Parse stream in x length chunks based on uint32 id

Question

I have little C/Qt experience and have a small parser that I need to port to Python. Is anyone able to explain how I can implement the below in Python? I understand what the result is, just can't understand how to achieve the uint32 instantiation and shift that results in different part lengths based on the 4 byte "id". I'm hoping to parse it using just native Python 3.5+ packages, numpy or similar is fine if it makes the typing convenient.

QDataStream stream(item);
stream.setByteOrder(QDataStream::LittleEndian);
Items parts;
while (!stream.atEnd()) {
    quint32 partId;
    stream >> id;
    char *bytes;
    uint length;
    stream.readBytes(bytes, length);
    parts.append(QPair<quint32, QByteArray>(id, QByteArray(bytes, length)));
    delete bytes;
}
return parts;

Answer 1

Since in python the numeric types do not match those of C++ then QDataStream no longer uses the ">>" operator to obtain the value but has specific methods such as readUInt32.

Considering the following code that generates the data:

#include <QDataStream>
#include <QFile>
#include <QDebug>

int main()
{
    QFile file("file.dat");
    if(!file.open(QIODevice::WriteOnly)){
        qDebug() << file.error() << file.errorString();
        return EXIT_FAILURE;
    }
    QDataStream stream(&file);
    stream.setByteOrder(QDataStream::LittleEndian);
    stream.setVersion(QDataStream::Qt_5_15);

    QVector <QPair<quint32, QByteArray>> data;
    data.append({1, "One"});
    data.append({2, "Two"});
    data.append({3, "Three"});

    for(const QPair<quint32, QByteArray> & d: qAsConst(data)){
        stream << d.first;
        stream.writeBytes(d.second.constData(), d.second.size());
    }
    return EXIT_SUCCESS;
}

The following code gets the data:

import sys

from PyQt5.QtCore import QByteArray, QDataStream, QFile, QIODevice


file = QFile("file.dat")
if not file.open(QIODevice.ReadOnly):
    print(file.error(), file.errorString())
    sys.exit(-1)

items = []

stream = QDataStream(file)
stream.setByteOrder(QDataStream.LittleEndian)
stream.setVersion(QDataStream.Qt_5_15)
while not stream.atEnd():
    id_ = stream.readUInt32()
    data = stream.readBytes()
    items.append((id_, QByteArray(data)))
print(items)

Output:

[(1, PyQt5.QtCore.QByteArray(b'One')), (2, PyQt5.QtCore.QByteArray(b'Two')), (3, PyQt5.QtCore.QByteArray(b'Three'))]

If PySide2 is used then the implementation changes a bit.

import sys

from PySide2.QtCore import QByteArray, QDataStream, QFile, QIODevice


file = QFile("file.dat")
if not file.open(QIODevice.ReadOnly):
    print(file.error(), file.errorString())
    sys.exit(-1)

items = []

stream = QDataStream(file)
stream.setByteOrder(QDataStream.LittleEndian)
stream.setVersion(QDataStream.Qt_5_15)
while not stream.atEnd():
    id_ = stream.readUInt32()
    data = QByteArray()
    stream >> data
    items.append((id_, data))
print(items)

Output:

[(1, PySide2.QtCore.QByteArray(b'One')), (2, PySide2.QtCore.QByteArray(b'Two')), (3, PySide2.QtCore.QByteArray(b'Three'))]

Update:

It is not possible to obtain the data if you do not use QDataStream since Qt uses its own format for each type of data, and this format is not a standard that can change with each version without notifying it. For this reason, the byteorder and the version of QDataStream used must be indicated.

Update 2

Assuming that the format that QDataStream uses to pack quint32 and bytes is QDataStream.Qt_5_15 then a possible implementation is:

import sys
import struct

items = []

with open("file.dat", "rb") as f:
    while True:
        try:
            (id_,) = struct.unpack("I", f.read(4))
            (length,) = struct.unpack("I", f.read(4))
            data = f.read(length)
        except (EOFError, struct.error) as e:
            break
        else:
            items.append((id_, data))

print(items)

Output:

[(1, b'One'), (2, b'Two'), (3, b'Three')]