SQLite3内存数据库到Dis的纯Python备份

Question

在不安装其他模块的情况下，如何使用SQLite backup API将内存中的数据库备份到磁盘上的数据库？我成功地执行了磁盘到磁盘的备份，但是将已经存在的内存连接传递给sqlite3_backup_init函数似乎是个问题。在

我的玩具例子，改编自https://gist.github.com/achimnol/3021995并缩减到最小值，如下所示：

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)

我在第49行（p_backup = sqlitedll.sqlite3_backup_init(p_dst_db, 'main', sourceconn, 'main')）收到一个错误ctypes.ArgumentError: argument 3: <type 'exceptions.TypeError'>: Don't know how to convert parameter 3）。不知何故，我需要将内存数据库的引用传递给sqlite3_backup_init函数。在

我不知道足够的C来理解API本身的细节。在

安装程序：Windows7、ActiveState Python2.7

Answer 1

内存中的数据库只能通过创建它的SQLite库访问（在本例中，是Python的内置SQLite）。在

Python的sqlite3模块不提供对备份API的访问，因此无法复制内存中的数据库。在

首先，您需要安装一个附加模块，或者使用磁盘上的数据库。在

Answer 2

@cl错了。我使用ctypes来提取Python连接对象的底层C指针，并创建了一个小包：https://pypi.org/project/sqlite3-backup/

源代码位于https://sissource.ethz.ch/schmittu/sqlite3_backup

Answer 3

从python3.7开始，这个功能在standard library中可用。以下是一些直接从官方文件中复制的例子：

Example 1, copy an existing database into another:

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()

Example 2, copy an existing database into a transient copy:

^{pr2}$

要回答将内存中的数据库备份到磁盘的具体问题，这看起来是可行的。下面是一个使用标准库backup方法的快速脚本：

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()

并且backup.db数据库可以加载到sqlite3中并进行检查：

$ 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!