python基础之字符串常用方法

str常用命令：
字符操作：.capitalize() .upper() .lower() .title() .swapcase()
判断：.startwith() .endwith() .isalnum() .isalpha() .isdigit()
统计：.count() len()
索引：.find() .index()
结构：.format() .strip() .lstrip() .rstrip() .split()  .replace .center() .expandtabs()
循环：for
1，首字母大写：
s.capitalize()

s1 = 'python is good'
s2 = s1.capitalize()
print(s2)

2，全部大写：
s.upper()

s1 = 'python is good'
s2 = s1.upper()
print(s2)

3，全部小写
s.lower()

s1 = 'Python iS good'
s2 = s1.lower()
print(s2)

result:python is good
4，大小写翻转：
s.swapcase()

s1 = 'Python iS good'
s2 = s1.swapcase()
print(s2)

result:pYTHON Is GOOD
5，每个分隔的单词首字母大写：
s.title()

s1 = 'Python iS good'
s2 = s1.title()
print(s2)

result:Python Is Good
6，字符串居中：
s.center(20)--20代表输出宽度

s1 = 'Python iS good'
s2 = s1.center(20)
print(s2)

s.center(20,'-')--符号代表填充内容

s1 = 'Python iS good'
s2 = s1.center(20,'-')
print(s2)

7，自动填充，当字符串中有\t（tab键），前面不足8位的补充到8位，不足16未补充到16位：
s.expandtabs()
8，字符串长度：
len(s)--长度

s1 = 'Python iS good'
s2 = len(s1)
print(s2)

result:14
9，判断字符串以什么开头/结尾：
s.startwith('e',start='',end='')--e表示查找对象，start/end切片定位，返回true/faulse

s1 = 'Python iS good'
s2 = s1.startswith('o')
print(s2)

result:False

s1 = 'Python iS good'
s2 = s1.startswith('P')
print(s2)

result:true

s1 = 'Python iS good'
s2 = s1.startswith('P',2,5)
print(s2)

result:False
s.endwith('')--同理
10.根据元素找索引：
s.find('')--返回元素位置索引，未找到返回-1.

s1 = 'Python iS good'
s2 = s1.find('S')
print(s2)

result:8

s1 = 'Python iS good'
s2 = s1.find('w')
print(s2)

result:-1
s.index('')--同理，未找到报错

s1 = 'Python iS good'
s2 = s1.index('w')
print(s2)

result:ValueError: substring not found

s1 = 'Python iS good'
s2 = s1.index('S')
print(s2)

result:8
11，去掉空格：常用于有用户输入的地方
s.strip()--去掉前后空格

s1 = ' Python iS good '
s2 = s1.strip()
print(s2)

result:Python iS good
s.strip('%')--%表示要去掉的所有元素，可多个元素-%q，也可有空格- %q
 
s.lstrip()--去掉前端空格

s1 = ' Python iS good '
s2 = s1.lstrip()
print(s2)

result:Python iS good
s.rstrip()--去掉尾部空格

s1 = ' Python iS good '
s2 = s1.rstrip()
print(s2)

result: Python iS good
*strip的坑：--前后相关字段都去掉了

s = 'ab12aaab123a33b'
print(s.strip('ab')) 

result:12aaab123a33
12，统计元素的个数：
s.count('e')--返回e元素个数

s1 = ' Python iS good '
s2 = s1.count('o')
print(s2)

result:3

#按索引切片统计

s1 = ' Python iS good '
s2 = s1.count('o',6,14)
print(s2)

result:2
13，分割元素：
s.split()--默认以空格分隔，返回元素列表

s1 = ' Python iS good '
s2 = s1.split()
print(s2)

result:['Python', 'iS', 'good']
s.split(',')--以，分隔

s1 = ' Python iS good,english is good too '
s2 = s1.split(',')
print(s2)

result:[' Python iS good', 'english is good too ']
14，格式化输出：format三种方法
s=‘我叫{}，今年{}'.format('wang','18')--{}表示占位符，等同于%s，format依次补充内容

s1 = '我叫{}，今年{}'.format('wang','')
print(s1)

result:我叫wang，今年18
s=‘我叫{0}，今年{1}，再说一下我叫{0}'.format('wang','18')--使用索引标识输出位

s1 = '我叫{0}，今年{1}，再说一下我叫{0}'.format('wang',''
print(s1

result:我叫wang，今年18，再说一下我叫wang
s=‘我叫{name}，今年{age}'.format(name='wang',age='18')--使用变量标识输出内容

s1 = '我叫{name}，今年{age}'.format(name='wang',age='')
print(s1)

result:我叫wang，今年18
15，元素替换：
s.replace(old,new,count=None)--old-原元素，new-新元素，count-替换个数，默认替换全部

s1 = 'python is good'
s2 = s1.replace('o','t')
print(s2)

result:pythtn is gttd

s1 = 'python is good'
s2 = s1.replace('o','t',2)
print(s2)

result:pythtn is gtod
16，判断：
s.isalnum()--由字母与数字组成，返回true/false

s1 = ''
s2 = s1.isalnum()
print(s2)

result:True

s1 = 'aa133'
s2 = s1.isalnum()
print(s2)

result:True
s.isalpha()--由字母组成

s1 = 'aassss'
s2 = s1.isalpha()
print(s2)

result:True
s.isdigit()--由数字组成

s1 = 'aassss'
s2 = s1.isdigit()
print(s2)

result:False
17，循环：
for i in s:
     print(i)--依次遍历

s1 = 'python is good'
for i in s1:
    print(i)

result:
p
y
t
h
o
n
..省略.