Passing Protocol buffer serialized datas from C++ to Python via LevelDB

Question

Though I've followed the excellent Protocol Buffer documentation and tutorials for C++ and Python, I can't achieve my goal which is : - to serialize datas from a C++ process. - insert it into LevelDB from that same process. - extract the serialized datas from a Python process - Deseralize it from this same Python process - Use those deseralized datas in Python

I can serialize my datas using protocol buffer in C++ (using a std::string container). I can insert it into LevelDB. But, when I levelDB->Get my serialized datas, though Python seems to recognize it as a String, and showing me their raw content, whenever I deserialize it into a Python String, it is empty!

Here is how I serialize and insert my datas in C++ :

int                             main(int arg, char** argv)
 {
     GOOGLE_PROTOBUF_VERIFY_VERSION;

     leveldb::DB*                  db;
     leveldb::Options              options;
     leveldb::Status               status;
     tutorial::AddressBook         address_book;
     tutorial::Person*             person1;
     tutorial::Person*             person2;

     options.create_if_missing = true;
     status = leveldb::DB::Open(options, "test_db", &db);
     assert(status.ok());

     person1 = address_book.add_person();
     person1->set_id(1);
     person1->set_name("ME");
     person1->set_email("me@me.com");

     person2 = address_book.add_person();
     person2->set_id(2);
     person2->set_name("SHE");
     person2->set_email("she@she.com");

     std::string                   test;
     if (!address_book.SerializeToString(&test))
     {
         std::cerr << "Failed to write address book" << std::endl;
         return -1;
     }

     if (status.ok()) status = db->Put(leveldb::WriteOptions(), "Test", test);

And here is how I try to deserialize it in Python:

address_book = addressbook_pb2.AddressBook()
db = leveldb.LevelDB('test_db')
ab = address_book.ParseFromString(db.Get("Test"))

ad var type is NoneType

Edit : before the db.Get(), ab.ByteSize() returns 0, 76 after the ParseFromString(), I assume it's a Type problem then... + ab.ListFields() returns a unexploitable list of the contained field: succesfully couting two person instances, but unable to let me acces to it.

Any clues, any ideas of what I didn't understand, what I'm doing wrong here?

Many thanks!

Answer 1

Ok, so this was my bad.

I went back into the Protocol Buffers Python documentation, and the fact is that even if the AdressBook object I was retrieving did not showed any description, it was still able to be iterated over and even had a .str() method.

so, if anyone comes to that problem again, just try to explore your ProtocolBuffers object using iPython like I did, and you'll find that every of your proto elements are fields of your object. Using my example:

ab = adress_book.ParseFromString(db.Get('Test'))
ab.__str__()  # Shows a readable version of my object
for person in adress_book.person:  # I'm even able to iterate over any of my ab fields values
    print person.id
    print person.name