python高级特性总结之切片 迭代 生成器 迭代器

切片：

>>> L = list(range(100))
>>> L
[0, 1, 2, 3, ..., 99]
>>> L[:10]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> L[-10:]
[90, 91, 92, 93, 94, 95, 96, 97, 98, 99]
>>> L[10:20]
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
#每隔2个取
>>> L[:10:2]
[0, 2, 4, 6, 8]
#每隔5个取
>>> L[::5]
[0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95]
#复制一份
>>> L[:]
[0, 1, 2, 3, ..., 99]

迭代：

使用for循环可以快速进行迭代

>>> for x, y in [(1, 1), (2, 4), (3, 9)]:
...     print(x, y)
def findMinAndMax(L):
    if len(L) == 0:
        return (None, None)
    min = L[0]
    max = L[0]
    for x in L:
        if x > max:
            max = x
        if x < min:
            min = x
    return (min, max)

列表生成器list

>>> list(range(1, 11))
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> [x * x for x in range(1, 11)]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
>>> [x * x for x in range(1, 11) if x % 2 == 0]
[4, 16, 36, 64, 100]
>>> [m + n for m in 'ABC' for n in 'XYZ']
['AX', 'AY', 'AZ', 'BX', 'BY', 'BZ', 'CX', 'CY', 'CZ']
#迭代key value
>>> d = {'x': 'A', 'y': 'B', 'z': 'C' }
>>> for k, v in d.items():
...     print(k, '=', v)
...
y = B
x = A
z = C

#for 和 if 结合，注意没有else
>>> [x for x in range(1, 11) if x % 2 == 0]
[2, 4, 6, 8, 10]
或者这样 必须有else
>>> [x if x % 2 == 0 else -x for x in range(1, 11)]
[-1, 2, -3, 4, -5, 6, -7, 8, -9, 10]

L1 = ['Hello', 'World', 18, 'Apple', None]
L2 = []
for x in L1:
    if isinstance(x, str) and x!= None:
        L2.append(x.lower())

生成器generator

generator保存的是算法，每次调用next(g)，就计算出g的下一个元素的值，直到计算到最后一个元素，没有更多的元素时，抛出StopIteration的错误。

通过for循环来迭代

#list 
>>> L = [x * x for x in range(10)]
#generator
>>> g = (x * x for x in range(10))
>>> for n in g:
...     print(n)
#也可以用函数来生成
def fib(max):
    n, a, b = 0, 0, 1
    while n < max:
        yield b
        a, b = b, a + b
        n = n + 1
    return 'done'
#每次调用函数 执行到yield结束 下次调用直接从这里开始执行
>>> g = fib(6)
>>> while True:
...     try:
...         x = next(g)
...         print('g:', x)
...     except StopIteration as e:
...         print('Generator return value:', e.value)
...         break

生成杨辉三角形

 def triangles(): 
    n = 1
    L = [1]
    while n < 11:
        yield L
        L2 = L.copy()
        L2.append(1)      
        if len(L2) <= 2:
            pass
        else:
            for x in range(len(L2)):
                if x ==0 or x == len(L2)-1:
                    pass
                else:
                    L2[x]=L[x-1]+L[x]
        L = L2
        n = n + 1 
    return 'done'  

results = []
for t in triangles():
    results.append(t)
    n = n + 1
    if n == 10:
        break

for t in results:
    print(t)

if results == [
    [1],
    [1, 1],
    [1, 2, 1],
    [1, 3, 3, 1],
    [1, 4, 6, 4, 1],
    [1, 5, 10, 10, 5, 1],
    [1, 6, 15, 20, 15, 6, 1],
    [1, 7, 21, 35, 35, 21, 7, 1],
    [1, 8, 28, 56, 70, 56, 28, 8, 1],
    [1, 9, 36, 84, 126, 126, 84, 36, 9, 1]
]:
    print('测试通过!')
else:
    print('测试失败!')

可迭代对象Iterable和迭代器Iterator

可迭代对象不一定是迭代器

>>> from collections.abc import Iterable
>>> isinstance([], Iterable)
True
>>> isinstance({}, Iterable)
True
>>> isinstance('abc', Iterable)
True
>>> isinstance((x for x in range(10)), Iterable)
True
>>> isinstance(100, Iterable)
False

>>> from collections.abc import Iterator
>>> isinstance((x for x in range(10)), Iterator)
True
>>> isinstance([], Iterator)
False
>>> isinstance({}, Iterator)
False
>>> isinstance('abc', Iterator)
False

>>> isinstance(iter([]), Iterator)
True
>>> isinstance(iter('abc'), Iterator)
True

生成器都是Iterator对象，但list、dict、str虽然是Iterable，却不是Iterator。

把list、dict、str等Iterable变成Iterator可以使用iter()函数

参考来源：https://www.liaoxuefeng.com/wiki/1016959663602400/1017323698112640