Day 8: File I/O and Exception Handling

What You'll Learn Today

• Reading and writing files
• Context managers (with statement)
• Various file formats (text, CSV, JSON)
• Exception handling (try-except)
• Custom exceptions

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Basic File Operations

Opening Files

# Basic approach
file = open('example.txt', 'r')  # Open in read mode
content = file.read()
file.close()  # Must close

# Using with statement (recommended)
with open('example.txt', 'r') as file:
    content = file.read()
# File is automatically closed

File Modes

Mode	Description
'r'	Read (default)
'w'	Write (overwrite)
'a'	Append
'x'	Create new (error if exists)
'b'	Binary mode
't'	Text mode (default)
'+'	Read and write

# Combinations
'rb'   # Binary read
'w+'   # Read/write (overwrite)
'ab'   # Binary append

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Reading and Writing Text Files

Reading Files

# Read entire file
with open('example.txt', 'r', encoding='utf-8') as f:
    content = f.read()
    print(content)

# Read line by line
with open('example.txt', 'r', encoding='utf-8') as f:
    for line in f:
        print(line.strip())  # Remove newline

# Read as list
with open('example.txt', 'r', encoding='utf-8') as f:
    lines = f.readlines()
    print(lines)

# Read specified bytes
with open('example.txt', 'r', encoding='utf-8') as f:
    chunk = f.read(100)  # First 100 characters

Writing Files

# Create new or overwrite
with open('output.txt', 'w', encoding='utf-8') as f:
    f.write('Hello, World!\n')
    f.write('Python is great!\n')

# Write multiple lines at once
lines = ['Line 1\n', 'Line 2\n', 'Line 3\n']
with open('output.txt', 'w', encoding='utf-8') as f:
    f.writelines(lines)

# Append
with open('output.txt', 'a', encoding='utf-8') as f:
    f.write('Additional line\n')

---

Path Operations (pathlib)

The recommended path manipulation module since Python 3.4.

from pathlib import Path

# Create path
path = Path('folder') / 'subfolder' / 'file.txt'
print(path)  # folder/subfolder/file.txt

# Current directory
current = Path.cwd()
print(current)

# Home directory
home = Path.home()
print(home)

# Path information
p = Path('example.txt')
print(p.name)      # example.txt
print(p.stem)      # example
print(p.suffix)    # .txt
print(p.parent)    # .
print(p.exists())  # True/False
print(p.is_file()) # True/False
print(p.is_dir())  # True/False

# Create directory
Path('new_folder').mkdir(exist_ok=True)
Path('a/b/c').mkdir(parents=True, exist_ok=True)

# List files
for file in Path('.').glob('*.py'):
    print(file)

# Recursive search
for file in Path('.').rglob('*.txt'):
    print(file)

flowchart TB
    subgraph Path["Path Object"]
        A["name: filename"]
        B["stem: without extension"]
        C["suffix: extension"]
        D["parent: parent directory"]
    end

    P["Path('folder/file.txt')"]
    P --> A
    P --> B
    P --> C
    P --> D

    style Path fill:#3b82f6,color:#fff

---

Working with CSV Files

csv Module

import csv

# Read CSV
with open('data.csv', 'r', encoding='utf-8') as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)  # Get as list

# Read as dictionary
with open('data.csv', 'r', encoding='utf-8') as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row)  # {'column1': 'value1', ...}

# Write CSV
data = [
    ['Name', 'Age', 'City'],
    ['Taro', 25, 'Tokyo'],
    ['Hanako', 30, 'Osaka']
]

with open('output.csv', 'w', newline='', encoding='utf-8') as f:
    writer = csv.writer(f)
    writer.writerows(data)

# Write from dictionaries
data = [
    {'Name': 'Taro', 'Age': 25, 'City': 'Tokyo'},
    {'Name': 'Hanako', 'Age': 30, 'City': 'Osaka'}
]

with open('output.csv', 'w', newline='', encoding='utf-8') as f:
    fieldnames = ['Name', 'Age', 'City']
    writer = csv.DictWriter(f, fieldnames=fieldnames)
    writer.writeheader()
    writer.writerows(data)

---

Working with JSON Files

import json

# Read JSON
with open('data.json', 'r', encoding='utf-8') as f:
    data = json.load(f)
    print(data)

# Write JSON
data = {
    'name': 'Taro',
    'age': 25,
    'hobbies': ['reading', 'programming']
}

with open('output.json', 'w', encoding='utf-8') as f:
    json.dump(data, f, indent=2)

---

Exception Handling

What Are Exceptions?

