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PRACTICAL PYTHON SCRIPTING - CHALLENGE 3 SOLUTION
Chapter 3: Database Connections
Challenge: DatabaseConnection Class (Production Pattern)
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PROBLEM:
Create a reusable DatabaseConnection class that:
1. Accepts configuration parameters
2. Has connect/disconnect methods
3. Has execute_query method for SELECT
4. Has execute_update method for INSERT/UPDATE/DELETE
5. Handles all errors gracefully

SOLUTION:
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import mysql.connector
from mysql.connector import Error
from typing import List, Tuple, Optional, Any

class DatabaseConnection:
    """
    Reusable database connection wrapper for MySQL/MariaDB.

    Features:
    - Connection pooling and reuse
    - Automatic error handling
    - Type hints for clarity
    - Parameterized queries (SQL injection prevention)
    - Detailed logging and error reporting
    """

    def __init__(self, host: str, user: str, password: str, database: str,
                 port: int = 3306, charset: str = 'utf8mb4'):
        """
        Initialize database connection parameters.

        Args:
            host: Database server hostname
            user: Database username
            password: Database password
            database: Database name
            port: Database port (default: 3306)
            charset: Character set (default: utf8mb4)
        """
        self.config = {
            'host': host,
            'user': user,
            'password': password,
            'database': database,
            'port': port,
            'charset': charset
        }
        self.connection = None
        self.last_error = None

    def connect(self) -> bool:
        """
        Establish connection to database.

        Returns:
            True if connection successful, False otherwise
        """
        try:
            self.connection = mysql.connector.connect(**self.config)

            if self.connection.is_connected():
                print(f"✅ Connected to {self.config['database']} "
                      f"on {self.config['host']}")
                return True

        except Error as e:
            self.last_error = e
            print(f"❌ Connection failed: {e.errno} - {e.msg}")
            return False

    def disconnect(self) -> bool:
        """
        Close connection to database.

        Returns:
            True if closed successfully, False otherwise
        """
        if self.connection and self.connection.is_connected():
            try:
                self.connection.close()
                print("✅ Connection closed")
                return True
            except Error as e:
                self.last_error = e
                print(f"❌ Error closing connection: {e}")
                return False

        return True

    def is_connected(self) -> bool:
        """Check if currently connected to database."""
        return self.connection is not None and self.connection.is_connected()

    def execute_query(self, query: str, params: Tuple = None) -> Optional[List[Tuple]]:
        """
        Execute a SELECT query and return results.

        Args:
            query: SQL SELECT query with %s placeholders for parameters
            params: Tuple of parameters to bind (optional)

        Returns:
            List of result rows (tuples), or None if error
        """
        if not self.is_connected():
            print("❌ Not connected to database")
            return None

        try:
            cursor = self.connection.cursor()

            # Execute with or without parameters
            if params:
                cursor.execute(query, params)
            else:
                cursor.execute(query)

            # Fetch all results
            results = cursor.fetchall()
            cursor.close()

            return results

        except Error as e:
            self.last_error = e
            print(f"❌ Query failed: {e}")
            return None

    def fetch_one(self, query: str, params: Tuple = None) -> Optional[Tuple]:
        """
        Execute a SELECT query and return first row only.

        Args:
            query: SQL SELECT query with %s placeholders
            params: Tuple of parameters to bind (optional)

        Returns:
            First result row (tuple), or None if no results
        """
        if not self.is_connected():
            return None

        try:
            cursor = self.connection.cursor()

            if params:
                cursor.execute(query, params)
            else:
                cursor.execute(query)

            result = cursor.fetchone()
            cursor.close()

            return result

        except Error as e:
            self.last_error = e
            print(f"❌ Query failed: {e}")
            return None

    def execute_update(self, query: str, params: Tuple = None) -> bool:
        """
        Execute INSERT, UPDATE, or DELETE query.

