code: def _struct_get_field(expr, field_name):
      
      return ops.StructField(expr, field_name).to_expr().name(field_name)
docs: Get the `field_name` field from the ``Struct`` expression `expr`.
  
      Parameters
      ----------
      field_name : str
          The name of the field to access from the ``Struct`` typed expression
          `expr`. Must be a Python ``str`` type; programmatic struct field
          access is not yet supported.
  
      Returns
      -------
      value_expr : ibis.expr.types.ValueExpr
          An expression with the type of the field being accessed.