apache/beam · error · io.UnsupportedOperation

JsonRowWriter is not readable

Error message

JsonRowWriter is not readable

What it means

JsonRowWriter wraps a file opened for writing JSON rows for BigQuery loads; its read() raises io.UnsupportedOperation('JsonRowWriter is not readable') because a write-only writer cannot serve reads. It signals an attempt to read from a stream that only supports writing.

Solutions

1. Reopen the underlying file path with open(path, 'rb') (or the reader class) to read the written JSON records
2. Use the corresponding reader (e.g. JsonReader) instead of the writer
3. Access writer._file_handle only if you know it's opened in a read-capable mode
4. Restructure code so reading happens after closing the writer, on a fresh handle

Example fix

// before
rows = writer.read()  # raises UnsupportedOperation
// after
writer.flush()
with open(writer._file_handle.name, 'r') as f:
    rows = [json.loads(line) for line in f]

if getattr(obj, 'writable', lambda: False)() and not getattr(obj, 'readable', lambda: True)():
    raise TypeError('stream is write-only; reopen the file to read')

def is_readable_stream(o):
    return hasattr(o, 'read') and callable(getattr(o, 'readable', None)) and o.readable()

try:
    data = writer.read()
except io.UnsupportedOperation:
    writer.flush()
    with open(writer._file_handle.name, 'r') as f:
        data = f.read()

When it happens

Trigger: Calling .read() (or passing the writer where a readable file-like object is expected) on a JsonRowWriter instance; e.g. mistakenly handing the writer to code that reads records back.

Common situations: Confusing the writer with the reader class; code written for a real file object reused on the writer; debugging code attempting to inspect written contents directly from the writer instead of reopening the file.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• AvroRowWriter is not readable
• Output stream must be writable
• Please specify a BigQuery table to read from.
• A BigQuery table or a query must be specified
• A schema is required to write non-schema'd data.

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/1097e16c870bf3d3. Report an issue: GitHub.

Appendix: source

Thrown at sdks/python/apache_beam/io/gcp/bigquery_tools.py:1487

    """
    if not file_handle.writable():
      raise ValueError("Output stream must be writable")

    self._file_handle = file_handle
    self._coder = RowAsDictJsonCoder()

  def close(self):
    self._file_handle.close()

  @property
  def closed(self):
    return self._file_handle.closed

  def flush(self):
    self._file_handle.flush()

  def read(self, size=-1):
    raise io.UnsupportedOperation("JsonRowWriter is not readable")

  def tell(self):
    return self._file_handle.tell()

  def writable(self):
    return self._file_handle.writable()

  def write(self, row):
    return self._file_handle.write(self._coder.encode(row) + b'\n')


class AvroRowWriter(io.IOBase):
  """
  A writer which provides an IOBase-like interface for writing table rows
  (represented as dicts) as Avro records.
  """
  def __init__(self, file_handle, schema):
    """Initialize an AvroRowWriter.

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