yield列表反转 islice切片（2.6）

# yield列表反转 islice切片
### 列表反转
```python
l1 = [i for i in range(10)]
print(l1)
print(l1[::2])
l1.reverse()
# 注： python2里列表reverse是返回一个新的列表
print(l1)
print(l1[::-1])
for x in reversed(l1):
print(x)
```
output:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[0, 2, 4, 6, 8]
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
0
1
2
3
4
5
6
7
8
9

实际上,for循环要求l1有这个函数， __iter__ 反向循环reveresd要求l1有__reveresd__

l1.__iter__()
l1.__reversed__()

自己实现一个可以反转的列表

class FloatRange:
    def __init__(self, start, end, step):
        self.start = start
        self.end = end
        self.step = step

    def __iter__(self):
        res = self.start
        while res <= self.end:
            yield res
            res += self.step

    def __reversed__(self):
        res = self.end
        while res >= self.start:
            yield res
            res -= self.step

for i in FloatRange(1, 10, 0.5):
    print(i)
print('#============')
for i in reversed(FloatRange(1, 10, 0.5)):
    print(i)

本文和前文有很多yiled的例子，也讲了读文件的分片，介绍一个可以分片的函数 itertools.islice

islice(iter, start=0, end, step=1)

# 运行这个代码可以看到with 0和1输出相同
# f 0 1输出不同，是因为islice会消耗iter
from itertools import islice
print("#======with 0======\n\n")
with open('2.5生成器.md', 'r') as f:
    for i in islice(f, 100, 110):
        print(i)
print("#======with 1======\n\n")
with open('2.5生成器.md', 'r') as f:
    for i in islice(f, 100, 110):
        print(i)

print("#======f 0======\n\n")
# 对同一个f进行操作
f = open('2.5生成器.md', 'r')
for i in islice(f, 0, 10):
    print(i)
print("#======f 1======\n\n")
for i in islice(f, 0, 10):
    print(i)

简单的例子展示islice消耗iter

l = range(10)
l = iter(l)
for i in islice(l, 0, 5, 2):
    print(i)

print('一直输出到结束')
for i in islice(l, 0, None):
    print(i)

output:

0
2
4
一直输出到结束
5
6
7
8
9