python生成器调用方法抛出异常

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