File Handling in Python

# File Handling in Python

Welcome to Chapter 16! File handling is essential for real-world programming — saving data, reading configurations, processing CSV files, and more.

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1. Learning Objectives #

• Open, read, write, and close files.

• Use context managers (with statement).

• Work with different file modes.

• Read and write CSV files.

• Build a notes manager project.

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2. Opening and Reading Files #

```python id="py16_ex1" # Method 1: Open and close manually file = open("example.txt", "r") content = file.read() print(content) file.close()

# Method 2: Context manager (recommended!) with open("example.txt", "r") as file: content = file.read() print(content) # File is automatically closed after the block

### Reading Methods

python id="py16_ex2" with open("example.txt", "r") as f: # Read entire file content = f.read() # Read one line f.seek(0) # Reset position line = f.readline() # Read all lines as list f.seek(0) lines = f.readlines() # Iterate line by line (memory efficient) f.seek(0) for line in f: print(line.strip())

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## 3. Writing to Files

python id="py16_ex3" # Write mode ('w') — overwrites existing content with open("output.txt", "w") as f: f.write("Hello, World!\n") f.write("Python File Handling\n")

# Append mode ('a') — adds to end with open("output.txt", "a") as f: f.write("This line is appended!\n")

# Write multiple lines lines = ["Line 1\n", "Line 2\n", "Line 3\n"] with open("output.txt", "w") as f: f.writelines(lines)

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## 4. File Modes

| Mode | Description |
|------|-------------|
| `r` | Read (default). Error if file doesn't exist |
| `w` | Write. Creates or overwrites file |
| `a` | Append. Creates file if doesn't exist |
| `x` | Create. Error if file already exists |
| `r+` | Read and write |
| `rb` | Read binary |
| `wb` | Write binary |

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## 5. Working with CSV Files

python id="py16_ex4" import csv

# Writing CSV with open("students.csv", "w", newline="") as f: writer = csv.writer(f) writer.writerow(["Name", "Age", "Grade"]) writer.writerow(["Alice", 25, "A"]) writer.writerow(["Bob", 22, "B"]) writer.writerow(["Charlie", 23, "A"])

# Reading CSV with open("students.csv", "r") as f: reader = csv.reader(f) for row in reader: print(row)

# DictReader — read as dictionaries with open("students.csv", "r") as f: reader = csv.DictReader(f) for row in reader: print(f"{row['Name']}: Grade {row['Grade']}")

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## 6. File Operations with os

python id="py16ex5" import os

# Directory operations # os.mkdir("newfolder") # os.listdir(".")

# pathlib (modern approach) from pathlib import Path

p = Path("output.txt") print(p.exists()) print(p.stem) # filename without extension print(p.suffix) # extension print(p.parent) # parent directory

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## 7. Mini Project: Notes Manager

python id="py16project" import os from datetime import datetime

NOTESFILE = "notes.txt"

def addnote(): note = input(" Enter note: ") timestamp = datetime.now().strftime("%Y-%m-%d %H:%M") with open(NOTESFILE, "a") as f: f.write(f"[{timestamp}] {note}\n") print(" ✅ Note saved!")

def viewnotes(): if not os.path.exists(NOTESFILE): print(" 📭 No notes yet!") return with open(NOTESFILE, "r") as f: notes = f.readlines() if not notes: print(" 📭 No notes yet!") else: print("\n 📝 Your Notes:") for i, note in enumerate(notes, 1): print(f" {i}. {note.strip()}")

def clearnotes(): confirm = input(" Clear all notes? (yes/no): ") if confirm.lower() == "yes": open(NOTESFILE, "w").close() print(" 🗑️ All notes cleared!")

while True: print("\n 1. Add Note") print(" 2. View Notes") print(" 3. Clear Notes") print(" 4. Exit") choice = input("\n Choose (1-4): ") if choice == "1": addnote() elif choice == "2": viewnotes() elif choice == "3": clearnotes() elif choice == "4": print(" 👋 Goodbye!") break ``

1. 1. Not closing files: Always use with (context manager).

1. 2. Using w mode accidentally: Overwrites everything. Use a to append.

1. 3. FileNotFoundError: Check with os.path.exists() before reading.

1. 4. Encoding issues: Use open("file.txt", "r", encoding="utf-8").

Q2: 'w' mode does: A) Reads only B) Appends C) Overwrites/creates D) Creates only Answer: C

Q3: readlines() returns: A) String B) List of strings C) Tuple D) Generator Answer: B

Q5: csv.DictReader returns rows as: A) Lists B) Tuples C) Dicts D) Sets Answer: C

Q6: with statement ensures: A) Faster reading B) File is auto-closed C) File is encrypted D) Binary mode Answer: B

Q8: 'rb' mode is for: A) Read text B) Read binary C) Read backward D) Read raw Answer: B

Q9: pathlib.Path is: A) Old-style B) Modern OOP file path handling C) Only for Linux D) Deprecated Answer: B

Q10: newline="" in csv.writer prevents: A) Errors B) Extra blank lines C) Unicode issues D) Slow writing Answer: B

1. 1. What is a context manager? An object implementing _enter and exit_ methods, used with with to manage resources.

1. 2. Difference between read(), readline(), readlines()? read() returns entire content, readline() one line, readlines() list of all lines.

1. 3. How to read large files efficiently? Iterate line by line with for line in file: instead of read() or readlines().

1. 5. How to handle file encoding? Specify encoding="utf-8" in open().

• Use with open() (context manager) for automatic file closing.

• Modes: r (read), w (write/overwrite), a (append), x (create).

• Use csv module for CSV files.

• Use os.path or pathlib for file system operations.

• Always handle FileNotFoundError` gracefully.

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12. Next Chapter Recommendation #

In Chapter 17: Exception Handling, you'll learn to handle errors gracefully with try/except/finally! 🚀