<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def MonSQL(host=None, port=None, username=None, password=None, dbname=None, dbpath=None, dbtype=None):
 	""" 	Initialize and return a Database instance 	"""

	if dbtype is None:
		raise MonSQLException('Database type must be specified')

	if dbtype == DB_TYPES.MYSQL:
		return MySQLDatabase(host, port, username, password, dbname)
	elif dbtype == DB_TYPES.SQLITE3:
		return SQLite3Database(dbpath)
	elif dbtype == DB_TYPES.POSTGRESQL:
		return PostgreSQLDatabase(host, port, username, password, dbname)
	else:
		raise MonSQLException('Database type %s not supported' %dbtype)