python3.x 基础一：str字符串方法

*字符串不能更改值

数据类型字符串str

| capitalize(...)   返回字符串中第一个字母大写
|      S.capitalize() -> str
|
|      Return a capitalized version of S, i.e. make the first character
|      have upper case and the rest lower case.
```
>>> str1='i am chinese'

>>> str1.capitalize()

'I am chinese'
```
| casefold(...) 将大写转换成小写
|      S.casefold() -> str
|
|      Return a version of S suitable for caseless comparisons.
```
>>> str2='I AM A CHINESE'

>>> str2.casefold()

'i am a chinese'
```
| center(...) 第一个参数是总字符长度，第二个是补全字符，原字符串居中
|      S.center(width[, fillchar]) -> str
|
|      Return S centered in a string of length width. Padding is
|      done using the specified fill character (default is a space)
```
>>> str1.center(50,'#')

'###################i am chinese###################'
```
| count(...) 计算某个字符/元素出现次数
|      S.count(sub[, start[, end]]) -> int
|
|      Return the number of non-overlapping occurrences of substring sub in
|      string S[start:end]. Optional arguments start and end are
|      interpreted as in slice notation.
```
>>> str1.count('e')
2
>>> str1.count('i')
2
>>> str1.count('a')
1
```
| endswith(...) 是否以某个后缀字符串结束
|      S.endswith(suffix[, start[, end]]) -> bool
|
|      Return True if S ends with the specified suffix, False otherwise.
|      With optional start, test S beginning at that position.
|      With optional end, stop comparing S at that position.
|      suffix can also be a tuple of strings to try.
```
>>> str2.endswith('SE')

True

>>> str2.endswith('S')

False
```
```
>>> str = "this is string example....wow!!!";

>>> suffix = "is"

>>> print (str.endswith(suffix, 2, 6))

False

>>> print(str[2:7])

is is

#字符串含有指定后缀，为什么不是返回真？
#理解错误，起始位置是左闭右开
>>> print (str.endswith(suffix, 2,7))
True 
```
| expandtabs(...) 扩展tab字符n个
|      S.expandtabs(tabsize=8) -> str
|
|      Return a copy of S where all tab characters are expanded using spaces.
|      If tabsize is not given, a tab size of 8 characters is assumed.
```
>>> str4='I am \t Chinese'

>>> str4.expandtabs(10)

'I am       Chinese'
```
| find(...) 返回查找到的第一个字符位置，否则返回-1
|      S.find(sub[, start[, end]]) -> int
|
|      Return the lowest index in S where substring sub is found,
|      such that sub is contained within S[start:end]. Optional
|      arguments start and end are interpreted as in slice notation.
|
|      Return -1 on failure.
```
>>> str2

'I AM A CHINESE'

>>> str2.find('M')

3

>>> str2.find('A')

2

>>> str2.find('B')

-1
```

| index(...) 同上，查找不到则报错
|      S.index(sub[, start[, end]]) -> int
|
|      Like S.find() but raise ValueError when the substring is not found.

>>> str2

'I AM A CHINESE'

>>> str2.index('A')

2

>>> str2.index('B')

