python 之 Strings （字符串）

https://www.programiz.com/python-programming/string

Python不支持字符类型，它们被视为长度为1的字符串，也被视为子字符串

字符串是一系列字符。它可以通过使用双引号在python中声明。字符串是不可变的，它表明，一旦将String绑定到变量，它能被删除，但不能被修改

通过在引号中放置一系列字符来创建字符串。字符串可以用单引号，双引号或三引号括起来，它们由三个单引号或三个双引号组成

三引号中的字符串可以跨越多行而不使用转义字符

默认情况下，Python 2中的字符串是非Unicode（ASCII），但也支持Unicode。

另一方面，Python 3字符串都是Unicode（UTF-8）

Strings in Python 2.

>>> print(type('Python String'))
<type 'str'>
>>> print(type(u'Python Unicode String'))
<type 'unicode'>

Strings in Python 3.

>>> print(type('Python String'))
<class 'str'>
>>> print(type(u'Python Unicode String'))
<class 'str'>

Python中的字符串运算符

字符串索引
查看字母x在字符串中的索引位置
string.index('x')

字符串切片Slicing [ ]  Range Slicing [X:y]
使用方括号与索引或索引范围一起切​​片以获得子串

string = "python"
print(string[4])
slice运算符在给定索引处打印字符

print(string[2:4])
打印给定范围内的字符
print(string[::-1])

字符串串联Concatenation (+)

string1+string2

将两个字符串合并为一个

字符串重复Repetition (*)
string*9
通过重复给定次数来创建新字符串，它实质上是一个多重串联
>>> ".-." * 4

'.-..-..-..-.'

成员资格Membership( in，not in)

检查字符串中是否存在某些字符，返回true,false

string="Long" 

print("Lo" in string)  

#True

print("Ln" in string)

#False

print("Ln" not in string)

#False

迭代Iterating (For)

使用此运算符，我们可以遍历字符串的所有字符

# example
for var in var1:print (var, end ="")

原始字符串Raw String (r/R)

# example
print (r'\n')
# \n
print (R'\n')
# \n

Python中的内置字符串函数

字符串替换  replace(old,new[,count])

var='This is a good example'
str='was'
print (var.replace('is',str))
# Thwas was a good example
print (var.replace('is',str,1))
# Thwas is a good example

string.replace('旧','新')
oldstring = 'I like You!'
newstring = oldstring.replace('like', 'love')
print(newstring)


字符串大写
string.upper()

var = 'TechBeamers'
print (var.upper())
# TECHBEAMERS

字符串小写
string.lower()

var = 'TechBeamers'
print (var.lower())
# techbeamers

字符串首字母大写
string.capitalize()
返回首个单词大写字母，其余小写字母的字符串

var = 'welcome to Python programming'
print (var.capitalize())
# Welcome to python programming

string.title()
所有单词均以大写字母开头，单词中的其余字符均以小写字母开头

var = 'welcome to Python programming'
print (var.title())
# Welcome To Python Programming

字符串大小写互换
string.swapcase()

var = 'TechBeamers'
print (var.swapcase())
# tECHbEAMERS

字符串计数count( str[, beg [, end]])

如果给出了beg和end索引，则返回子字符串'str'在[beg，end]范围内出现的次数，否则搜索将以完整字符串继续进行，字符串搜索区分大小写

var='TechBeamers'
str='e'
print (var.count(str))
# 3

var1='Eagle Eyes'
print (var1.count('e'))
# 2

var2='Eagle Eyes'
print (var2.count('E',0,5))
# 1

rjust(width[,fillchar])
返回String的填充版本，其中原始String左对齐到width列的总和 默认情况下，填充使用空格。否则，“ fillchar”指定填充字符。

var='Python'
print (var.rjust(10))
#    Python

print (var.rjust(10,'-'))
# ----Python

ljust(width[,fillchar])返回String的填充版本，其中原始String左对齐到width列的总和 默认情况下，填充使用空格。否则，“ fillchar”指定填充字符

var='Python'
print (var.ljust(10))
# Python      

print (var.ljust(10,'-'))
# Python----

center(width[,fillchar])在将原始内容推入中心的同时返回填充有输入字符的字符串。 默认情况下，填充使用空格。否则，“ fillchar”指定填充字符。

var='Python'
print (var.center(20))
#      Python

print (var.center(20,'*'))
# *******Python*******

zfill(width)返回填充有原始内容的字符串，该内容在左侧用零填充，以使String的总长度等于输入大小
如果字符串中存在前导符号（+/-），则使用此功能将在符号之后而不是符号之前开始填充

var='Python'
print (var.zfill(10))
# 0000Python

var='+Python'
print (var.zfill(10))
# +000Python

find(str [,i [,j]])在完整的字符串中搜索"str''。如果找到"str''，则此函数返回索引，否则返回"-1'' 
i = search从此索引开始，j = search从此索引结束

var="Tech Beamers"
str="Beam"
print (var.find(str))
# 5

var="Tech Beamers"
str="Beam"
print (var.find(str,4))
# 5

var="Tech Beamers"
str="Beam"
print (var.find(str,7))
# -1

index(str[,i [,j]])与“find”方法相同。唯一的区别是，如果不存在“ str”，它将引发“ ValueError”异常

var='Tech Beamers'
str='Beam'
print (var.index(str))
# 5

var='Tech Beamers'
str='Beam'
print (var.index(str,4))
# 5

var='Tech Beamers'
str='Beam'
print (var.index(str,7))
# ValueError: substring not found

rfind(str[,i [,j]])这与find（）相同，只是此函数返回找到“str”的最后一个索引。如果未找到“str”，则返回“-1”

var='This is a good example'
str='is'
print (var.rfind(str,0,10))
# 5

print (var.rfind(str,10))
# -1

count(str[,i [,j]])返回字符串中子字符串“ str”的出现次数
i = search从此索引开始，j = search从此索引结束

var='This is a good example'
str='is'
print (var.count(str))
# 2

print (var.count(str,4,10))
# 1

反转字符串
string="123456"
string[::-1]
或者"".join(reversed(string))


