class_docstr: Base object for pipeline variables
  
  PipelineVariable subclasses must implement the expr property. Its subclasses include:
  :class:`~sagemaker.workflow.parameters.Parameter`,
  :class:`~sagemaker.workflow.properties.Properties`,
  :class:`~sagemaker.workflow.functions.Join`,
  :class:`~sagemaker.workflow.functions.JsonGet`,
  :class:`~sagemaker.workflow.execution_variables.ExecutionVariable`.
  :class:`~sagemaker.workflow.step_outputs.StepOutput`.
docstr: Get the expression structure for workflow service calls.
docstr_tok: ["Get","the","expression","structure","for","workflow","service","calls","."]
code: def expr(self) -> RequestType:
      """Get the expression structure for workflow service calls."""
  
imports: from __future__ import absolute_import
  import abc
  from enum import EnumMeta
  from typing import Any, Dict, List, Union, TYPE_CHECKING
label_desc: Use image node_id 6 for calling the PipelineVariable obj's underlying member method code with example usage: obj.expr() without return types