重要的内置函数

reversed()
l = [1,2,3,4,5]
l.reverse()
print(l)
l = [1,2,3,4,5]
l2 = reversed(l)
print(l2)
# 保留原列表，返回一个反向的迭代器

l = (1,2,23,213,5612,342,43) #切片函数
sli = slice(1,5,2)  #切片规则，从1到5隔2
print(l[sli])
print(l[1:5:2])

# print(format('test', '<20'))
# print(format('test', '>40'))
# print(format('test', '^40'))

#bytes 转换成bytes类型
# 我拿到的是gbk编码的，我想转成utf-8编码
# print(bytes('你好',encoding='GBK'))     # unicode转换成GBK的bytes
# print(bytes('你好',encoding='utf-8'))   # unicode转换成utf-8的bytes

# 网络编程 只能传二进制
# 照片和视频也是以二进制存储
# html网页爬取到的也是编码
# b_array = bytearray('你好',encoding='utf-8')
# print(b_array)
# print(b_array[0])
# '\xe4\xbd\xa0\xe5\xa5\xbd'
# s1 = 'alexa'
# s2 = 'alexb'

# l = 'ahfjskjlyhtgeoahwkvnadlnv'
# l2 = l[:10]

# 切片 —— 字节类型  不占内存
# 字节 —— 字符串 占内存

# print(ord('好'))    #字符按照unicode转数字
# print(ord('1'))
# print(chr(97))    #数字按照unicode转字符

# print(ascii('好'))
# print(ascii('1'))

name = 'egg'
print('你好%r'%name)
print(repr('1'))   #带着符号输出
print(repr(1))

# print(all(['a','',123]))
# print(all(['a',123]))
# print(all([0,123]))

# print(any(['',True,0,[]]))

l = [1,2,3,4,5]
l2 = ['a','b','c','d']
l3 = ('*','**',[1,2])
d = {'k1':1,'k2':2}
for i in zip(l,l2,l3,d): #拉链，把多个值拉到一起
    print(i)

#filter  过滤器，筛选
def is_odd(x):
    return x % 2 == 1
def is_str(s):
    return s and str(s).strip()
ret = filter(is_odd, [1,  6, 7, 12, 17])  #只传函数名，不调用
ret = filter(is_str, [1, 'hello','','  ',None,[], 6, 7, 'world', 12, 17])
#print(ret)
for i in ret:
    print(i)
[i for i in [1, 4, 6, 7, 9, 12, 17] if i % 2 == 1]


#过滤100以内平方根是整数的
from math import sqrt
def func(num):
    res = sqrt(num) #开平方
    return res % 1 == 0
ret = filter(func,range(1,101))
for i in ret:
    print(i)

#map
ret = map(abs,[1,-4,6,-8])
print(ret)
for i in ret:
    print(i)


# filter 执行了filter之后的结果集合 <= 执行之前的个数
        #filter只管筛选，不会改变原来的值
# map 执行前后元素个数不变
      # 值可能发生改变

l = [1,-4,6,5,-10]
l.sort(key = abs)   # 在原列表的基础上进行排序
print(l)

print(sorted(l,key=abs,reverse=True))      # 生成了一个新列表 不改变原列表 占内存
print(l)

#根据列表的每一个元素的长度排序
l = ['   ',[1,2],'hello world']
new_l = sorted(l,key=len)
print(new_l)