python之yield表达式

yield表达式用于generator function

调用generator function时，返回一个iterator（函数内语句不被会执行），调用iterator函数时，执行到yield表达式，

当前函数暂停执行，返回表达式的值到调用者，继续调用iterator函数，从暂停处恢复执行。、

遇到yield表达式，与遇到其他表达式差不多，yield表达式也有值，一般为None。

与其他表达式的不同之处在于yield表达式会在yield处返回表达式的值

官方文档描述如下：

官方例子：

>>> def echo(value=None):
...     print("Execution starts when 'next()' is called for the first time.")
...     try:
...         while True:
...             try:
...                 value = (yield value)
...             except Exception as e:
...                 value = e
...     finally:
...         print("Don't forget to clean up when 'close()' is called.")
...
>>> generator = echo(1)
>>> print(next(generator))
Execution starts when 'next()' is called for the first time.
1
>>> print(next(generator))
None
>>> print(generator.send(2))
2
>>> generator.throw(TypeError, "spam")
TypeError('spam',)
>>> generator.close()
Don't forget to clean up when 'close()' is called.

模拟个iterator版range函数

def my_range(start, stop=None, step=1):
    if not stop:
        stop = start
        start = 0
    while start < stop:
        yield start
        start += step


if __name__ == '__main__':
    for i in my_range(10):
        print(i)
    for i in my_range(0, 10):
        print(i)
    for i in my_range(0, 10, 2):
        print(i)