Errors that occur during program execution are called "exceptions."

flowchart TB
    A["Execute code"] --> B{Error occurred?}
    B -->|Yes| C["Exception raised"]
    B -->|No| D["Normal completion"]
    C --> E{Handler exists?}
    E -->|Yes| F["Handle error"]
    E -->|No| G["Program crashes"]

    style C fill:#ef4444,color:#fff
    style F fill:#22c55e,color:#fff
    style G fill:#ef4444,color:#fff

Basic try-except

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")

# Program continues
print("Continuing execution")

Catching Multiple Exceptions

try:
    num = int(input("Enter a number: "))
    result = 10 / num
except ValueError:
    print("Please enter a valid number")
except ZeroDivisionError:
    print("Cannot divide by zero")

# Catch multiple at once
try:
    # Some operation
    pass
except (ValueError, TypeError) as e:
    print(f"Error: {e}")

else and finally

try:
    result = 10 / 2
except ZeroDivisionError:
    print("Division by zero error")
else:
    # Executed if no exception occurred
    print(f"Result: {result}")
finally:
    # Always executed (cleanup, etc.)
    print("Processing complete")

Common Built-in Exceptions

Exception	Description
ValueError	Invalid value
TypeError	Invalid type
KeyError	Dictionary key not found
IndexError	Index out of range
FileNotFoundError	File not found
ZeroDivisionError	Division by zero
AttributeError	Attribute not found
ImportError	Import failed

Raising Exceptions

def validate_age(age):
    if age < 0:
        raise ValueError("Age must be 0 or greater")
    if age > 150:
        raise ValueError("Invalid age")
    return age

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

---

Custom Exceptions

Create your own exception classes:

class ValidationError(Exception):
    """Validation error"""
    pass

class InsufficientFundsError(Exception):
    """Insufficient funds error"""
    def __init__(self, balance, amount):
        self.balance = balance
        self.amount = amount
        message = f"Cannot withdraw ${amount} with balance of ${balance}"
        super().__init__(message)

class BankAccount:
    def __init__(self, balance):
        self.balance = balance

    def withdraw(self, amount):
        if amount > self.balance:
            raise InsufficientFundsError(self.balance, amount)
        self.balance -= amount
        return amount

# Usage
account = BankAccount(1000)
try:
    account.withdraw(1500)
except InsufficientFundsError as e:
    print(f"Error: {e}")
    print(f"Current balance: ${e.balance}")

---

File Operation Best Practices

Safe File Reading with Error Handling

from pathlib import Path

def read_file_safely(filepath):
    """Read file safely with error handling"""
    path = Path(filepath)

    if not path.exists():
        print(f"File not found: {filepath}")
        return None

    if not path.is_file():
        print(f"Not a file: {filepath}")
        return None

    try:
        with open(path, 'r', encoding='utf-8') as f:
            return f.read()
    except PermissionError:
        print(f"No permission to read file: {filepath}")
        return None
    except UnicodeDecodeError:
        print(f"Invalid file encoding: {filepath}")
        return None

# Usage
content = read_file_safely('example.txt')
if content:
    print(content)

Using Temporary Files

import tempfile

# Create temporary file
with tempfile.NamedTemporaryFile(mode='w', delete=False, suffix='.txt') as f:
    f.write('Temporary data')
    temp_path = f.name

print(f"Temp file: {temp_path}")

# Create temporary directory
with tempfile.TemporaryDirectory() as tmpdir:
    print(f"Temp directory: {tmpdir}")
    # Work inside directory
# Auto-deleted when exiting block

---

Summary

Concept	Description	Example
open()	Open file	open('file.txt', 'r')
with statement	Auto-close	with open(...) as f:
pathlib	Path operations	Path('folder') / 'file'
try-except	Exception handling	try: ... except: ...
raise	Raise exception	raise ValueError("...")

Key Takeaways

1. Always use with statement for files
2. Explicitly specify encoding (utf-8)
3. Wrap file operations in exception handling
4. Use pathlib for path operations
5. Use custom exceptions for meaningful error messages

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Practice Exercises

Exercise 1: Basics

Write a program that reads a text file and displays the character count for each line.

Exercise 2: CSV Operations

Read a CSV file in this format and calculate total and average scores:

Name,Math,English,Science
Taro,80,75,90
Hanako,95,88,85

Challenge

Create a function that reads a JSON config file and uses default values if settings are missing. Handle exceptions appropriately for missing files or invalid JSON.

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References

• Python Docs - File I/O
• Python Docs - pathlib
• Python Docs - Exceptions

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Next Up: In Day 9, you'll learn about "Standard Library Utilities." Master more powerful modules!