How to code and wrap with ctypes a python function

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 perform the iteration. I have coded the following with a lot of help from internet:

UPDATED with @eryksun pièces of advice

import ctypes

def extractor(unused, num_columns, pcolumn, pcolumn_name):
    for column in range(0,num_columns):
        if pcolumn[i] != None:
            print pcolumn[i]

sqlite3DLL = ctypes.CDLL("C:\\Python\\PYTHON\\DLLs\\sqlite3.dll")
SQLITE_OPEN_READONLY = 1 
null_ptr = ctypes.c_void_p(None)
p_src_db = ctypes.c_void_p(None)

ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p, ctypes.POINTER(ctypes.c_char_p), ctypes.POINTER(ctypes.c_char_p))
callback_func = callback_type(extractor)

connect = sqlite3DLL.sqlite3_open_v2(DatabasePath, ctypes.byref(p_src_db), SQLITE_OPEN_READONLY, null_ptr)
connect = sqlite3DLL.sqlite3_exec(DatabasePath, "SELECT * FROM *", callback_func, None, null_ptr)
sqlite3DLL.sqlite3_close(DatabasePath)

Before moving on to the python callback function I have some doubts:

1. Is "SELECT * FROM *" a possible SQL statement to avoid providing the name of a table (because I do not know it)?
2. Is the first argument of both functions sqlite3_open_v2 and sqlite3_exec the path to the database?

If all that is ok we can move on to the callback function. According to what I found on the net, the C callback function should be somewhat similar to:

callback(void *NotUsed, int argc, char **argv, char **azColName)
    for (int i = 0; i < argc; i++) {printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL")}

That is why I coded the CFUNCTYPE you see on my code. How can I code a python function which matches the callback needed to maybe fill a list with the database contents?

I have added the proposed changes on the code, the callback function just prints the value to verify the output. But It wont work I get an error:

con = sqlite3DLL.sqlite3_exec(FastenerLibraryPath, "SELECT * FROM *", callback_func, None, null_ptr)
WindowsError: exception: access violation writing 0x0000000000000009

Many thanks in advance!

FINAL WORKING EDITION (@eryksun comments and @MarkTolonen solution)

import ctypes

def extractor(unused, num_columns, pcolumn, pcolumn_name):
    print ','.join(["''" if x is None else "'"+x+"'" for x in pcolumn[:num_columns]])
    return 0

sqlite3DLL = ctypes.CDLL("C:\\Python\\PYTHON\\DLLs\\sqlite3.dll")
SQLITE_OPEN_READONLY = 1 
null_ptr = ctypes.c_void_p(None)
p_src_db = ctypes.c_void_p(None)

ctypes.CFUNCTYPE(ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p, ctypes.c_int, ctypes.POINTER(ctypes.c_char_p), ctypes.POINTER(ctypes.c_char_p))
callback_func = callback_type(extractor)

connect = sqlite3DLL.sqlite3_open_v2(DatabasePath, ctypes.byref(p_src_db), SQLITE_OPEN_READONLY, None)
connect = sqlite3DLL.sqlite3_exec(p_src_db, b"SELECT * FROM Pin", callback_func, None, None)
sqlite3DLL.sqlite3_close(p_src_db)

This is working, however, I will look into argtypes and the opaque type.

Thank you all!

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