Inline Begin transction for RW transactions

Author: surbhigarg92

google/cloud/spanner_v1/transaction.py

@@ -61,9 +61,7 @@ def _check_state(self):
         :raises: :exc:`ValueError` if the object's state is invalid for making
                  API requests.
         """
-        if self._transaction_id is None:
-            raise ValueError("Transaction is not begun")
-
+        
         if self.committed is not None:
             raise ValueError("Transaction is already committed")
 
@@ -78,7 +76,11 @@ def _make_txn_selector(self):
         :returns: a selector configured for read-write transaction semantics.
         """
         self._check_state()
-        return TransactionSelector(id=self._transaction_id)
+        
+        if self._transaction_id is None:
+            return TransactionSelector(begin=TransactionOptions(read_write=TransactionOptions.ReadWrite()))
+        else:
+            return TransactionSelector(id=self._transaction_id)
 
     def begin(self):
         """Begin a transaction on the database.
@@ -302,6 +304,10 @@ def execute_update(
             response = api.execute_sql(
                 request=request, metadata=metadata, retry=retry, timeout=timeout
             )
+
+        if self._transaction_id is None and response.metadata.transaction is not None:
+            self._transaction_id = response.metadata.transaction.id
+
         return response.stats.row_count_exact
 
     def batch_update(self, statements, request_options=None):