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

ValueError: substring not found

| isalnum(...) 判断字符串是否由字母数字组成
|      S.isalnum() -> bool
|
|      Return True if all characters in S are alphanumeric
|      and there is at least one character in S, False otherwise.
```
>>> str5='I am 1 chinese'

>>> str5.isalnum()

False

>>> str6='iAm1Chinese'

>>> str6.isalnum()

True
```
| isalpha(...) 判断字符串是否全部是字符组成
|      S.isalpha() -> bool
|
|      Return True if all characters in S are alphabetic
|      and there is at least one character in S, False otherwise.
```
>>> str7='iamChinese'

>>> str7.isalpha()

True

>>> str3.isalpha()

False
```
| isdecimal(...) 如果字符串是否只包含十进制字符返回True，否则返回False
|      S.isdecimal() -> bool
|
|      Return True if there are only decimal characters in S,
|      False otherwise.
```
>>> str8=u'asdfasdf1000'

>>> str8.isdecimal()

False

>>> stru=u'1000'

>>> stru.isdecimal()

True
```
| isdigit(...) 判断字符串中是否全部是数字
|      S.isdigit() -> bool
|
|      Return True if all characters in S are digits
|      and there is at least one character in S, False otherwise.
```
>>> str9,str10='abc123','123'

>>> str9.isdigit()

False

>>> str10.isdigit()

True
```
| isidentifier(...) 判断变量明明是否合法
|      S.isidentifier() -> bool
|
|      Return True if S is a valid identifier according
|      to the language definition.
|
|      Use keyword.iskeyword() to test for reserved identifiers
|      such as "def" and "class".
| islower(...) 判断字符串中所有字母是否小写
|      S.islower() -> bool
|
|      Return True if all cased characters in S are lowercase and there is
|      at least one cased character in S, False otherwise.
```
>>> str11,str12,str13='abcd','AbCd','Ab123'

>>> str11.islower()

True

>>> str12.islower()

False

>>> str13.islower()

False

>>> str14='aa123'

>>> str14.islower()

True
```
| isnumeric(...) 判断字符串中所有字符是否是数字
|      S.isnumeric() -> bool
|
|      Return True if there are only numeric characters in S,
|      False otherwise.
```
>>> str10,str14

('123', 'aa123')

>>> str10.isnumeric()

True

>>> str14.isnumeric()

False
```
| isspace(...) 判断字符是否是空格
|      S.isspace() -> bool
|
|      Return True if all characters in S are whitespace
|      and there is at least one character in S, False otherwise.
```
>>> str1,str14,str15
('i am chinese', 'aa123', ' ')
>>> str1.isspace(),str14.isspace(),str15.isspace()
(False, False, True)
```
| istitle(...) 判断字符串是否是标题，标题单词首字母均大写
|      S.istitle() -> bool
|
|      Return True if S is a titlecased string and there is at least one
|      character in S, i.e. upper- and titlecase characters may only
|      follow uncased characters and lowercase characters only cased ones.
|      Return False otherwise.
```
>>> str16,str17='I Am Chinese','I am chinese'

>>> str16.istitle(),str17.istitle()

(True, False)
```
| isupper(...) 判断字符串中字母是否全部是大写
|      S.isupper() -> bool
|
|      Return True if all cased characters in S are uppercase and there is
|      at least one cased character in S, False otherwise.
```
>>> str18,str19='ABC123','Abc123'

>>> str18.isupper(),str19.isupper()

(True, False)
```
| join(...) 将其他可迭代对象转换成字符串，S表示字符串分隔符
|      S.join(iterable) -> str
|
|      Return a string which is the concatenation of the strings in the
|      iterable. The separator between elements is S.
```
>>> list1=['a','b','c','d']
>>> ','.join(list1)
'a,b,c,d'
>>> ''.join(list1)
'abcd'
>>> ' '.join(list1)
'a b c d'
```
| ljust(...) 共width个位置，字符串靠左，右边以字符填充
|      S.ljust(width[, fillchar]) -> str
|
|      Return S left-justified in a Unicode string of length width. Padding is
|      done using the specified fill character (default is a space).
```
>>> str10

'123'

>>> str10.ljust(10,'#')

'123#######'
```
| lower(...) 将字符串中所有大写字母转换成小写
|      S.lower() -> str
|
|      Return a copy of the string S converted to lowercase.
| lstrip(...) 将左边空格剪掉，可以指定其他被剪掉空格，\n \t
|      S.lstrip([chars]) -> str
|
|      Return a copy of the string S with leading whitespace removed.
|      If chars is given and not None, remove characters in chars instead.
```
>>> str20='  123abc  ABC  '

>>> str20.lstrip()

'123abc  ABC  '
```
strip()将左右两边空格剪掉，rstrip()将右边空格剪掉
| partition(...) 以某个字符切片字符串，返回三元组，中间是分隔符，找不到则返回源字符串和2个空格
|      S.partition(sep) -> (head, sep, tail)
|
|      Search for the separator sep in S, and return the part before it,
|      the separator itself, and the part after it. If the separator is not
|      found, return S and two empty strings.
```
>>> str21='http://www.baidu.com'

>>> str21.partition('://'... )

('http', '://', 'www.baidu.com')

>>> str21.partition('abc')

('http://www.baidu.com', '', '')
```
| replace(...) 字符替换，默认全部替换，如指定计数则替换指定计数个数字符
