python 坑1
1.编码解码
编码:将文字转换成字节形式 encode
name = '小冯'
print(name.encode('utf-8')) # b'\xe5\xb0\x8f\xe5\x86\xaf'
解码:将字节转换成文字形式 decode
name = '小冯'
msg = name.encode('utf-8')
print(msg.decode()) # 小冯
2.基础数据类型补充:
2.1 str:
首字母大写
name = 'xiao,feng'
print(name.capitalize()) # Xiao,feng
**每个单词首字母大写 **
print(name.title()) # Xiao,Feng
**大小写反转 **
name = 'xiao,fenG'
print(name.swapcase()) # XIAO,FENg
居中 -- 填充
print(name.center(20,'*')) # *****xiao,feng******
查找:find index
print(name.find('f')) # 返回索引值 5
print(name.find('y')) # 如果没有 则返回 -1
print(name.index('f')) # 返回索引值 5
print(name.index('y')) # 报错 ValueError: substring not found
**拼接 **
print('_'.join(name)) # x_i_a_o_,_f_e_n_g
格式化: name.format
msg = '啦啦啦{},{},{}'
print(msg.format(1,2,3)) # 啦啦啦1,2,3
msg = '啦啦啦{2},{0},{1}'
print(msg.format(1,2,3)) # 啦啦啦3,1,2
msg = '啦啦啦{q},{w},{e}'
print(msg.format(q = 5,e = 9,w = 3)) # 啦啦啦5,3,9
2.2list:
**排序(默认是升序) 降序sort(reverse=True) **
lst = [1,2,3,4,5]
lst.sort(reverse = True)
print(lst) # [5, 4, 3, 2, 1]
lst.sort()
print(lst) # [1, 2, 3, 4, 5]
反转
lst.reverse()
print(lst) # [5, 4, 3, 2, 1]
查找 index
print(lst.index(4)) # 3
print(lst.index(9)) # 报错 ValueError: 9 is not in list
**统计 count **
print(lst.count(5)) # 1
+ - * 元素都是共用的 会开辟一个新的内存空间
面试：
lst = [[]]
new_lst = lst * 5
new_lst[0].append(10)
print(new_lst) # [[10], [10], [10], [10], [10]]
lst = [1,[]]
new_lst = lst * 5
new_lst[0] = 10
print(new_lst) # [10, [], 1, [], 1, [], 1, [], 1, []]
lst = [1,[]]
new_lst = lst * 5
new_lst[1] = 10
print(new_lst) # [1, 10, 1, [], 1, [], 1, [], 1, []]
lst = [1,2,3]
new_lst = lst * 5
print(id(new_lst), id(new_lst[0])) # 1813566569096 1756851216
2.3tuple:
**(1,) # 元组 **
print(type((1,))) # <class 'tuple'>
**(1) # 括号里数据本身 **
print(type(1)) # <class 'int'>
2.4dict:
popitem 随机删除字典中的键值对
dic = {"key":1,"key2":2,"key3":56}
print(dic.popitem()) # ('key3', 56)
print(dic) # {'key': 1, 'key2': 2}
**fromkeys("可迭代的键",共用的值) -- 坑 **
dic = {}
dic.fromkeys("123",[23]) # 批量添加键值对{"1":[23],"2":[23],"3":[23]}
print(dic) # {}
dic = dict.fromkeys("123456789",1) # 批量添加键值对"键是可迭代对象",值 -- 会被共用
dic["1"] = 18
print(dic) # {'1': 18, '2': 1, '3': 1, '4': 1, '5': 1, '6': 1, '7': 1, '8': 1, '9': 1}
dic(key = 1,key2 =2)
print(dict(key = 1,key2 = 2)) # {'key': 1, 'key2': 2}
2.5set:
**set() -- 空集合 **
set("alex") # 迭代添加
3.坑
print(list('qwe')) # ['q', 'w', 'e']
print(tuple('qwe')) # ('q', 'w', 'e')
print(dict('qwe')) # ValueError: dictionary update sequence element #0 has length 1; 2 is required
print(set('qwe')) # {'q', 'e', 'w'}
lst = [1,2]
for i in lst:
lst.append(3)
print(lst) # 死循环
lst = [1,2,3,4]
for i in lst:
lst.pop()
print(lst) # [1, 2]
lst = [1,2,3,4]
for i in lst:
lst.pop()
print(lst) # [1, 2]
lst = [1,2,3,4]
for i in lst:
lst.pop(0)
print(lst) # [3, 4]
lst = [1,2,3,4]
for i in lst:
lst.remove(i)
print(lst) # [2, 4]
列表删除 -- 从后向前删除
lst = [1,2,3,4,5]
for i in range(len(lst)):
lst.pop()
print(lst)
lst = [1,2,3,4,6]
for i in range(len(lst)-1,-1,-1):
del lst[i]
print(lst)
lst = [1,2,3,4,6]
for i in range(len(lst)):
del lst[-1]
print(lst)
创建一个新的列表,删除旧的列表
lst = [1,2,3,4,5,6]
lst1 = lst.copy()
for i in lst1:
lst.remove(i)
print(lst)
**字典删除 -- 循环的时候不能改变源数据的大小 (可以改变值) **
dict.fromkeys('123456',[])
**创建一个新的字典,删除旧的字典 **
dic = dict.fromkeys("12345",1)
dic1 = dic.copy()
for i in dic1:
dic.pop(i)
print(dic)
集合删除 -- 循环的时候不能改变源数据的大小
4.类型转换:
**list -- str join **
str -- list split
set - list
list - set
5.数据类型：
可变:list ,dict ,set
不可变:int bool str tuple
有序:list,tuple,str,int,bool
无序:dict,set
取值方式:
索引: str list tuple
直接: set ,int ,bool
**键: dict **
bool: False
数字: 0
字符串: ""
列表:[]
元组:()
字典:{}
集合: set()
其他: None
