class_docstr: Interface for defining a custom :class:`~ray.data.Dataset` datasource.
  
  To read a datasource into a dataset, use :meth:`~ray.data.read_datasource`.
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docstr: Callback for when a write job starts.
  
  Use this method to perform setup for write tasks. For example, creating a
  staging bucket in S3.
  
  Args:
      write_args: Additional kwargs to pass to the datasource impl.
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code: def on_write_start(self, **write_args) -> None:
      """Callback for when a write job starts.
  
      Use this method to perform setup for write tasks. For example, creating a
      staging bucket in S3.
  
      Args:
          write_args: Additional kwargs to pass to the datasource impl.
      """
      pass
  
imports: import warnings
  from typing import Any, Callable, Iterable, List, Optional
  import numpy
  import ray
  from ray.data._internal.execution.interfaces import TaskContext
  from ray.data._internal.util import _check_pyarrow_version
  from ray.data.block import Block, BlockAccessor, BlockMetadata
  from ray.data.context import DataContext
  from ray.types import ObjectRef
  from ray.util.annotations import Deprecated, DeveloperAPI, PublicAPI
label_desc: Use image node_id 3 for calling the Datasource obj's underlying member method code with example usage: obj.on_write_start() without return types