Pure Python Backup of SQLite3 In-Memory Database to Disk

Question

Without installing additional modules, how can I use the SQLite backup API to backup an in-memory database to an on-disk database? I have managed to successfully perform a disk-to-disk backup, but passing the already-extant in-memory connection to the sqlite3_backup_init function appears to be the problem.

My toy example, adapted from https://gist.github.com/achimnol/3021995 and cut down to the minimum, is as follows:

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)

I receive an error ctypes.ArgumentError: argument 3: <type 'exceptions.TypeError'>: Don't know how to convert parameter 3 on line 49 (p_backup = sqlitedll.sqlite3_backup_init(p_dst_db, 'main', sourceconn, 'main')). Somehow, I need to pass a reference to the in-memory database to that sqlite3_backup_init function.

I do not know enough C to grasp the specifics of the API itself.

Setup: Windows 7, ActiveState Python 2.7

Answer 1

It looks like as of Python 3.7, this functionality is available within the standard library. Here are some examples copied directly out of the official docs:

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:

import sqlite3

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

To answer your specific question of backing up an in-memory database to disk, it looks like this works. Here's a quick script using the standard library backup method:

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()

And the backup.db database can be loaded into sqlite3 and inspected:

$ 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!

Answer 2

An in-memory database can be accessed only through the SQLite library that created it (in this case, Python's built-in SQLite).

Python's sqlite3 module does not give access to the backup API, so it is not possible to copy an in-memory database.

You would need to install an additional module, or use an on-disk database in the first place.

Answer 3

While this isn't strictly a solution to your questions (as it's not using the backup API) it serves as a minimal effort approach and works well for small in memory databases.

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()