        Args:
            query: SQL INSERT/UPDATE/DELETE query with %s placeholders
            params: Tuple of parameters to bind (optional)

        Returns:
            True if successful, False otherwise
        """
        if not self.is_connected():
            print("❌ Not connected to database")
            return False

        try:
            cursor = self.connection.cursor()

            # Execute with or without parameters
            if params:
                cursor.execute(query, params)
            else:
                cursor.execute(query)

            # Get the number of affected rows
            affected = cursor.rowcount

            # Commit the changes
            self.connection.commit()

            cursor.close()

            print(f"✅ Query executed: {affected} row(s) affected")
            return True

        except Error as e:
            self.last_error = e
            print(f"❌ Update failed: {e}")

            # Rollback on error
            try:
                self.connection.rollback()
            except:
                pass

            return False

    def execute_many(self, query: str, data: List[Tuple]) -> bool:
        """
        Execute multiple INSERT/UPDATE/DELETE queries (batch).

        Args:
            query: SQL query with %s placeholders
            data: List of parameter tuples

        Returns:
            True if successful, False otherwise
        """
        if not self.is_connected():
            print("❌ Not connected to database")
            return False

        try:
            cursor = self.connection.cursor()
            cursor.executemany(query, data)

            # Get total affected rows
            affected = cursor.rowcount

            # Commit changes
            self.connection.commit()
            cursor.close()

            print(f"✅ Batch executed: {affected} row(s) affected")
            return True

        except Error as e:
            self.last_error = e
            print(f"❌ Batch failed: {e}")

            try:
                self.connection.rollback()
            except:
                pass

            return False

    def get_error(self) -> Optional[str]:
        """Return the last error message."""
        if self.last_error:
            return f"{self.last_error.errno}: {self.last_error.msg}"
        return None

    def __enter__(self):
        """Context manager entry."""
        self.connect()
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        """Context manager exit."""
        self.disconnect()


# Example usage and testing
if __name__ == "__main__":
    from dotenv import load_dotenv
    import os

    load_dotenv()

    # Create database connection
    db = DatabaseConnection(
        host=os.environ.get('DB_HOST'),
        user=os.environ.get('DB_USER'),
        password=os.environ.get('DB_PASS'),
        database=os.environ.get('DB_NAME')
    )

    # Example 1: Manual connect/disconnect
    print("=" * 60)
    print("EXAMPLE 1: Manual Connection")
    print("=" * 60 + "\n")

    if db.connect():
        # Execute a SELECT query
        results = db.execute_query("SELECT TABLE_NAME FROM information_schema.TABLES WHERE TABLE_SCHEMA = DATABASE()")

        if results:
            print(f"\n📊 Tables in database:")
            for row in results:
                print(f"  - {row[0]}")

        db.disconnect()

    # Example 2: Using context manager (automatic cleanup)
    print("\n" + "=" * 60)
    print("EXAMPLE 2: Context Manager (Auto-close)")
    print("=" * 60 + "\n")

    with DatabaseConnection(
        host=os.environ.get('DB_HOST'),
        user=os.environ.get('DB_USER'),
        password=os.environ.get('DB_PASS'),
        database=os.environ.get('DB_NAME')
    ) as db:

        # Query for single row
        result = db.fetch_one(
            "SELECT COUNT(*) as table_count FROM information_schema.TABLES WHERE TABLE_SCHEMA = DATABASE()"
        )

        if result:
            print(f"📊 Total tables: {result[0]}")

    # Example 3: Batch insert (if you have a pages table)
    print("\n" + "=" * 60)
    print("EXAMPLE 3: Batch Operation")
    print("=" * 60 + "\n")

    db = DatabaseConnection(
        host=os.environ.get('DB_HOST'),
        user=os.environ.get('DB_USER'),
        password=os.environ.get('DB_PASS'),
        database=os.environ.get('DB_NAME')
    )

    if db.connect():
        # Example: Insert multiple records (commented out, modify for your table)
        # data = [
        #     ('Chapter 1', 'chapter-1'),
        #     ('Chapter 2', 'chapter-2'),
        #     ('Chapter 3', 'chapter-3'),
        # ]
        # db.execute_many(
        #     "INSERT INTO pages (title, slug) VALUES (%s, %s)",
        #     data
        # )

        db.disconnect()

    print("\n✅ Examples complete!")

