Python Advanced: Reading & Writing Files

Goal of the Day

Today you will learn:

• How to work with text files
• Store data that persists across program runs
• Read, write, and append files

By the end, you will be able to write programs that save data permanently, e.g., a shopping list, high scores, or notes.

Step 1: Opening Files

To work with files, use the open() function.

file = open("filename.txt", "mode")

Modes:

• "w" → write (overwrites existing file)
• "r" → read
• "a" → append (keeps existing content)

Step 2: Writing to a File

file = open("test.txt", "w")
file.write("Hello World\n")
file.close()

Explanation:

• "test.txt" → file name
• "w" → write mode
• write() → writes text to the file
• \n → new line
• close() → closes the file; very important, otherwise data may not be saved

Step 3: Reading a File

file = open("test.txt", "r")
content = file.read()
print(content)
file.close()

Explanation:

• "r" → read mode
• read() → reads the entire content
• close() → closes the file

Step 4: Appending to a File

file = open("test.txt", "a")
file.write("New line\n")
file.close()

Explanation: New data is added at the end of the file; old data remains.

Step 5: Practice – Saving User Input

A program that saves input and reads it on the next run:

# Save input
inputs = input("Enter something to save: ")
file = open("storage.txt", "a")  # append
file.write(inputs + "\n")
file.close()

# Read inputs at start
file = open("storage.txt", "r")
content = file.read()
print("Previously saved inputs:")
print(content)
file.close()

Explanation:

• Each input is appended to the end of the file
• On the next run, the program reads all previous inputs

Practice Exercise

Write a program that saves a list of tasks (To-Do list)

• Each time the program starts:
• Display existing tasks
• Add new tasks
• Save the file
• Optional: Use with open("file.txt", "r") as file: → Python closes the file automatically

with open("storage.txt", "r") as file:
    content = file.read()
    print(content)

Step 6: Tips & Best Practices

• Always close files: use close() or with
• Append instead of overwrite if you want to keep old data
• Separate lines using \n
• Name files safely → no spaces, simple names