How to code and wrap with ctypes a python function needed as callback by sqlite3?

Question

I\'m trying to perform sqlite3_exec from python to extract line by line the contents of a database. According to the C API, I need a Callback function which will pe

Answer 1

Tested in Python 2.7 and Python 3.6 (if you change the DLL path) using the following database:

create table tbl1(one varchar(10), two smallint);
insert into tbl1 values('hello',10);
insert into tbl1 values('goodbye',20);

Code:

# I know, bad form, but it makes the code easier to read for an example
from ctypes import *

# This was missing the 2nd c_int parameter.
CALLBACK = CFUNCTYPE(c_int, c_void_p, c_int, POINTER(c_char_p), POINTER(c_char_p))

@CALLBACK
def extractor(unused, num_columns, pcolumn, pcolumn_name):
    print(pcolumn[:num_columns])
    return 0 # needs to return 0 from callback or will abort.

sqlite3DLL = CDLL(r"C:\Python27\DLLs\sqlite3.dll")
SQLITE_OPEN_READONLY = 1 
p_src_db = c_void_p()

sqlite3DLL.sqlite3_open_v2(b'test.db', byref(p_src_db), SQLITE_OPEN_READONLY, None)
# pass the handle returned by above as first parameter below
sqlite3DLL.sqlite3_exec(p_src_db, b'SELECT * FROM tbl1', extractor, None, None)
sqlite3DLL.sqlite3_close(p_src_db)

Output:

['hello', '10']
['goodbye', '20']

I also recommend setting argtypes because it helps catch type errors and for some parameter types (like c_double) it is required.

sqlite3DLL.sqlite3_open_v2.argtypes = c_char_p, POINTER(c_void_p), c_int,c_char_p
sqlite3DLL.sqlite3_open_v2.restype = c_int

sqlite3DLL.sqlite3_exec.argtypes = c_void_p,c_char_p,CALLBACK,c_void_p,POINTER(c_char_p)
sqlite3DLL.sqlite3_open_v2.restype = c_int