Rolling append to Avro file with size threshold in Python

Question

I have a bunch of objects that I'd like to export to Avro files. The amount of data can be arbitrarily large, so I'd like to write them into fixed-sized parts where each part is smaller than a THRESHOLD. For example, say I have 13 GB of data that I'd like two write into 5GB volumes, I'd like my files output files to be

/out/objects-001.avro size: 5GB
/out/objects-002.avro size: 5GB
/out/objects-003.avro size: 3GB

Now my problem is that when I use DataFileWriter to append the objects to the file, there's no method that can return the size of the file written so far in bytes. So I don't know when the current output file crosses the threshold size.

I was looking to write something along the lines of:

THRESHOLD = 500000000
writer = DataFileWriter(open("users.avro", "wb"), DatumWriter(), schema)
#.
#.
#.
while objects_iter.hasNext():
  if writer.bytesWritternSoFar() < THRESHOLD:
    writer.append(objects_iter.nextObject())
  else:
#   closr the current writer and open a new one 

I'd like to mention that the size of the objects is not fixed, so I cannot approximate the size on a disk by counting the objects.

Any idea how to know the file size without resorting to forking the Avro code and exposing this size?