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HOW IT WORKS:
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1. __init__ METHOD
   - Stores connection parameters
   - Initializes connection to None
   - Stores last error for debugging

2. connect() METHOD
   - Uses mysql.connector.connect(**config)
   - Checks is_connected()
   - Returns True/False
   - Stores error if it fails

3. execute_query() METHOD
   - Checks if connected first
   - Creates cursor
   - Executes SELECT with parameters
   - Fetches all results
   - Returns list of tuples

4. execute_update() METHOD
   - Checks connection
   - Executes INSERT/UPDATE/DELETE
   - Gets rowcount (affected rows)
   - Commits changes
   - Rollback on error

5. Parameterized Queries
   - Uses %s as placeholders
   - Passes params as tuple
   - Prevents SQL injection
   - Example:
     cursor.execute("SELECT * FROM pages WHERE id = %s", (42,))

6. Context Manager (__enter__/__exit__)
   - Allows "with" statement
   - Automatically calls connect() on entry
   - Automatically calls disconnect() on exit
   - Guarantees cleanup even if errors occur

7. Error Handling
   - Catches mysql.connector.Error
   - Stores error for later inspection
   - Rolls back transactions on error
   - Prints helpful messages

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TESTING THE SOLUTION:
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Test 1: Using manual connect/disconnect
from db_utils import DatabaseConnection

db = DatabaseConnection("localhost", "philip", "password", "learning_blog")

if db.connect():
    results = db.execute_query("SELECT TABLE_NAME FROM information_schema.TABLES LIMIT 3")
    if results:
        for row in results:
            print(row[0])
    db.disconnect()

Output:
✅ Connected to learning_blog on localhost

pages
page_content
subjects

✅ Connection closed

---

Test 2: Using context manager
with DatabaseConnection("localhost", "philip", "password", "learning_blog") as db:
    count = db.fetch_one("SELECT COUNT(*) FROM information_schema.TABLES")
    print(f"Tables: {count[0]}")

Output:
✅ Connected to learning_blog on localhost
Tables: 4
✅ Connection closed

---

Test 3: Error handling
db = DatabaseConnection("localhost", "philip", "wrong_password", "learning_blog")

if db.connect():
    print("Connected")
else:
    print(f"Error: {db.get_error()}")

Output:
❌ Connection failed: 1045 - Access denied for user 'philip'@'localhost' (using password: YES)
Error: 1045: Access denied for user 'philip'@'localhost' (using password: YES)

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WHY THIS PATTERN:
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Class-based approach advantages:

1. REUSABILITY
   - Create once, use everywhere
   - Don't duplicate connection code

2. STATE MANAGEMENT
   - Tracks connection status
   - Stores last error
   - Manages cursor lifecycle

3. ERROR HANDLING
   - Consistent error handling
   - Automatic cleanup
   - Detailed error reporting

4. PARAMETER BINDING
   - All methods support parameterized queries
   - Prevents SQL injection
   - Cleaner code

5. FLEXIBILITY
   - Methods for different query types
   - Works with or without parameters
   - Supports batch operations

6. CONTEXT MANAGERS
   - Automatic resource cleanup
   - Works with "with" statement
   - Fail-safe pattern

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COMPARISON: BEFORE AND AFTER:
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❌ BEFORE (No class):
try:
    conn = mysql.connector.connect(...)
    cursor = conn.cursor()
    cursor.execute(query)
    results = cursor.fetchall()
    cursor.close()
    conn.close()
except Error as e:
    print(e)

✅ AFTER (With class):
with DatabaseConnection(...) as db:
    results = db.execute_query(query)

Much cleaner and safer!

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KEY TAKEAWAYS:
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✅ Create reusable database wrapper classes
✅ Use type hints for clarity (Optional, Tuple, List, etc.)
✅ Always parameterize queries (%s placeholders)
✅ Use try/except for all database operations
✅ Return True/False for operations, not exceptions
✅ Store last error for debugging
✅ Implement context managers (__enter__, __exit__)
✅ Commit changes for INSERT/UPDATE/DELETE
✅ Rollback on errors
✅ Support batch operations (executemany)

Class Pattern Checklist:
- __init__ stores config ✅
- connect() establishes connection ✅
- disconnect() closes connection ✅
- is_connected() checks status ✅
- execute_query() for SELECT ✅
- fetch_one() for single row ✅
- execute_update() for INSERT/UPDATE/DELETE ✅
- execute_many() for batch ✅
- Error handling with try/except ✅
- Context manager support ✅

This is the production pattern used in real applications!

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