Exception Handling try, except, finally, Custom Exceptions

Python Basics: File Handling

Learn to read and write text, CSV, and JSON files – your gateway to data persistence!

1. Text Files

Reading Text Files

Use open() and read() to read files. Always close files with close() or use with for auto-closing.

Example: Read a story from a file.

# Method 1: Using with (recommended)  
with open("story.txt", "r") as file:  
    content = file.read()  
    print(content)  

# Method 2: Manual close  
file = open("story.txt", "r")  
print(file.readlines())  # Read as a list of lines  
file.close()  

            

Writing to Text Files

"w": Overwrite the file.

"a": Append to the file.

Example: Save a to-do list.

tasks = ["Homework", "Practice coding", "Read a book"]  

with open("todo.txt", "w") as file:  
    for task in tasks:  
        file.write(task + "\n")  # \n adds a new line  

# Append a new task  
with open("todo.txt", "a") as file:  
    file.write("Water the plants 🌱")  

            

2. CSV Files (Comma-Separated Values)

Use the csv module for spreadsheet-like data.

Reading CSV Files

Example: Read game high scores.

import csv  

with open("scores.csv", "r") as file:  
    reader = csv.reader(file)  
    for row in reader:  
        print(f"Player: {row[0]}, Score: {row[1]}")  

            

Writing CSV Files

Example: Save student grades.

import csv  

grades = [  
    ["Alice", "A"],  
    ["Bob", "B"],  
    ["Charlie", "C"]  
]  

with open("grades.csv", "w", newline="") as file:  
    writer = csv.writer(file)  
    writer.writerow(["Name", "Grade"])  # Header  
    writer.writerows(grades)  

            

Pro Tip: Use DictReader/DictWriter for column names:

with open("grades.csv", "r") as file:  
    reader = csv.DictReader(file)  
    for row in reader:  
        print(row["Name"], row["Grade"])  

            

3. JSON Files (Structured Data)

Use the json module for data like dictionaries.

Reading JSON Files

Example: Load game settings.

import json  

with open("settings.json", "r") as file:  
    settings = json.load(file)  
    print(f"Volume: {settings['volume']}%")  

            

Writing JSON Files

Example: Save user profiles.

import json  

profile = {  
    "name": "Alex",  
    "level": 5,  
    "items": ["sword", "shield"]  
}  

with open("profile.json", "w") as file:  
    json.dump(profile, file, indent=4)  # indent for readability  

            

Real-World Projects

Project 1: To-Do List Manager (Text Files)

def add_task(task):  
    with open("todo.txt", "a") as file:  
        file.write(task + "\n")  

add_task("Walk the dog 🐕")  

            

Project 2: Grade Book (CSV)

def add_student(name, grade):  
    with open("grades.csv", "a", newline="") as file:  
        writer = csv.writer(file)  
        writer.writerow([name, grade])  

add_student("Diana", "A")  

            

Project 3: Game Save System (JSON)

def save_game(level, score):  
    data = {"level": level, "score": score}  
    with open("save.json", "w") as file:  
        json.dump(data, file)  

save_game(3, 1500)  

            

Common Mistakes

• ❌ Forgot to close files: Use with to avoid this.
• ❌ File not found: Double-check file paths.
• ❌ Wrong mode: Using "w" instead of "a" deletes old data.

Key Takeaways

• ✅ Text Files: Use open(), read(), write(), and with.
• ✅ CSV Files: Use the csv module for rows/columns.
• ✅ JSON Files: Use json.load() and json.dump() for structured data.

Practice Quiz

Which mode appends to a text file?

Answer: "a".

What’s the output of this code?

import json  
data = '{"name": "Alice"}'  
print(json.loads(data)["name"])  

            

Answer: Alice.

Fun Activity: Build a Story Generator

Create a words.json file with lists of adjectives, nouns, and verbs.

Read the JSON file and randomly generate silly sentences!

import json  
import random  

with open("words.json", "r") as file:  
    words = json.load(file)  

adj = random.choice(words["adjectives"])  
noun = random.choice(words["nouns"])  
verb = random.choice(words["verbs"])  

print(f"The {adj} {noun} loves to {verb}!")  

            

Example Output:

The fluffy cloud loves to dance!  

            

What’s Next?

Learn exception handling to gracefully manage file errors (e.g., missing files)!

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