python生成器调用方法引发异常

def fn():

    a = 0

    while True:

        a += 1

        r = yield # r 就是接收 send 参数的变量

        print('{} => {}'.format(a, r))

>>> f = fn()

>>> f.send('a') # 不传递 None 的后果

Traceback (most recent call last):

  File "/opt/python3/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 2910, in run_code

    exec(code_obj, self.user_global_ns, self.user_ns)

  File "<ipython-input-31-6f758a7cad28>", line 1, in <module>

    f.send('a')

TypeError: can't send non-None value to a just-started generator

>>> next(f) # 也可以不传递 None 而是使用 next 执行，两种方式都可以

>>> f.send('a')

1 => a

>>> f.send('b')

2 => b