logo标签列表

Python纯代码备份SQLite3内存数据库到磁盘

pythonsqlitedatabase-backups
9

如何在不安装其他模块的情况下，使用SQLite备份API将内存数据库备份到磁盘数据库？我已成功执行了磁盘到磁盘备份，但将已存在的内存连接传递给sqlite3_backup_init函数似乎是问题所在。

我的玩具示例，改编自https://gist.github.com/achimnol/3021995并缩减到最小，如下：

import sqlite3
import ctypes

# Create a junk in-memory database
sourceconn = sqlite3.connect(':memory:')
cursor = sourceconn.cursor()
cursor.execute('''CREATE TABLE stocks
             (date text, trans text, symbol text, qty real, price real)''')
cursor.execute("INSERT INTO stocks VALUES ('2006-01-05','BUY','RHAT',100,35.14)")
sourceconn.commit()

target = r'C:\data\sqlite\target.db'
dllpath = u'C:\\Python27\DLLs\\sqlite3.dll'

# Constants from the SQLite 3 API defining various return codes of state.
SQLITE_OK = 0
SQLITE_ERROR = 1
SQLITE_BUSY = 5
SQLITE_LOCKED = 6
SQLITE_OPEN_READONLY = 1
SQLITE_OPEN_READWRITE = 2
SQLITE_OPEN_CREATE = 4

# Tweakable variables
pagestocopy = 20
millisecondstosleep = 100

# dllpath = ctypes.util.find_library('sqlite3') # I had trouble with this on Windows
sqlitedll = ctypes.CDLL(dllpath)
sqlitedll.sqlite3_backup_init.restype = ctypes.c_void_p

# Setup some ctypes
p_src_db = ctypes.c_void_p(None)
p_dst_db = ctypes.c_void_p(None)
null_ptr = ctypes.c_void_p(None)

# Check to see if the first argument (source database) can be opened for reading.
# ret = sqlitedll.sqlite3_open_v2(sourceconn, ctypes.byref(p_src_db), SQLITE_OPEN_READONLY, null_ptr)
#assert ret == SQLITE_OK
#assert p_src_db.value is not None

# Check to see if the second argument (target database) can be opened for writing.
ret = sqlitedll.sqlite3_open_v2(target, ctypes.byref(p_dst_db), SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE, null_ptr)
assert ret == SQLITE_OK
assert p_dst_db.value is not None

# Start a backup.
print 'Starting backup to SQLite database "%s" to SQLite database "%s" ...' % (sourceconn, target)
p_backup = sqlitedll.sqlite3_backup_init(p_dst_db, 'main', sourceconn, 'main')
print '    Backup handler: {0:#08x}'.format(p_backup)
assert p_backup is not None

# Step through a backup.
while True:
    ret = sqlitedll.sqlite3_backup_step(p_backup, pagestocopy)
    remaining = sqlitedll.sqlite3_backup_remaining(p_backup)
    pagecount = sqlitedll.sqlite3_backup_pagecount(p_backup)
    print '    Backup in progress: {0:.2f}%'.format((pagecount - remaining) / float(pagecount) * 100)
    if remaining == 0:
        break
    if ret in (SQLITE_OK, SQLITE_BUSY, SQLITE_LOCKED):
        sqlitedll.sqlite3_sleep(millisecondstosleep)

# Finish the bakcup
sqlitedll.sqlite3_backup_finish(p_backup)

# Close database connections
sqlitedll.sqlite3_close(p_dst_db)
sqlitedll.sqlite3_close(p_src_db)

我收到一个错误 ctypes.ArgumentError: argument 3: <type 'exceptions.TypeError'>: 不知道如何转换参数3，在第49行 (p_backup = sqlitedll.sqlite3_backup_init(p_dst_db, 'main', sourceconn, 'main'))。不知何故，我需要将内存数据库的引用传递给sqlite3_backup_init函数。

我不懂足够的C语言来理解API本身的细节。

设置：Windows 7，ActiveState Python 2.7

4个回答
16

看起来从Python 3.7开始，这个功能可以在标准库中使用。以下示例直接从官方文档中复制：

示例1：将现有的数据库复制到另一个数据库中：

import sqlite3

def progress(status, remaining, total):
    print(f'Copied {total-remaining} of {total} pages...')

con = sqlite3.connect('existing_db.db')
bck = sqlite3.connect('backup.db')
with bck:
    con.backup(bck, pages=1, progress=progress)
bck.close()
con.close()

例子2，将现有数据库复制到瞬时副本中：

import sqlite3

source = sqlite3.connect('existing_db.db')
dest = sqlite3.connect(':memory:')
source.backup(dest)

回答你具体的问题：将内存数据库备份到磁盘上，似乎这样做可以。下面是一个使用标准库中备份方法的快速脚本：backup。

import sqlite3


source = sqlite3.connect(':memory:')
dest = sqlite3.connect('backup.db')

c = source.cursor()
c.execute("CREATE TABLE test(id INTEGER PRIMARY KEY, msg TEXT);")
c.execute("INSERT INTO test VALUES (?, ?);", (1, "Hello World!"))
source.commit()

source.backup(dest)

dest.close()
source.close()

可以将backup.db数据库加载到sqlite3中进行检查：

$ sqlite3 backup.db
SQLite version 3.24.0 2018-06-04 14:10:15
Enter ".help" for usage hints.
sqlite> .schema
CREATE TABLE test(id INTEGER PRIMARY KEY, msg TEXT);
sqlite> SELECT * FROM test;
1|Hello World!
1
虽然这不是解决您问题的严格方法（因为它没有使用备份API），但它作为一种最小化工作量的方法，并且适用于内存中的小型数据库。
import os
import sqlite3

database = sqlite3.connect(':memory:')

# fill the in memory db with your data here

dbfile = 'dbcopy.db'
if os.path.exists(dbfile):
    os.remove(dbfile) # remove last db dump

new_db = sqlite3.connect(dbfile)
c = new_db.cursor() 
c.executescript("\r\n".join(database.iterdump()))
new_db.close()
1
1

内存数据库只能通过创建它的SQLite库（在本例中为Python内置的SQLite）进行访问。

Python的sqlite3模块无法访问备份API，因此不可能复制内存数据库。

您需要安装其他模块或首先使用磁盘上的数据库。

3
此回答已过时，仅适用于Python 3.7及以前版本。 - Wolfgang Fahl
网页内容由stack overflow 提供, 点击上面的
可以查看英文原文，
原文链接