Writes given Evaluation or Validation data using given writer PTransform.

evaluation_or_validation Evaluation or Validation data.
key Key for Evaluation or Validation output to write. It is valid for the key to not exist in the dict (in which case the write is a no-op).
ptransform PTransform to use for writing.

ValueError If Evaluation or Validation is empty. The key does not need to exist in the Evaluation or Validation, but the dict must not be empty.

The result of the underlying beam write PTransform. This makes it possible for interactive environments to execute your writer, as well as for downstream Beam stages to make use of the files that are written.