class_docstr: Fake for datastore transaction.
  
  See https://cloud.google.com/datastore/docs/concepts/transactions
  for details of how transactions work in Cloud Datastore.
class_docstr_tok: ["Fake","for","datastore","transaction",".","See","https",":","\/\/cloud.google.com\/datastore\/docs\/concepts\/transactions","for","details","of","how","transactions","work","in","Cloud","Datastore","."]
docstr: Helper method to check that transaction has been started.
docstr_tok: ["Helper","method","to","check","that","transaction","has","been","started","."]
code: def _check_transaction_started(self):
      """Helper method to check that transaction has been started."""
      if self._state != "started":
          raise ValueError(
              (
                  "Invalid state of transaction, "
                  "expected started, was %s"
              )
              % self._state
          )
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import copy
  from io import StringIO
  import six
label_desc: Use image node_id 2 for calling the FakeDatastoreClientTransaction obj's underlying member method code with example usage: obj._check_transaction_started() without return types