Simple key-value storage based on SQLite

sqlite_storage.py

# coding: utf-8
#
# Copyright (c) 2017 Roman Miroshnychenko <roman1972@gmail.com>
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
"""
This module provides SQLite-based key-value storage

It is compatible with both Python 2 and 3.
"""
from __future__ import print_function
try:
    import cPickle as pickle
except ImportError:
    import pickle
import sqlite3

try:
    basestring
except NameError:
    basestring = str


class SqliteStorage(object):
    """
    SQLite-based key-value storage

    It can work as a context manager.

    Example::

        with SqliteStorage('storage.db') as stor:
            stor.set_value('foo', 'bar')
    """
    def __init__(self, db_path, table='storage'):
        """
        :param db_path: full path to the datebase including filename and extention
        :type db_path: str
        :param table: table name (optional)
        :type table: str
        """
        self._table = table
        self._dirty = False
        self._connection = sqlite3.connect(db_path)
        self._cursor = self._connection.cursor()
        self._cursor.execute('SELECT name FROM sqlite_master WHERE type="table" AND name=?', (self._table,))
        if self._cursor.fetchone() is None:
            self._cursor.execute('CREATE TABLE {0} (skey TEXT PRIMARY KEY, value BLOB)'.format(self._table))
            self._dirty = True

    def set_value(self, key, value):
        """
        Store a value with the given key

        .. note:: ``value`` must be picklable

        :param key: storage key
        :type key: str
        :param value: a value to store
        :raises TypeError: if the key is not of class:`str` type
        """
        if not isinstance(key, basestring):
            raise TypeError('key must be of str type!')
        self._cursor.execute('INSERT OR REPLACE INTO {0} VALUES (?,?)'.format(self._table), (key, bytes(pickle.dumps(value))))
        self._dirty = True

    def get_value(self, key):
        """
        Retrieve a value from the storage by the given key

        :param key: storage key
        :type key: str
        :return: a stored value
        :raises TypeError: if the key is not of :class:`str` type
        :raises KeyError: if the key is not found in the storage
        """
        if not isinstance(key, basestring):
            raise TypeError('key must be of str type!')
        self._cursor.execute('SELECT value FROM {0} WHERE skey=?'.format(self._table), (key,))
        row = self._cursor.fetchone()
        if row is None:
            raise KeyError(key)
        return pickle.loads(bytes(row[0]))

    def close(self):
        """Close storage and commit changes if any"""
        if self._dirty:
            self._connection.commit()
        self._connection.close()

    def keys(self):
        """
        :return: iterator for storage keys
        :rtype: types.GeneratorType
        """
        self._cursor.execute('SELECT skey FROM {0}'.format(self._table))
        for row in self._cursor.fetchall():
            yield row[0]

    def values(self):
        """
        :return: iterator stored values
        :rtype: types.GeneratorType
        """
        self._cursor.execute('SELECT value FROM {0}'.format(self._table))
        for row in self._cursor.fetchall():
            yield pickle.loads(bytes(row[0]))

    def items(self):
        """
        :return: iterator for (key, value) tuples
        :rtype: types.GeneratorType
        """
        self._cursor.execute('SELECT * FROM {0}'.format(self._table))
        for row in self._cursor.fetchall():
            yield row[0], pickle.loads(bytes(row[1]))

    def __iter__(self):
        return self.keys()

    def __enter__(self):
        return self

    def __exit__(self, t, v, tb):
        self.close()

    def __contains__(self, key):
        try:
            self.get_value(key)
        except KeyError:
            return False
        return True