PyQt5: Loading an extension into sqlite

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I am getting started with PyQt5 and its Sql classes with sqlite. I want to load an extension into sqlite. To do that, extension loading must be enabled at runtime for sqlite. In the python module sqlite3, this is enabled through enable_load_extension.

In C++ the sqlite handle can be obtained like this (taken from https://doc.qt.io/qt-5/qsqldriver.html#handle):

QSqlDatabase db = QSqlDatabase::database();
QVariant v = db.driver()->handle();
if (v.isValid() && (qstrcmp(v.typeName(), "sqlite3*") == 0)) {
    // v.data() returns a pointer to the handle
    sqlite3 *handle = *static_cast<sqlite3 **>(v.data());
    if (handle) {
        // ...
    }
}

The python equivalent is

from PyQt5.QtSql import QSqlDatabase

db = QSqlDatabase.addDatabase('QSQLITE')
db.driver().handle()
-> TypeError: unable to convert a C++ 'sqlite3*' instance to a Python object

As a side note, in Pyside2, the handle method is not exposed.

It seems this is the wrong way to go about it. Is there any way I could load my sqlite extension through PyQt5?

pyqt5 pyqt sqlite python
1 Answers

One possible solution is to create a library that is loaded using ctypes.

In this case I show the solution for ubuntu linux but I suppose that similar steps can be applied to other OS.

Compile Library

qsqlite.pro

QT -= gui
QT += sql
TEMPLATE = lib
DEFINES += QSQLITE_LIBRARY
CONFIG += unversioned_libname unversioned_soname
CONFIG += c++11
SOURCES += \
    qsqlite.cpp

HEADERS += \
    qsqlite_global.h \
    qsqlite.h

LIBS += -lsqlite3

qsqlite_global.h

#ifndef QSQLITE_GLOBAL_H
#define QSQLITE_GLOBAL_H

#if defined(_MSC_VER) || defined(WIN64) || defined(_WIN64) || defined(__WIN64__) || defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__)
#  define Q_DECL_EXPORT __declspec(dllexport)
#  define Q_DECL_IMPORT __declspec(dllimport)
#else
#  define Q_DECL_EXPORT     __attribute__((visibility("default")))
#  define Q_DECL_IMPORT     __attribute__((visibility("default")))
#endif

#if defined(QSQLITE_LIBRARY)
#  define QSQLITE_EXPORT Q_DECL_EXPORT
#else
#  define QSQLITE_EXPORT Q_DECL_IMPORT
#endif

#endif // QSQLITE_GLOBAL_H

qsqlite.h

#ifndef QSQLITE_H
#define QSQLITE_H

#include "qsqlite_global.h"

class QSqlDriver;

extern "C" {
    bool QSQLITE_EXPORT enable_extension(QSqlDriver *ptr, bool enabled);
}

#endif // QSQLITE_H

qsqlite.cpp

#include "qsqlite.h"

#include <sqlite3.h>

#include <QSqlDriver>
#include <QVariant>

bool enable_extension(QSqlDriver *driver, bool enabled)
{
    if(!driver)
        return false;
    QVariant v = driver->handle();
    if (!v.isValid() || !(qstrcmp(v.typeName(), "sqlite3*")==0))
        return false;
    if(sqlite3 *db_handle = *static_cast<sqlite3 **>(v.data())){
        sqlite3_initialize();
        sqlite3_enable_load_extension(db_handle, enabled);
        return true;
    }
    return false;
}

qsqlite/
├── qsqlite.cpp
├── qsqlite_global.h
├── qsqlite.h
└── qsqlite.pro

To compile you must use Qt so in this case I will use aqtinstall(python -m pip install aqtinstall) by executing the following command:

python -m aqt install 5.15.0 linux desktop --outputdir qt
qt/5.15.0/gcc_64/bin/qmake qsqlite
make

Note: To compile the library it is necessary to have the headers of sqlite3 for this you must install with libsqlite3-dev in ubuntu: sudo apt install -y --no-install-recommends libsqlite3-dev.

