Appending data to a file

Objective
By the end of this lesson, students will understand how to add content to an existing file in Python using the 'a' mode. They will learn to use write() and writelines() to append data effectively and will understand the difference between appending and overwriting.


1. Introduction to file appending:

Appending data allows you to add new content to the end of an existing file without deleting the existing content. In Python, this is accomplished by opening the file in 'a' (append) mode.


2. Using 'a' mode for appending:

When a file is opened in 'a' mode:

- New data is added after existing content.
- If the file doesn’t exist, Python will create it.
  

with open("notes.txt", "a") as file:
    file.write("Appending a new line.\n")

In this example, "Appending a new line.\n" is added to the end of notes.txt. If notes.txt does not exist, it will be created.


3. Appending multiple lines with writelines():

The writelines() method can be used in 'a' mode to add multiple lines at once. Just like with write(), writelines() does not add newlines automatically, so they must be included in each string if needed.

lines = ["First appended line\n", "Second appended line\n"]
with open("notes.txt", "a") as file:
    file.writelines(lines)

This code appends each line from the list to notes.txt.


4. Comparing 'w' Mode and 'a' mode:

It’s important to choose the mode carefully to avoid unintended data loss.


5. Practical use cases for appending data:

6. Practical examples and exercises:

Exercise 1: Creating a log file
1. Open a file in 'a' mode.
2. Append a timestamped log entry each time the code is run.

Exercise 2: Collecting user data
1. Ask the user to enter their name and favorite color.
2. Append their response to a file in 'a' mode.

Exercise 3: Experimenting with newlines
1. Open a file in 'a' mode and append lines without '\n'.
2. Re-run the code with '\n' added, and observe the difference.

Exercise 4: Appending lists
1. Use writelines() to append a list of names to a file.
2. Test the code with and without newlines.


Conclusion
In this lesson, students learned how to append data to files in Python using 'a' mode. They explored using write() and writelines() in append mode and saw practical examples of when to use file appending over overwriting. Appending data is valuable for logging, user input collection, and gradually building up a file’s contents over time.