File Operations

File Handling in Python: Your Digital Filing Cabinet 📁

Imagine you have a magical filing cabinet in your room. This cabinet can hold letters, pictures, homework, and secrets. But to use it, you need to know how to:

• Open the drawer (open the file)
• Read what’s inside (read from files)
• Put new things in (write to files)
• Close the drawer properly (close the file)

Python gives you this exact magical cabinet for your computer!

The Big Picture

graph TD
    A["Your Python Code"] --> B["Open File"]
    B --> C{Choose Mode}
    C --> D["Read 'r'"]
    C --> E["Write 'w'"]
    C --> F["Append 'a'"]
    D --> G["Get Content"]
    E --> H["Create/Replace"]
    F --> I["Add to End"]
    G --> J["Close File"]
    H --> J
    I --> J

Opening Files: Unlocking the Cabinet

Before reading or writing, you must open the file. Think of it like unlocking a drawer with a key.

The Magic Spell

file = open("diary.txt", "r")

What’s happening?

• "diary.txt" → The name of your file
• "r" → The mode (read mode)
• file → Your handle to work with it

Real Example

# Open a file called "story.txt"
my_file = open("story.txt", "r")
print("File opened!")
my_file.close()  # Always close!

Why close? Imagine leaving your cabinet drawer open forever. Bad things happen! Files can get corrupted. Always close what you open.

File Modes: Choosing Your Superpower

When you open a file, you pick a mode. Each mode gives you different powers:

The Key Difference

# WRITE mode - Erases everything first!
file = open("notes.txt", "w")
file.write("New content only")
file.close()

# APPEND mode - Adds to the end!
file = open("notes.txt", "a")
file.write("\nMore content added")
file.close()

Remember: "w" is like getting a blank page. "a" is like adding to your list.

Reading from Files: Peeking Inside

There are THREE ways to read. Pick the one that fits!

Method 1: Read Everything

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

This grabs ALL the text at once. Great for small files!

Method 2: Read Line by Line

file = open("shopping.txt", "r")
first_line = file.readline()
second_line = file.readline()
print(first_line)
print(second_line)
file.close()

Each readline() grabs ONE line. Like reading one item from your shopping list.

Method 3: Get All Lines as a List

file = open("names.txt", "r")
all_lines = file.readlines()
print(all_lines)
# Output: ['Alice\n', 'Bob\n', 'Charlie\n']
file.close()

Now you have a list! Each line is one item.

Loop Through Lines (Best Practice)

file = open("tasks.txt", "r")
for line in file:
    print(line.strip())
file.close()

.strip() removes the \n (newline character) at the end.

Writing to Files: Leaving Your Mark

Writing puts YOUR words into a file. It’s like writing in your diary.

Write a Single Line

file = open("message.txt", "w")
file.write("Hello, World!")
file.close()

Important: write() doesn’t add newlines automatically!

Write Multiple Lines

file = open("list.txt", "w")
file.write("Apples\n")
file.write("Bananas\n")
file.write("Cherries\n")
file.close()

Or use writelines():

fruits = ["Apples\n", "Bananas\n", "Cherries\n"]
file = open("list.txt", "w")
file.writelines(fruits)
file.close()

The with Statement: Your Safety Net

Here’s the best way to work with files. It’s like having a robot that ALWAYS closes the file for you!

The Old Way (Risky)

file = open("data.txt", "r")
content = file.read()
# What if error happens here?
file.close()  # Might never run!

The Smart Way (Safe)

with open("data.txt", "r") as file:
    content = file.read()
    print(content)
# File closes automatically here!

Magic! When the with block ends, Python closes the file. Even if an error happens!

Why with is Amazing

graph TD
    A["with open..."] --> B["File Opens"]
    B --> C["Your Code Runs"]
    C --> D{Error?}
    D -->|Yes| E["File Still Closes!"]
    D -->|No| F["File Closes"]
    E --> G["Safe!"]
    F --> G

Pro tip: Always use with for file handling. It’s the Pythonic way!

File Position Methods: Where Am I?

When you read a file, Python uses a cursor (like your finger pointing at words while reading).

Key Methods

See It in Action

with open("sample.txt", "r") as file:
    print(file.read(5))     # Read first 5 chars
    print(file.tell())      # Position: 5

    file.seek(0)            # Go back to start
    print(file.tell())      # Position: 0

    print(file.read())      # Read everything

The Cursor Journey

Imagine your file contains: HELLO

Position: 0   1   2   3   4   5
Content:  H   E   L   L   O  [end]
          ^
          Cursor starts here

After read(3):

Position: 0   1   2   3   4   5
Content:  H   E   L   L   O  [end]
                  ^
                  Now here!

Binary vs Text Files: Two Worlds

Files come in two flavors:

Text Files

• Human-readable
• Contains letters, numbers, symbols
• Examples: .txt, .csv, .json, .py

with open("story.txt", "r") as file:
    text = file.read()
    print(text)  # Readable!

Binary Files

• Computer-readable
• Contains raw bytes (0s and 1s)
• Examples: .jpg, .mp3, .pdf, .exe

with open("photo.jpg", "rb") as file:
    data = file.read()
    print(data[:20])  # Shows bytes
# Output: b'\xff\xd8\xff\xe0\x00\x10JFIF...'

Copying a Binary File

# Copy an image
with open("original.jpg", "rb") as source:
    data = source.read()

with open("copy.jpg", "wb") as dest:
    dest.write(data)
print("Image copied!")

Quick Comparison

Putting It All Together

Here’s a complete example that shows everything working:

# Create a to-do list app

# 1. Write tasks to file
with open("todos.txt", "w") as file:
    file.write("Buy groceries\n")
    file.write("Do homework\n")
    file.write("Call mom\n")

# 2. Read and display tasks
print("Your Tasks:")
with open("todos.txt", "r") as file:
    for i, line in enumerate(file, 1):
        print(f"{i}. {line.strip()}")

# 3. Add a new task
with open("todos.txt", "a") as file:
    file.write("Learn Python\n")

# 4. Show updated list
print("\nUpdated Tasks:")
with open("todos.txt", "r") as file:
    print(file.read())

Output:

Your Tasks:
1. Buy groceries
2. Do homework
3. Call mom

Updated Tasks:
Buy groceries
Do homework
Call mom
Learn Python

Common Mistakes to Avoid

1. Forgetting to close files

# BAD
file = open("data.txt")
data = file.read()
# Forgot to close!

# GOOD
with open("data.txt") as file:
    data = file.read()
# Auto-closes!

2. Wrong mode for operation

# BAD - Can't write in read mode
file = open("data.txt", "r")
file.write("Hello")  # Error!

# GOOD
file = open("data.txt", "w")
file.write("Hello")  # Works!

3. File not found

# Handle missing files gracefully
try:
    with open("missing.txt", "r") as file:
        content = file.read()
except FileNotFoundError:
    print("Oops! File doesn't exist.")

Key Takeaways

1. Always open before using - Use open()
2. Choose the right mode - r, w, a, or binary versions
3. Use with statement - It closes files automatically
4. Text vs Binary - Know which type you’re working with
5. tell() and seek() - Navigate within files

You now have the keys to Python’s filing cabinet! Practice these concepts, and you’ll be a file-handling master in no time.