python的高级特性

切片

L = ['Michael', 'Sarah', 'Tracy', 'Bob', 'Jack']
print(L[0:3])
print(L[:3])
print(L[-2:])#依然是从左到右

['Michael', 'Sarah', 'Tracy']
['Michael', 'Sarah', 'Tracy']
['Bob', 'Jack']

L = list(range(100))
print(L[:10])
print(L[-10:])
print(L[10:20])
print(L[:10:2])
print(L[::5])
print(L[:])

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[90, 91, 92, 93, 94, 95, 96, 97, 98, 99]
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
[0, 2, 4, 6, 8]
[0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99]

列表生成式

列表生成式在使用的时候先需要用中括号括住

[x * x for x in range(1, 11)]#生成1*1，2*2，...,10*10

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

加上条件判断

在某个条件下生成

[x*x for x in range(1,11) if x%2==0]

[4, 16, 36, 64, 100]

根据条件生成对应的东西

[x if x % 2 == 0 else -x for x in range(1, 11)]

[-1, 2, -3, 4, -5, 6, -7, 8, -9, 10]

两层循环生成列表

[m + n for m in 'ABC' for n in 'XYZ']

['AX', 'AY', 'AZ', 'BX', 'BY', 'BZ', 'CX', 'CY', 'CZ']

在字典中使用列表生成式

d = {'x': 'A', 'y': 'B', 'z': 'C' }
[k+'='+v for k,v in d.items() ]

['x=A', 'y=B', 'z=C']

生成器

简介：如果列表很大避免每次生成完整的列表而是先根据需要生辰给其中的一部分

L = [x * x for x in range(10)]
print(L)
g = (x * x for x in range(10))#这个是列表生成器
print(g)

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
<generator object <genexpr> at 0x000001E243330CC8>

g = (x * x for x in range(10))#for循环完成一次之后就会使得g没有内容
for n in g :
    print(n,end = " ")
print('\n----for complete once----')
for n in g :
    print(n,end = " ")
print('\n----for complete twice----')

0 1 4 9 16 25 36 49 64 81 
----for complete once----

----for complete twice----

使用类似函数的方式构建生成器

def fib(max):
    n, a, b = 0, 0, 1
    while n < max:
        yield(b)#改成print(b)正常的函数
        a, b = b, a + b
        n = n + 1
    return 'done'


f = fib(6)
print(f)
for i in f:
    print(i,end=" ")

<generator object fib at 0x000001E243330BC8>
1 1 2 3 5 8