生成器

生成器作用：节省内存空间。通过一边循环一边计算的机制称为生成器：generator

创建生成器：

1、将列表推导式的[]改为()，就创建了一个生成器。通过next()函数或for语句获取generator的值。

>>> l = [x*x for x in range(10)]
>>> l
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> g = (x*x for x in range(10))
>>> g
<generator object <genexpr> at 0x026F50A8>
>>> next(g)
0
>>> next(g)
1
>>> next(g)
4
>>> next(g)
9
>>> next(g)
16
>>> next(g)
25
>>> next(g)
36
>>> next(g)
49
>>> next(g)
64
>>> next(g)
81
>>> next(g)
Traceback (most recent call last):
  File "<pyshell#14>", line 1, in <module>
    next(g)
StopIteration
>>> g
<generator object <genexpr> at 0x026F50A8>
>>> for n in g:
    print(n)    
>>> 
>>> 
>>> g = (x*x for x in range(10))
>>> g
<generator object <genexpr> at 0x026F8788>
>>> for n in g:
    print(n)    
0
1
4
9
16
25
36
49
64
81
>>

2、通过函数的方式创建生成器：若一个函数中包含yield关键字，则此函数为一个generator

def fib(max):
    n,a,b = 0,1,1
    while n < max:
        print(b)
        a, b = b, a+b
        n = n+1
    return "done"

def fib1(max):
    n,a,b = 0,1,1
    while n < max:
        yield(b)
        a, b = b, a+b
        n = n+1
    return "done"
>>> fib(6)
1
2
3
5
8
13
'done'
>>> fib1(6)
<generator object fib1 at 0x03225210>
>>> 
>>> next(fib(6))
1
2
3
5
8
13
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    next(fib(6))
TypeError: 'str' object is not an iterator
>>> next(fib1(6))
1
>>> for n in fib1(6)
SyntaxError: invalid syntax
>>> for n in fib1(6):
    print(n)
1
2
3
5
8
13
>>> 

def odd():
    print("step 1")
    yield 1
    print('step 2')
    yield 3
    print('step 3')
    yield 5
>>> odd()
<generator object odd at 0x02D051C0>
>>> for i in odd():
    print(i)    
step 1
1
step 2
3
step 3
5
>>> next(odd())
step 1
1
>>> next(odd())
step 1
1
>>> next(odd())
step 1
1
>>> o = odd()
>>> next(o)
step 1
1
>>> next(o)
step 2
3
>>> next(o)
step 3
5
>>> next(o)
Traceback (most recent call last):
  File "<pyshell#12>", line 1, in <module>
    next(o)
StopIteration

注意：使用for循环调用生成器时，不能获取生成器return语句的返回值。返回值包含在stopIteration的value