This creates the libqsqlite.so library that must be copied next to the script, for example the following code loads the spatialite module(sudo apt install -y --no-install-recommends libsqlite3-mod-spatialite).

main.py

from ctypes import CDLL, c_void_p
import os

from PyQt5.QtSql import QSqlDatabase, QSqlQuery

import sip

CURRENT_DIR = os.path.dirname(os.path.realpath(__file__))


def load_spatialite():
    queries = (
        "SELECT load_extension('mod_spatialite')",
        "SELECT InitSpatialMetadata(1)",
    )
    q = QSqlQuery()
    for query in queries:
        if not q.exec_(query):
            print(
                f"Error: cannot load the Spatialite extension ({q.lastError().text()})"
            )
            return False
    return True


def main():
    db = QSqlDatabase.addDatabase("QSQLITE")

    db.setDatabaseName("foo.sqlite")
    if not db.open():
        sys.exit(-1)

    lib = CDLL(os.path.join(CURRENT_DIR, "libqsqlite.so"))
    lib.enable_extension(c_void_p(sip.unwrapinstance(db.driver()).__int__()), True)
    load_spatialite()

    query = QSqlQuery()

    query.exec_("CREATE TABLE my_line(id INTEGER PRIMARY KEY)")
    query.exec_(
        """SELECT AddGeometryColumn("my_line","geom" , 4326, "LINESTRING", 2)"""
    )

    polygon_wkt = "POLYGON ((11 50,11 51,12 51,12 50,11 50))"

    XA = 11
    YA = 52
    XB = 12
    YB = 49

    line_wkt = "LINESTRING({0} {1}, {2} {3})".format(XA, YA, XB, YB)

    query.prepare("""INSERT INTO my_line VALUES (?,GeomFromText(?, 4326))""")

    query.addBindValue(1)
    query.addBindValue(line_wkt)
    query.exec_()

    query.prepare(
        """SELECT astext(st_intersection(geom, GeomFromText(?, 4326))) from my_line WHERE st_intersects(geom, GeomFromText(?, 4326))"""
    )
    query.addBindValue(polygon_wkt)
    query.addBindValue(polygon_wkt)
    query.exec_()

    while query.next():
        for i in range(query.record().count()):
            print(query.value(i))


if __name__ == "__main__":
    main()

├── main.py
└── libqsqlite.so

Output:

LINESTRING(11.333333 51, 11.666667 50)

The same library can be used for PySide2:

from ctypes import CDLL, c_void_p
import os

from PySide2.QtSql import QSqlDatabase, QSqlQuery

import shiboken2

CURRENT_DIR = os.path.dirname(os.path.realpath(__file__))


def load_spatialite():
    queries = (
        "SELECT load_extension('mod_spatialite')",
        "SELECT InitSpatialMetadata(1)",
    )
    q = QSqlQuery()
    for query in queries:
        if not q.exec_(query):
            print(
                f"Error: cannot load the Spatialite extension ({q.lastError().text()})"
            )
            return False
    return True


def main():
    db = QSqlDatabase.addDatabase("QSQLITE")

    db.setDatabaseName("foo.sqlite")
    if not db.open():
        sys.exit(-1)

    lib = CDLL(os.path.join(CURRENT_DIR, "libqsqlite.so"))
    lib.enable_extension(c_void_p(shiboken2.getCppPointer(db.driver())[0]))
    load_spatialite()

    query = QSqlQuery()

    query.exec_("CREATE TABLE my_line(id INTEGER PRIMARY KEY)")
    query.exec_(
        """SELECT AddGeometryColumn("my_line","geom" , 4326, "LINESTRING", 2)"""
    )

    polygon_wkt = "POLYGON ((11 50,11 51,12 51,12 50,11 50))"

    XA = 11
    YA = 52
    XB = 12
    YB = 49

    line_wkt = "LINESTRING({0} {1}, {2} {3})".format(XA, YA, XB, YB)

    query.prepare("""INSERT INTO my_line VALUES (?,GeomFromText(?, 4326))""")

    query.addBindValue(1)
    query.addBindValue(line_wkt)
    query.exec_()

    query.prepare(
        """SELECT astext(st_intersection(geom, GeomFromText(?, 4326))) from my_line WHERE st_intersects(geom, GeomFromText(?, 4326))"""
    )
    query.addBindValue(polygon_wkt)
    query.addBindValue(polygon_wkt)
    query.exec_()

    while query.next():
        for i in range(query.record().count()):
            print(query.value(i))


if __name__ == "__main__":
    main()

For the test I have used docker that you can find here.

user_66225870
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