yield關(guān)鍵字

用法

普通函數(shù)：

>>> def f1(): return 'f1'
>>> f1()
'f1'

帶有yield關(guān)鍵字的函數(shù)：

>>> def f2(): yield 'f2'
>>> f2()
<generator object f2 at 0x7f453255e4c0>
>>> next(f2())
'f2'

yield將函數(shù)的返回值變成了迭代器,其相當(dāng)于獲取函數(shù)運(yùn)算結(jié)果的入口
next()為python的內(nèi)置函數(shù)，用于從迭代器獲取函數(shù)運(yùn)算結(jié)果

機(jī)制

>>> def func():
...     i = 0
...     while True:
...             yield i
...             i += 1
... 
>>> f = func()
>>> [next(f), next(f), next(f)]
[0, 1, 2]

相當(dāng)于將函數(shù)“暫?！痹趛ield處，再次調(diào)用next()則執(zhí)行到下一個(gè)yield

用法舉例

for...in

>>> def func():
...     i = 0
...     while i < 3:
...             yield i
...             i += 1
...     raise StopIteration # 該行替換成return或者不寫(xiě)有同樣的效果
... 
>>> for i in func():
...     print(i)
... 
0
1
2

與range(n)相比省很大內(nèi)存空間

內(nèi)置send()函數(shù)

>>> def func():
...     i = 0
...     while True:
...             i += yield i
... 
>>> f = func()
>>> next(f)
0
>>> f.send(5)
5
>>> f.send(5)
10
>>> f.send(5)
15

send(X)調(diào)用將X作為上次執(zhí)行到的yield表達(dá)式的值(此處相當(dāng)于i += X)，并再次執(zhí)行next()。用于介入迭代過(guò)程

注意第一次執(zhí)行next()后函數(shù)停止的位置，此時(shí)“+=”運(yùn)算還未被執(zhí)行：

image.png-14kB

!!必須執(zhí)行至少一次next()后才能執(zhí)行send()，如下：

>>> f.send(5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't send non-None value to a just-started generator

類的_iter() 、_next()方法

除了包含yield的函數(shù)，還可以自己定義迭代器對(duì)象

內(nèi)置函數(shù)iter(obj)調(diào)用相當(dāng)于通過(guò)執(zhí)行obj對(duì)象的_iter()方法獲取迭代器對(duì)象
內(nèi)置函數(shù)next(obj)調(diào)用相當(dāng)于通過(guò)執(zhí)行obj對(duì)象的_next()方法獲取迭代結(jié)果

>>> class counter:
    def __init__(self):
        self.i = 0

    def __iter__(self):
        return self

    def __next__(self):
        self.i += 1
        if self.i <= 3: return self.i
        else: raise StopIteration


>>> f = iter(counter())
>>> print([next(f), next(f), next(f)])
[1, 2, 3]