Python追加到文件

先决条件：

• 文件处理基础
• 访问模式

在读取或写入文件时，访问模式控制打开的文件中可能的操作类型。它指的是文件打开后将如何使用。这些模式还定义了文件句柄在文件中的位置。文件句柄就像一个游标，它定义了必须从哪里读取或写入文件中的数据。

要将新行追加到现有文件，请以追加模式打开文件，使用'a'或'a+'作为访问模式。这些访问模式的定义如下：

• Append Only ('a')：打开文件进行写入。如果文件不存在，则创建该文件。句柄位于文件的末尾。正在写入的数据将插入到末尾，在现有数据之后。
• Append and Read ('a+')：打开文件进行读写。如果文件不存在，则创建该文件。句柄位于文件的末尾。正在写入的数据将插入到末尾，在现有数据之后。

当文件以追加模式打开时，句柄位于文件末尾。正在写入的数据将插入到末尾，在现有数据之后。让我们看下面的示例来阐明写入模式和附加模式之间的区别。

例子：

# Python program to illustrate
# Append vs write mode
file1 = open("myfile.txt", "w")
L = ["This is Delhi \n", "This is Paris \n", "This is London"]
file1.writelines(L)
file1.close()
   
# Append-adds at last
file1 = open("myfile.txt", "a")  # append mode
file1.write("Today \n")
file1.close()
   
file1 = open("myfile.txt", "r")
print("Output of Readlines after appending")
print(file1.read())
print()
file1.close()
   
# Write-Overwrites
file1 = open("myfile.txt", "w")  # write mode
file1.write("Tomorrow \n")
file1.close()
   
file1 = open("myfile.txt", "r")
print("Output of Readlines after writing")
print(file1.read())
print()
file1.close()

输出：

Output of Readlines after appending
This is Delhi
This is Paris
This is LondonToday


Output of Readlines after writing
Tomorrow

从新行追加数据

在上面的例子中，可以看出数据不是从新行追加的。这可以通过将字符'\n'写入文件来完成。

注意： '\n'被视为两个字节的特殊字符。

例子：

# Python program to illustrate
# append from new line
  
file1 = open("myfile.txt", "w")
L = ["This is Delhi \n", "This is Paris \n", "This is London"]
file1.writelines(L)
file1.close()
  
# Append-adds at last
# append mode
file1 = open("myfile.txt", "a")  
  
# writing newline character
file1.write("\n")
file1.write("Today")
  
# without newline character
file1.write("Tomorrow")
  
  
file1 = open("myfile.txt", "r")
print("Output of Readlines after appending")
print(file1.read())
print()
file1.close()

输出：

Output of Readlines after appending
This is Delhi
This is Paris
This is London
TodayTomorrow

带声明

Python中的with语句用于异常处理，以使代码更简洁、更具可读性。它简化了文件流等公共资源的管理。与上述实现不同，使用with语句时无需调用file.close() 。 with语句本身确保正确获取和释放资源。

例子：

# Program to show various ways to
# append data to a file using
# with statement
  
L = ["This is Delhi \n", "This is Paris \n", "This is London \n"]
  
# Writing to file
with open("myfile.txt", "w") as file1:
    # Writing data to a file
    file1.write("Hello \n")
    file1.writelines(L)
  
# Appending to file
with open("myfile.txt", 'a') as file1:
    file1.write("Today")
  
  
# Reading from file
with open("myfile.txt", "r+") as file1:
    # Reading form a file
    print(file1.read())

输出：

Hello
This is Delhi
This is Paris
This is London
Today

注意：要了解有关 with 声明的更多信息，请单击此处。