Python basics - Day 12

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Day 12 – File Input & Output (File I/O)

Project: Build a “Simple Notepad App” that writes and reads text files.

01. Learning Goal

By the end of this lesson, you will be able to:

Open, read, and write files in Python
Understand file modes (r, w, a)
Use with statements for safe file handling
Build a simple note-taking program

02. Problem Scenario

You need to save and read data from files — such as notes, logs, or user data.

Your goal today: learn how to handle files safely and efficiently in Python.

03. Step 1 – Opening Files

f = open("test.txt", "w")   # Open file for writing
f.write("Hello File!")      # Write content
f.close()                   # Close file

open("filename", "mode")

"w" : Write (overwrites existing content)
"a" : Append (adds new content at the end)
"r" : Read

04. Step 2 – Writing to Files

f = open("example.txt", "w", encoding="utf-8")
f.write("First line\n")
f.write("Second line\n")
f.close()

encoding="utf-8" prevents text corruption when working with non-English characters.

05. Step 3 – Reading Files

Read the entire file:

f = open("example.txt", "r", encoding="utf-8")
data = f.read()
print(data)
f.close()

Read line by line:

f = open("example.txt", "r", encoding="utf-8")
line1 = f.readline()
line2 = f.readline()
print(line1, line2)
f.close()

Read all lines as a list:

f = open("example.txt", "r", encoding="utf-8")
lines = f.readlines()
for line in lines:
    print(line.strip())   # Remove newline
f.close()

06. Step 4 – Using with (Recommended Way)

with automatically closes the file, even if an error occurs.

with open("example.txt", "r", encoding="utf-8") as f:
    for line in f:
        print(line.strip())

07. Step 5 – Writing a List to a File

fruits = ["apple", "banana", "grape"]

with open("fruits.txt", "w", encoding="utf-8") as f:
    for fruit in fruits:
        f.write(fruit + "\n")

08. Step 6 – Reading and Processing File Data

with open("fruits.txt", "r", encoding="utf-8") as f:
    fruits = f.readlines()

fruits = [f.strip() for f in fruits]  # Remove line breaks
print(fruits)   # ['apple', 'banana', 'grape']

09. Step 7 – Practice Examples

Example 1: Write Notes

note = input("Enter your note: ")

with open("note.txt", "a", encoding="utf-8") as f:
    f.write(note + "\n")

Example 2: Read Notes

with open("note.txt", "r", encoding="utf-8") as f:
    print(f.read())

10. Step 8 – Mini Project: Simple Notepad App

Create a text-based notepad that saves and reads user notes.

while True:
    choice = input("1: Write note, 2: Read notes, 0: Exit → ")

    if choice == "1":
        note = input("Enter your note: ")
        with open("notes.txt", "a", encoding="utf-8") as f:
            f.write(note + "\n")
        print("Note saved!\n")

    elif choice == "2":
        with open("notes.txt", "r", encoding="utf-8") as f:
            print("\nYour Notes:")
            print(f.read())

    elif choice == "0":
        print("Exiting Notepad. Goodbye!")
        break