拆分字符串split([sep[,maxsplit]]) 
返回用“ sep”作为分隔符分割字符串后获得的子字符串列表
其中sep =分隔符，默认为空格，maxsplit =要进行的分割数

var = "This is a good example"
print (var.split())
# ['This', 'is', 'a', 'good', 'example']
print (var.split(' ', 3))
# ['This', 'is', 'a', 'good example']

word = "I love You!"
word.split(" ")
分隔符是空格，则返回['I', 'love', 'You!']

word.split("o")
分隔符是o，怎返回['I l', 've Y', 'u!']


join(seq)
返回在将序列“ seq”与定界符字符串连接后获得的字符串。

seq=('ab','bc','cd')
str='='
print (str.join(seq))
# ab=bc=cd

splitlines(num)
在换行符处拆分字符串，并在删除换行符后返回列表
其中，num =如果这是一个正值。它指示换行符将出现在返回的列表中

var='Print new line\nNextline\n\nMove again to new line'
print (var.splitlines())
# ['Print new line', 'Nextline', '', 'Move again to new line']
print (var.splitlines(1))
# ['Print new line\n', 'Nextline\n', '\n', 'Move again to new line']

lstrip([chars])
从字符串开头删除字符后，返回一个字符串，默认为空格字符

var=' This is a good example '
print (var.lstrip())
# This is a good example
var='*****This is a good example*****'
print (var.lstrip('*'))
# This is a good example**********

rstrip([chars])
从字符串末尾删除字符后，返回一个字符串
其中：Chars =这是要从字符串中修剪的字符。默认为空格字符

var=' This is a good example '
print (var.rstrip())
# This is a good example
var='*****This is a good example*****'
print (var.lstrip('*'))
# *****This is a good example

rindex(str[,i [,j]])
在完整的字符串（如果未定义i和j）或字符串的子字符串（如果已定义i和j）中搜索“ str”。此函数返回提供“ str”的最后一个索引
如果“ str”不存在，则会引发ValueError异常， 其中：i 此索引开始，j 索引结束

var='This is a good example'
str='is'
print (var.rindex(str,0,10))
# 5
print (var.rindex(str,10))
# ValueError: substring not found

len（string）
返回给定String的长度

var='This is a good example'
print (len(var))
# 22

>>> print("%(lang)s is fun!" % {"lang":"Python"})

Python is fun!

>>> "Python is as simple as {0}, {1}, {2}".format("a", "b", "c") 

'Python is as simple as a, b, c' 

>>> "Python is as simple as {1}, {0}, {2}".format("a", "b", "c") 

'Python is as simple as b, a, c' 

>>> xy = {"x":0, "y":10} 

>>> print("Graph a point at where x={x} and y={y}".format(**xy)) 

Graph a point at where x=0 and y=10



format()

# default(implicit) order
default_order = "{}, {} and {}".format('John','Bill','Sean')
print('\n--- Default Order ---')
print(default_order)

# order using positional argument
positional_order = "{1}, {0} and {2}".format('John','Bill','Sean')
print('\n--- Positional Order ---')
print(positional_order)

# order using keyword argument
keyword_order = "{s}, {b} and {j}".format(j='John',b='Bill',s='Sean')
print('\n--- Keyword Order ---')

print(keyword_order)


>>> # formatting integers
>>> "Binary representation of {0} is {0:b}".format(12)
'Binary representation of 12 is 1100'
>>> # formatting floats
>>> "Exponent representation: {0:e}".format(1566.345)
'Exponent representation: 1.566345e+03'
>>> # round off
>>> "One third is: {0:.3f}".format(1/3)
'One third is: 0.333'

# out 3.142
pi = 3.141592653589793
n = 3
print(f"%.{n}f" % pi)
print("{0:.{1}f}".format(pi, n))

print(f"{pi:.{n}f}")

>>> # string alignment   左对齐<，右对齐>或^将字符串居中
>>> "|{:<10}|{:^10}|{:>10}|".format('butter','bread','ham')

'|butter    |  bread   |       ham|'



f'Python {expr}'

expr可以是常量，表达式，字符串或数字或Python对象等形式的任何东西

version = 3.6
feature = 'f-string'

str_final = f'Python {feature} was introduced in version {version}.'
print(str_final)
# Python f-string was introduced in version 3.6.

from datetime import datetime

print(f'Date as F-string:\n{datetime.now()}')
print(fr'Date as raw string:\n{datetime.now()}')

lambda_test = f"{(lambda x: x * 25) (5)}"
print(lambda_test)
# 125

lambda_test = f"{{999}}"
print(lambda_test)
# {999}
lambda_test = f"{{{999}}}"
print(lambda_test)
# {999}