在Python 3 中使用字符串

在Python中，字符序列称为字符串。它在Python中用于记录文本信息，例如名称。 Python字符串是“不可变的”，这意味着它们在创建后无法更改。

创建字符串

可以使用单引号、双引号甚至三引号来创建字符串。 Python将单引号视为双引号。

Python3

# creating string
# with single Quotes
String = 'Hello Geek'
print("Creating string with single quotes :", String)
  
# Creating String
# with double Quotes
String = "yes, I am Geek"
print("Creating String with double quotes :", String)
  
# Creating String
# with triple Quotes
String = '''yes, I am Geek'''
print("Creating String with triple quotes :", String)

输出

Creating string with single quotes : Hello Geek
Creating String with double quotes : yes, I am Geek
Creating String with triple quotes : yes, I am Geek

注意：小心引号！

Python3

# creating string
# with single quotes
String = 'Yes' I am geek'
print(String)

输出

File "", line 3
    String = 'Yes' I am geek'
                   ^
SyntaxError: invalid syntax

上述错误的原因是Yes' I stop the 字符串中的单引号。如果你想在 python 中打印'WithQuotes' ，这不能只用单（或双）引号来完成，它需要同时使用两者。避免此错误的最佳方法是使用双引号。

例子：

Python3

# this code prints the output within quotes. 
# print WithQuotes within single quotes 
print("'WithQuotes'") 
print("Hello 'Python'") 
    
# print WithQuotes within single quotes 
print('"WithQuotes"') 
print('Hello "Python"')

输出

'WithQuotes'
Hello 'Python'
"WithQuotes"
Hello "Python"

注意：有关详细信息，请参阅单引号和双引号 | Python

字符串索引

字符串是一个字符序列，这意味着Python可以使用索引来调用序列的一部分。有两种索引方式。

• 正索引
• 负索引

正索引

Python3

# creating a string
String = "GEEK"
  
# Show first element in string
print("The 1st element is : ", String[0])
  
# Show 2nd element in string
print("The 2nd element is : ", String[1])
  
print("The 3rd element is : ", String[2])
print("The 4th element is : ", String[3])

输出

The 1st element is :  G
The 2nd element is :  E
The 3rd element is :  E
The 4th element is :  K

负索引

Python3

# creating a string
String = "GEEK"
  
# Show last element in string
print("The 4th element is : ", String[-1])
  
# Show all element in string
print("The 3rd element is : ", String[-2])
  
print("The 2nd element is : ", String[-3])
print("The 1th element is : ", String[-4])

输出

The 4th element is :  K
The 3rd element is :  E
The 2nd element is :  E
The 1th element is :  G

更新字符串

在 Python 中，不允许 Python或字符字符串中的字符。这将导致错误，因为不支持从字符串中分配项目或删除项目。 Python可以允许您将新字符串重新分配给现有的字符串。

Python3

# Creating string 
String = "Geeks"
  
# assign new character
String[0] = "Hi!, Geeks"

输出

Traceback (most recent call last):
 File “/home/b298782a4e04df4950426bf4bd5bee99.py”, line 5, in
   String[0] = “Hi!, Geeks”
TypeError: ‘str’ object does not support item assignment

更新整个字符串

Python3

# Creating string
String = "Hello Geeks"
print("Before updating : ", String)
  
# updating entire string
String = "Geeksforgeeks"
print("After updating : ", String)
  
# Update with indexing
String = 'Hello World!'
print("Updated String :- ", String[:6] + 'Python')

输出

Before updating :  Hello Geeks
After updating :  Geeksforgeeks
Updated String :-  Hello Python

字符串切片

Python切片是关于通过从头到尾分别切片来从给定字符串中获取子字符串。
Python切片可以通过两种方式完成。

• slice() 构造函数
• 扩展索引

Python3

# Creating a String
String = "Geekforgeeks"
  
s1 = slice(3)
  
# print everything except the first element
print(String[s1])
  
# print everything UP TO the 6th index
print(String[:6])
  
# print everything between both index
print(String[1:7])

输出

Gee
Geekfo
eekfor

使用负索引切片。

Python3

# Creating a String
String = "Geekforgeeks"
  
s1 = slice(-1)
  
# print everything except the last element
print(String[s1])
  
# print everything between both index
print(String[0:-3])

输出

Geekforgeek
Geekforge

我们可以使用 [ : : ] 来指定打印元素的频率。它指定从给定索引开始打印每个元素的步骤。如果没有给出任何内容，则它从第 0 个索引开始。

Python3

# Creating a String
String = "Geekforgeeks"
  
# print everything with step 1
print(String[::1])
  
# print everything with step 2
print(String[2::2])
  
# print a string backwards
print(String[::-1])

输出

Geekforgeeks
efrek
skeegrofkeeG

注意：有关详细信息，请参阅Python中的字符串切片

字符串格式

str.format() 和f-strings方法用于将格式化对象添加到打印的字符串语句中。 字符串 format() 方法格式化给定的字符串。它允许多次替换和值格式化。

Python3

# using format option in a simple string
String = 'Geeksforgeeks'
print("{}, A computer science portal for geeks."
      .format(String))
  
String = 'Geeks'
print("Hello {}, How are you ?".format(String))
  
# formatting a string using a numeric constant
val = 2
print("I want {} Burgers! ".format(val))

输出

Geeksforgeeks, A computer science portal for geeks.
Hello Geeks, How are you ?
I want 2 Burgers! 

注意：有关更多信息，请参阅Python |格式（）函数

格式化的 f 字符串字面量以 'f' 和花括号 { } 为前缀，其中包含将替换为其值的表达式。

Python3

# Creating string
String = 'GeekForGeeks'
print(f"{String}: A Computer Science portal for geeks")
  
# Creating string
String = 'Geek'
print(f"Yes, I am {String}")
  
# Manuplating int within {}
bags = 3
book_in_bag = 12
print(f'There are total {bags * book_in_bag} books')
  
# work with dictionaries in f-strings
Dic = {'Portal': 'Geeksforgeeks', 'for': 'Geeks'}
print(f"{Dic['Portal']} is a computer science portal for {Dic['for']}")

输出

GeekForGeeks: A Computer Science portal for geeks
Yes, I am Geek
There are total 36 books
Geeksforgeeks is a computer science portal for Geeks

注意：有关更多信息，请参阅Python 3 中的 f-strings – 格式化字符串字面量