[jira] [Created] (ARROW-1194) Trouble deserializing a pandas DataFrame from a PyArrow buffer.

["Robert Nishihara (JIRA)" <ji...@apache.org>]

Robert Nishihara created ARROW-1194:
---------------------------------------

             Summary: Trouble deserializing a pandas DataFrame from a PyArrow buffer.
                 Key: ARROW-1194
                 URL: https://issues.apache.org/jira/browse/ARROW-1194
             Project: Apache Arrow
          Issue Type: Bug
          Components: Python
    Affects Versions: 0.5.0
         Environment: Ubuntu 16.04
Python 3.6
            Reporter: Robert Nishihara


I'm running into the following problem.

Suppose I create a dataframe and serialize it.

{code:language=python}
import pyarrow as pa
import pandas as pd

df = pd.DataFrame({"a": [1, 2, 3]})
record_batch = pa.RecordBatch.from_pandas(df)
{code}

It's size is 352 according to

{code:language=python}
pa.get_record_batch_size(record_batch)  # This is 352.
{code}

However, if I write it using a stream_writer and then attempt to read it, the resulting buffer has size 928.

{code:language=python}
sink = pa.BufferOutputStream()
stream_writer = pa.RecordBatchStreamWriter(sink, record_batch.schema)
stream_writer.write_batch(record_batch)
new_buf = sink.get_result()
new_buf.size  # This is 928.
{code}

I'm running into this problem because I'm attempting to write the pandas DataFrame to the Plasma object store as follows (after Plasma has been started and a client has been created), so I need to know the size ahead of time.

{code:language=python}
data_size = pa.get_record_batch_size(record_batch)
object_id = plasma.ObjectID(np.random.bytes(20))

buf = client.create(object_id, data_size)  # Note that if I replace "data_size" by "data_size + 1000" then it works.
stream = plasma.FixedSizeBufferOutputStream(buf)
stream_writer = pa.RecordBatchStreamWriter(stream, record_batch.schema)
stream_writer.write_batch(record_batch)
{code}

The above fails because the buffer allocated in Plasma only has size 352, but 928 bytes are needed.

So my question is, am I determining the size of the record batch incorrectly? Or could there be a bug in {code:language=python}pa.get_record_batch_size{code}



--
This message was sent by Atlassian JIRA
(v6.4.14#64029)