Python3字符串

字符串是 Python 中最常用的数据类型。我们可以使用引号( ‘ 或 “ )来创建字符串。
创建字符串很简单，只要为变量分配一个值即可。例如：

var1 = 'Hello World!'
var2 = "Runoob"

Python 访问字符串中的值 (查）

Python 不支持单字符类型，单字符在 Python 中也是作为一个字符串使用。
Python 访问子字符串，可以使用方括号来截取字符串，如下实例：

var1 = 'Hello World!'
var2 = "Runoob"
print ("var1[0]: ", var1[0])
print ("var2[1:5]: ", var2[1:5])

Python 字符串更新 （改）

var1 = 'Hello World!'
print ("已更新字符串 : ", var1[:6] + 'Runoob!')

Python字符串运算符

下表实例变量a值为字符串 “Hello”，b变量值为 “Python”：

print( R’\n’ ) | | % | 格式字符串 | 请看下一节内容。 |

Unicode 字符串

Python中定义一个Unicode字符串和定义一个普通字符串一样简单：

>>> u'Hello World !'
u'Hello World !'

引号前小写的”u”表示这里创建一个Unicode字符串。如果你想加入一个特殊字符，可以使用Python的Unicode-Escape编码。如下例所示：

>>> u'Hello\u0020World !'
u'Hello World !'

IN and NOT IN

a = "Hello"
b = "Python"
print "a + b 输出结果：", a + b 
print "a * 2 输出结果：", a * 2 
print "a[1] 输出结果：", a[1] 
print "a[1:4] 输出结果：", a[1:4] 
if( "H" in a) :
    print "H 在变量 a 中" 
else :
    print "H 不在变量 a 中" 
if( "M" not in a) :
    print "M 不在变量 a 中" 
else :
    print "M 在变量 a 中"
print r'\n'
print R'\n'

循环（迭代）
#可以迭代的变量有str，list，dict，set，tuple

s10='hello world'
count=0
while count < len(s10):
    print(s10[count])
    count += 1
#输出
h
e
l
l
o
w
o
r
l
d

字符串循环

s10='hello world'
i=0
while i < len(s10):
    print(s10[i])
    i += 1

s10='hello world'
for c in  s10:
    print(c)

Python字符串格式化**
python字符串格式化符号：

格式化操作符辅助指令：

print("{} {}".format('hello', 'word'))  # 不设置指定位置，按默认顺序
hello word
print("{1} {0}".format('hello', 'word'))   # 设置指定位置
word hello
print("{1} {0} {1}".format('hello', 'word'))  # 设置指定位置
word hello word

print('my name is {name},my age is {age}'.format(name='jack', age=18))
my name is jack,my age is 18
# 通过字典设置参数
dic = {'name':'jack','age':18}
print('my name is {name},my age is {age}'.format(**dic))
my name is jack,my age is 18
# 通过列表索引设置参数
my_list = ['jack', 18]
print('my name is {0},my age is {1}'.format(*my_list))
my name is jack,my age is 18
my_list = ['jack', 18]
print('my name is {0[0]},my age is {0[1]}'.format(my_list)) # "0" 是必须的
my name is jack,my age is 18

案例1：

'Sam has {1:d} red balls and {0:d} yellow balls'.format(12, 31)
#输出
Sam has 31 red balls and 12 yellow balls

案例2： 通过列表索引设置参数

array = [34, 66, 12]
"A = {0}, B = {1}, C = {2}".format(*array)
#输出
A = 34, B = 66, C = 12

案例3： 通过字典设置参数

d = {
'hats' : 122,
'mats' : 42
}
"Sam had {hats} hats and {mats} mats".format(**d)
#输出
Sam had 122 hats and 42 mats

案例4：通过字典设置参数

'Sam has {red} red balls and {green} yellow balls'.format(red = 12, green = 31)

内置函数

startswith()判断字符串以某个字符串开头，返回boolean类型

name = "zhangsan"
isTrue = name.startswith("zh")
print(isTrue)
# 打印结果:True

endswith()判断字符串以某个字符串结尾，返回boolean类型

name = "zhangsan"
isTrue = name.endswith("san")
print(isTrue)
# 打印结果:True

find()在字符串中查找指定字符串，找不到时返回-1:

name = "zhangsan"
isIn = name.find("an")
print(isIn)
# 打印结果:2
# 返回字符串开始的下标

index()在字符串中查找指定的字符串,找不到时直接报错

name = "zhangsan"
isIn = name.index('h')
print(isIn)
# 打印结果:1

format()格式化输出字符串

name = "{} {} zhangsan" #"{} {}"是占位符，用下边format()里边的参数填充
name = name.format('I', 'am')
print(name)
# 打印结果:I am zhangsan

join()字符串连接

name = "zhangsan"
name = '*'.join(name)
print(name)
# 打印结果:z*h*a*n*g*s*a*n
name=['h','e','l','l','o']
newname=''.join(name)
print(newname)
#打印结果：hello

isalnum()判断字符串是否包含字母数字字符

name = "zhangsan1"
isTrue = name.isalnum()
print(isTrue)
# 打印结果:True
name = "zhangsan1*"
isTrue = name.isalnum()
print(isTrue)
# 打印结果:False

isalpha()判断是否只包含字母:

name = "zhangsan"
isTrue = name.isalpha()
print(isTrue)
# 打印结果:True
name = "zhangsan1"
isTrue = name.isalpha()
print(isTrue)
# 打印结果:False

isdigit()判断字符串只由数字构成:

name = "1234"
isTrue = name.isdigit()
print(isTrue)
# 打印结果:True
name = "zhangsan1234"
isTrue = name.isdigit()
print(isTrue)
# 打印结果:False

#错误3次就退出
restr=''
count=0
while count < 3:
    num = input("请%s输入：" % restr)
    if num.isdigit():
        num = int(num)
        print(num)
        break
    elif (count >= 2):
        print("错误次数太多")
        break
    else:
        print("请输入数字")
        restr = "再次"
        count += 1

isspace()判断字符串是否是空格:

name = " "
isTrue = name.isspace()
print(isTrue)
# 打印结果:True
name = "zhang san"
isTrue = name.isspase()
print(isTrue)
# 打印结果:False

isupper()判断字符串是否全是大写字符:

name = "ZHANGSAN"
isTrue = name.isupper()
print(isTrue)
# 打印结果:True

lower()将所有的大写字符转为小写字符:

name = "ZHANGSAN"
name = name.lower()
print(name)
# 打印结果:zhangsan

lstrip()去除字符串左边的空格:

name = "   zhangsan  "
name = name.lstrip()
print(name)
# 打印结果:zhangsan

rstrip()去除字符串右边的空格:

name = "   zhangsan  "
name = name.rstrip()
print(name)
# 打印结果:   zhangsan

strip()去除字符串2边的空格:

name = "   zhangsan   "
name = name.strip()
print(name)
# 打印结果:zhangsan

replace()字符串替换:

name = "zhangsan"
name = name.replace("zhang", "li")
print(name)
# 打印结果:lisan

split()字符串分割，默认是空格:

name = "zhang san"
name = name.split()
print(name)
# 打印结果:['zhang', 'san']