File Handling in Python

What is File Handling?

File handling lets a program save data to a file and read it back later — so data is not lost when the program ends. This is how real apps store notes, records and reports.

File Modes

Open, Read, Close

The basic flow: open() the file, do your work, then close() it. Better still, use with which closes automatically.

Program 1: Write to a File

f = open("notes.txt", "w")
f.write("Hello from Python\n")
f.write("This is line 2\n")
f.close()
print("File written successfully")

• "w" creates notes.txt (or erases it if it exists).
• \n moves to a new line; .close() saves the file.

Program 2: Read a File

f = open("notes.txt", "r")
content = f.read()
f.close()
print(content)

.read() returns the whole file as one string.

Program 3: Append Data

f = open("notes.txt", "a")
f.write("This line was added later\n")
f.close()
print("Line appended")

"a" keeps the old content and adds the new line at the end (unlike "w" which erases).

Program 4: Read Line by Line with `with`

with open("notes.txt", "r") as f:
    for line in f:
        print(line.strip())
# file closes automatically here

• with closes the file automatically — even if an error happens.
• Looping over f reads one line at a time (memory-friendly).
• .strip() removes the extra newline.

Common Mistakes

• Using "w" when you meant "a" — it erases the file!
• Forgetting to close the file (use with to be safe).
• Opening a non-existent file in "r" mode → FileNotFoundError.

Practice Tasks

1. Write 5 student names to a file, one per line.
2. Read the file and count how many lines it has.
3. Append today's date to a log file each time the program runs.
4. Read a file and print it in UPPERCASE.

Summary

• File handling saves and reads data permanently.
• Modes: "r" read, "w" write/erase, "a" append.
• Use with open(...) as f: — it closes the file automatically.