent call last):

  File "<ipython-input-86-14c313ab3ec4>", line 1, in <module>
    a1 = AstockTrading('ma', 20)

TypeError: AstockTrading() takes no arguments