Python Basics — Day 12 File I/O in Python

01. Opening a File

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

• open("filename", "mode")

  • "w" : Write (overwrites existing content)

  • "a" : Append (add new content at the end)

  • "r" : Read

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02. Writing to a File

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 (e.g., Korean) from breaking.

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03. Reading from a File

Read 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 characters
f.close()

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04. Using with (Recommended)

Files must always be closed after use.
Using with automatically closes the file.

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

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05. 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")

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06. Reading Data and Processing

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

fruits = [f.strip() for f in fruits]  # Remove newlines
print(fruits)   # ['Apple', 'Banana', 'Grape']

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07. Practice Examples

Example 1: Simple Notepad (Write)

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())