|      S.replace(old, new[, count]) -> str
|
|      Return a copy of S with all occurrences of substring
|      old replaced by new. If the optional argument count is
|      given, only the first count occurrences are replaced.
```
>>> str21

'http://www.baidu.com'

>>> str21.replace('baidu','jd')

'http://www.jd.com'

>>> str21.replace('w','z')

'http://zzz.baidu.com'

>>> str21.replace('w','z',1)

'http://zww.baidu.com'
```
| rfind(...) 从左往右查找，并返回最右边的字符索引位置
|      S.rfind(sub[, start[, end]]) -> int
|
|      Return the highest index in S where substring sub is found,
|      such that sub is contained within S[start:end]. Optional
|      arguments start and end are interpreted as in slice notation.
|
|      Return -1 on failure.
```
>>> str21

'http://www.baidu.com'

>>> str21.rfind('b')

11

>>> str21.rfind('w')

9
```
| rindex(...)
|      S.rindex(sub[, start[, end]]) -> int
|
|      Like S.rfind() but raise ValueError when the substring is not found.
| rjust(...) 字符靠右，左边补全字符，默认空格
|      S.rjust(width[, fillchar]) -> str
|
|      Return S right-justified in a string of length width. Padding is
|      done using the specified fill character (default is a space).
```
 |  rjust(...)

 |      S.rjust(width[, fillchar]) -> str

 |

 |      Return S right-justified in a string of length width. Padding is

 |      done using the specified fill character (default is a space).
```
```
>>> str10.rjust(10,'x')

'xxxxxxx123'
```
| rpartition(...) 无法找到切片字符串，则源字符串索引是2，前面是空格
|      S.rpartition(sep) -> (head, sep, tail)
|
|      Search for the separator sep in S, starting at the end of S, and return
|      the part before it, the separator itself, and the part after it. If the
|      separator is not found, return two empty strings and S.
```
>>> str21.rpartition('://')

('http', '://', 'www.baidu.com')

>>> str21.rpartition('abc')

('', '', 'http://www.baidu.com')
```
| split(...) 默认分隔符空格 \t \n，可以指定分割次数
|      S.split(sep=None, maxsplit=-1) -> list of strings
|
|      Return a list of the words in S, using sep as the
|      delimiter string. If maxsplit is given, at most maxsplit
|      splits are done. If sep is not specified or is None, any
|      whitespace string is a separator and empty strings are
|      removed from the result.
```
>>> str21.split('w')

['http://', '', '', '.baidu.com']

>>> str21.split('w',1)

['http://', 'ww.baidu.com']

>>> str21.split('w',2)

['http://', '', 'w.baidu.com']

>>> str21.split('w',3)

['http://', '', '', '.baidu.com']
```
| rsplit(...) 右边第一个分割符开始切分，默认全切分，可以指定分割次数
|      S.rsplit(sep=None, maxsplit=-1) -> list of strings
|
|      Return a list of the words in S, using sep as the
|      delimiter string, starting at the end of the string and
|      working to the front. If maxsplit is given, at most maxsplit
|      splits are done. If sep is not specified, any whitespace string
|      is a separator.
```
>>> str21.rsplit('w')

['http://', '', '', '.baidu.com']

>>> str21.rsplit('w',1)

['http://ww', '.baidu.com']

>>> str21.rsplit('w',2)

['http://w', '', '.baidu.com']

>>> str21.rsplit('w',3)

['http://', '', '', '.baidu.com']
```
| splitlines(...) 行分割
|      S.splitlines([keepends]) -> list of strings
|
|      Return a list of the lines in S, breaking at line boundaries.
|      Line breaks are not included in the resulting list unless keepends
|      is given and true.
```
>>> str22='www\nbaidu\ncom'

>>> str22.splitlines()

['www', 'baidu', 'com']
```
| startswith(...) 同endswith()
|      S.startswith(prefix[, start[, end]]) -> bool
|
|      Return True if S starts with the specified prefix, False otherwise.
|      With optional start, test S beginning at that position.
|      With optional end, stop comparing S at that position.
|      prefix can also be a tuple of strings to try.
| swapcase(...)   字符串中大小写转换，大写变小写，小写变大写--- upper()，lower()
|      S.swapcase() -> str
|
|      Return a copy of S with uppercase characters converted to lowercase
|      and vice versa.
```
>>> str23='asdf123'

>>> str23.swapcase()

'ASDF123'

>>> str24='ABcd222'

>>> str24.swapcase()

'abCD222'
```
| title(...) 返回一个titile标题
|      S.title() -> str
|
|      Return a titlecased version of S, i.e. words start with title case
|      characters, all remaining cased characters have lower case.
```
>>> str25='a good persion asdf 213'

>>> str25.title()

'A Good Persion Asdf 213'
```
| zfill(...) 总长width个字符，前面补0
|      S.zfill(width) -> str
|
|      Pad a numeric string S with zeros on the left, to fill a field
|      of the specified width. The string S is never truncated.
```
>>> str25.zfill(30)
'0000000a good persion asdf 213'
```

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