To write to an existing file in Python, you can use the open() function with specific modes. The two most common modes for writing are:

• Append Mode ("a"): Appends content to the end of the file without removing the existing content.
• Write Mode ("w"): Overwrites any existing content in the file.

 

Example: Appending to a File

Here's an example of how to append content to an existing file:

def main():     # Open the file "demofile2.txt" in append mode     f = open("demofile2.txt", "a")     # Write content to the file     f.write("Now the file has more content!")     # Close the file     f.close()     # Open and read the file after appending     f = open("demofile2.txt", "r")     # Print the content of the file     print(f.read())     # Close the file     f.close() if __name__=="__main__":     main()

 

Explanation:

• Open the File in Append Mode: Use open("demofile2.txt", "a") to open the file in append mode.
• Write to the File: The write() method appends the specified string to the end of the file.
• Close the File: Always close the file after writing to ensure all changes are saved and resources are freed.
• Read and Print the File: Reopen the file in read mode and print its content to verify that the new content has been appended.

 

Example: Overwriting a File

If you want to overwrite the existing content, you can use write mode ("w") instead:

def main():     # Open the file "demofile2.txt" in write mode     f = open("demofile2.txt", "w")     # Write content to the file, overwriting any existing content     f.write("This content will overwrite the existing content!")     # Close the file     f.close()     # Open and read the file after overwriting     f = open("demofile2.txt", "r")     # Print the content of the file     print(f.read())     # Close the file     f.close() if __name__=="__main__":     main()

 

Explanation:

• Open the File in Write Mode: Use open("demofile2.txt", "w") to open the file in write mode.
• Write to the File: The write() method overwrites any existing content with the specified string.
• Close the File: Always close the file after writing to ensure all changes are saved and resources are freed.
• Read and Print the File: Reopen the file in read mode and print its content to verify that the old content has been overwritten.

 

Best Practices

• Use with Statement: Using the with statement ensures that files are properly closed after their suite finishes, even if an exception is raised.

with open("demofile2.txt", "a") as f:     f.write("Now the file has more content!") with open("demofile2.txt", "r") as f:     print(f.read())

 

• Error Handling: Handle potential errors such as file not found or permission issues using try-except blocks.

try:     with open("demofile2.txt", "a") as f:         f.write("Now the file has more content!") except IOError as e:     print(f"An error occurred: {e}")