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Lazy initialization function that returns a singleton SQLite database connection, creating the database and required table schema if they don't exist.

Purpose

This function implements the singleton pattern for database connection management. It ensures only one SQLite connection is created and reused throughout the application lifecycle. On first call, it creates the cache directory structure, establishes a connection to a SQLite database file, and initializes a 'filedata' table with columns for id, version, property, and value. Subsequent calls return the existing connection. This is typically used for caching file metadata or properties with versioning support.

Source Code

def _get_conn():
    global _conn
    if _conn is not None:
        return _conn

    CACHE_FILE.parent.mkdir(parents=True, exist_ok=True)
    _conn = sqlite3.connect(CACHE_FILE)
    c = _conn.cursor()
    c.execute('''CREATE TABLE IF NOT EXISTS filedata
                 (id TEXT, version INTEGER, property TEXT, value BLOB,
                  UNIQUE(id, version, property))''')
    _conn.commit()
    return _conn

Return Value

Returns a sqlite3.Connection object representing an active connection to the SQLite database. This connection can be used to create cursors and execute SQL queries against the 'filedata' table. The same connection instance is returned on all subsequent calls (singleton pattern).

Dependencies

• sqlite3
• xdg

Required Imports

import sqlite3
from xdg import xdg_cache_home

Usage Example

import sqlite3
from pathlib import Path
from xdg import xdg_cache_home

# Module-level setup
_conn = None
CACHE_FILE = Path(xdg_cache_home()) / 'myapp' / 'cache.db'

def _get_conn():
    global _conn
    if _conn is not None:
        return _conn
    CACHE_FILE.parent.mkdir(parents=True, exist_ok=True)
    _conn = sqlite3.connect(CACHE_FILE)
    c = _conn.cursor()
    c.execute('''CREATE TABLE IF NOT EXISTS filedata
                 (id TEXT, version INTEGER, property TEXT, value BLOB,
                  UNIQUE(id, version, property))''')
    _conn.commit()
    return _conn

# Usage
conn = _get_conn()
cursor = conn.cursor()
cursor.execute('INSERT OR REPLACE INTO filedata VALUES (?, ?, ?, ?)', 
               ('file1', 1, 'size', b'1024'))
conn.commit()

Best Practices

• This function relies on global state (_conn) which can make testing difficult; consider dependency injection for better testability
• The connection is never explicitly closed; ensure proper cleanup on application shutdown
• The function is not thread-safe; use threading locks if accessing from multiple threads
• CACHE_FILE must be defined before calling this function, typically using xdg_cache_home() for cross-platform cache directory support
• The 'filedata' table uses a UNIQUE constraint on (id, version, property) to prevent duplicate entries
• Consider using context managers or connection pooling for production applications
• The BLOB type for 'value' column allows storing binary data; use appropriate serialization (pickle, json, etc.) when storing complex objects

Tags

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