python yield 函数是如何工作的？

Question

def func():
    output = 0
    while True:
        new = yield output
        output = new


genr = func()
print(next(genr))
print(next(genr))
print(next(genr))

输出：

0
None
None

我想的是：

genr=func() return 一个生成器，但实际上运行不是它。
first print(next(genr)) 运行从 func 开始到 yield output，但还没有分配回 new，所以输出 0 是有意义的。
第二个print(next(genr))从将output赋值回new开始，下一行output = new将output和new都赋值为0 ，接下来执行yield output应该return0，但为什么它return None实际上？

Answer 1

yield expression is the value sent in by the generator.send() 函数的结果，next(gen) 等同于 gen.send(None)。因此，每次调用 next() 时，new 都会收到值 None。

如果改为这样做：

gen = func()
print(next(gen))     # gets the first value of 'output'
print(next(gen))     # send in None, get None back
print(gen.send(10))  # send in 10, get 10 back
print(gen.send(20))  # send in 20, get 20 back

你会得到这个输出：

0
None
10
20

Answer 2

A yield 语句像 return 一样用于 return 一个值，但它不会破坏堆栈帧（部分知道当前行、局部变量和未决 try-statements) 的函数。这样可以让函数在yield后恢复。

当您调用一个包含 yield 的函数时，它 return 是一个 "generator" 允许您运行代码直到一个 yield 然后从它停止的地方恢复它。

>>> def squares(n):
        for i in range(n):
            yield i ** 2

>>> g = squares(5)             # create the generator
>>> g
<generator object squares at 0x106beef10>
>>> next(g)                    # run until the first yield
0
>>> next(g)                    # resume after the yield
1
>>> next(g)                    # resume after the yield
4
>>> next(g)                    # resume after the yield
9
>>> next(g)                    # resume after the yield
16
>>> next(g)                    # looping is terminated with a StopIteration
Traceback (most recent call last):
  File "<pyshell#13>", line 1, in <module>
    next(g)                    # looping is terminated with a StopIteration
StopIteration

有趣的是，生成器可以使用 send() 方法接受值。要为这种发电机启动泵，第一个调用应该是 next().

>>> def capitalize():
        word = 'start'
        while word != 'quit':
            word = yield word.capitalize()

>>> g = capitalize()
>>> next(g)                      # run to the first yield
'Start'
>>> g.send('three')              # send in a value to be assigned to word
'Three'
>>> g.send('blind')              # send in a value to be assigned to word
'Blind'
>>> g.send('mice')               # send in a value to be assigned to word
'Mice'
>>> g.send('quit')               # send in a control value
Traceback (most recent call last):
  File "<pyshell#28>", line 1, in <module>
    g.send('quit')               # send in a control value
StopIteration

你的例子中 figured-out 是 next(g) 与 g.send(None) 相同。

以下是the docs要说的：

The value of the yield expression after resuming depends on the method which resumed the execution. If __next__() is used (typically via either a for or the next() builtin) then the result is None. Otherwise, if send() is used, then the result will be the value passed in to that method

这里有一个 session 让所有这些都可见：

>>> def show_expression():
        for i in range(5):
            word = yield 10
            print('The word is %r' % word)

>>> g = show_expression()
>>> next(g)
10
>>> g.send('blue')
The word is 'blue'
10
>>> g.send('no')
The word is 'no'
10
>>> g.send('yellow')
The word is 'yellow'
10
>>> next(g)
The word is None
10
>>> g.send('done')
The word is 'done'
Traceback (most recent call last):
  File "<pyshell#44>", line 1, in <module>
    g.send('done')
StopIteration

希望从第一原理中解释所有的奥秘:-)

python yield 函数是如何工作的？

How does this python yield function work?

python

yield