Python 生成器

生成器（generator）就是对象，在每次调用它的next()方法时返回一个值，直到它抛出StopIteration

要创建一个生成器需要做的一切只是写一个普通的包含yield 语句的 Python 函数。 Python 会检测对yield 的使用并将这个函数标记为一个生成器。当函数执行到yield 语句 时，它会像return 语句那样返回一个值，但一个明显不同在于：解释器会保存对栈的引用， 它将被用来在下一次调用next 时恢复函数的执行

def my_range(first=0, last=10, step=1):
    number = first
    while number < last:
        yield number
        number += step

ranger = my_range(1, 5)
ranger # <generator object my_range at 0x7ff7182befc0>
for x in ranger:
    print(x) #1 #2 #3 #4

sum(range(1, 101)) # 5050

g = (x * x for x in range(10))
print(next(g)) #0
print(next(g)) #1
print(next(g)) #4
print(next(g)) #9
print(next(g)) #16

for n in g:
    print(n) # 25 36 49 64 81

def fib(max):
    n, a, b = 0, 0, 1 # n=0 a=0 b=1
    while n < max:
        #print(b)
        yield b
        a, b = b, a + b #a=b b=a+b
        n = n + 1
    #return 'done' 不能有return

f = fib(6)
print(next(f)) #1
print(next(f)) #1
print(next(f)) #2
print('----') 
for n in f: #3 5 8
    print(n)

def odd():
    print('step 1')
    yield 1
    print('step 2')
    yield 2
    print('step 3')
    yield(3)

o = odd()
print(next(o)) #1
print(next(o)) #2
print(next(o)) #3
print(next(o)) #StopIteration
#while True:
#    try:
#        x = next(o)
#        print('g', x)
#    except StopIteration as e:
#        print('Generator return value', e.value)
#        break

检查一个函数是否是生成器

import inspect

def mygenerator():
    yield 1

inspect.isgeneratorfunction(mygenerator)

生成器的当前状态

GEN_CREATED 正在等待第一次被执行
GEN_RUNNING 当前正在被解析器执行
GEN_SUSPENDED 等待被next()调用唤醒
GEN_CLOSED 已经结束运行

import inspect

def mygenerator():
    yield 1

gen = mygenerator()
inspect.getgeneratorstate(gen) # GEN_CREATED
next(gen)
inspect.getgeneratorstate(gen) # GEN_SUSPENDED
try:
    next(gen) # StopIteration
except StopIteration as e:
    print(inspect.getgeneratorstate(gen)) # GEN_CLOSED

生成器表达式

gen = (x.upper() for x in ['hello', 'world'])
gen # <generator object <genexpr> at 0x7ff7182bef68>
list(gen) # ['HELLO', 'WORLD']
#next(gen) # HELLO
#next(gen) # WORLD