Python File I/O & Error Handling Explained — Complete Guide with Examples

# Read entire file at once (small files only)
with open("data.txt", "r") as file:
    content = file.read()
    print(content)

# Line by line (memory efficient for large files)
with open("data.txt", "r") as file:
    for line in file:
        print(line.strip())  # strip() removes the newline character

# All lines as a list
with open("data.txt", "r") as file:
    lines = file.readlines()
    print(lines)

# Write (overwrites existing content!)
with open("output.txt", "w") as file:
    file.write("Hello, World!\n")
    file.write("Second line\n")

# Append (adds to the end without overwriting)
with open("output.txt", "a") as file:
    file.write("Third line\n")

# Write multiple lines
lines = ["apple\n", "banana\n", "cherry\n"]
with open("fruits.txt", "w") as file:
    file.writelines(lines)

# Manual close (easy to forget)
file = open("data.txt", "r")
content = file.read()
file.close()           # If you forget, the file stays open

# Automatic close (preferred!)
with open("data.txt", "r") as file:
    content = file.read()
# File is closed automatically here

try:
    number = int(input("Enter a number: "))
    result = 10 / number
    print(f"Result: {result}")
except ValueError:
    print("That's not a valid number!")
except ZeroDivisionError:
    print("Can't divide by zero!")
except Exception as e:
    print(f"Something went wrong: {e}")
else:
    print("No errors occurred!")
finally:
    print("This always runs.")

def set_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    if age > 150:
        raise ValueError("Age seems unrealistic")
    print(f"Age set to {age}")

try:
    set_age(-5)
except ValueError as e:
    print(f"Error: {e}")

import json

# Write Python dict to JSON file
data = {"name": "Alice", "scores": [85, 92, 78]}
with open("data.json", "w") as f:
    json.dump(data, f, indent=2)

# Read JSON file back to Python dict
with open("data.json", "r") as f:
    loaded = json.load(f)
    print(loaded["name"])  # "Alice"

import csv

# Write
with open("students.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "grade"])
    writer.writerow(["Alice", 85])
    writer.writerow(["Bob", 92])

# Read
with open("students.csv", "r") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)  # ["name", "grade"], ["Alice", "85"], ...

f = open("data.txt", "r")
data = f.read()
# forgot f.close() — file stays open

with open("data.txt") as f:
    content = f.read()     # reads entire file
    again = f.read()       # empty string — cursor at end!

with open("log.txt", "w") as f:  # Overwrites existing log!
    f.write("New entry")

try:
    result = 10 / int(user_input)
except Exception:     # Catches everything, even Ctrl+C!
    print("Error")

with open("data.csv", "w") as f:
    writer = csv.writer(f)   # Blank lines between rows!

data = json.loads(bad_json_string)  # Crashes on malformed JSON

import json

# Simulate reading/writing (in-memory)
data = {
    "users": [
        {"name": "Alice", "age": 25},
        {"name": "Bob", "age": 30},
    ]
}

json_str = json.dumps(data, indent=2)
print("=== JSON Output ===")
print(json_str)

# Error handling demo
def safe_divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return "Cannot divide by zero!"

print(f"\nsafe_divide(10, 2) = {safe_divide(10, 2)}")
print(f"safe_divide(10, 0) = {safe_divide(10, 0)}")

import json
from datetime import datetime

JOURNAL_FILE = "journal.json"

def load_entries() -> list:
    try:
        with open(JOURNAL_FILE, "r") as f:
            return json.load(f)
    except FileNotFoundError:
        return []

def save_entries(entries: list) -> None:
    with open(JOURNAL_FILE, "w") as f:
        json.dump(entries, f, indent=2)

def add_entry(text: str, entries: list) -> None:
    entry = {
        "date": datetime.now().strftime("%Y-%m-%d %H:%M"),
        "text": text,
    }
    entries.append(entry)
    save_entries(entries)
    print("Entry saved!")

def show_entries(entries: list) -> None:
    if not entries:
        print("No entries yet.")
        return
    print("\n=== MY JOURNAL ===")
    for i, entry in enumerate(entries, 1):
        print(f"{i}. [{entry['date']}] {entry['text']}")
    print("=" * 20)

# Try it out
journal = load_entries()
print(f"Loaded {len(journal)} existing entries.")

add_entry("Started learning Python file I/O", journal)
add_entry("Built my first journal app", journal)
show_entries(journal)