Add 'Transaction.current' staticmethod.

Returns top of stack IFF it is a transaction, or None.

gcloud/datastore/test_transaction.py

@@ -65,6 +65,31 @@ def test_ctor_with_env(self):
         self.assertEqual(xact.dataset_id, DATASET_ID)
         self.assertEqual(xact.connection, CONNECTION)
 
+    def test_current(self):
+        from gcloud.datastore.test_api import _NoCommitBatch
+        _DATASET = 'DATASET'
+        connection = _Connection()
+        xact1 = self._makeOne(_DATASET, connection)
+        xact2 = self._makeOne(_DATASET, connection)
+        self.assertTrue(xact1.current() is None)
+        self.assertTrue(xact2.current() is None)
+        with xact1:
+            self.assertTrue(xact1.current() is xact1)
+            self.assertTrue(xact2.current() is xact1)
+            with _NoCommitBatch(_DATASET, _Connection):
+                self.assertTrue(xact1.current() is None)
+                self.assertTrue(xact2.current() is None)
+            with xact2:
+                self.assertTrue(xact1.current() is xact2)
+                self.assertTrue(xact2.current() is xact2)
+                with _NoCommitBatch(_DATASET, _Connection):
+                    self.assertTrue(xact1.current() is None)
+                    self.assertTrue(xact2.current() is None)
+            self.assertTrue(xact1.current() is xact1)
+            self.assertTrue(xact2.current() is xact1)
+        self.assertTrue(xact1.current() is None)
+        self.assertTrue(xact2.current() is None)
+
     def test_begin(self):
         _DATASET = 'DATASET'
         connection = _Connection(234)

gcloud/datastore/transaction.py

@@ -110,6 +110,19 @@ def id(self):
         """
         return self._id
 
+    @staticmethod
+    def current():
+        """Return the topmost transaction.
+
+        .. note:: if the topmost element on the stack is not a transaction,
+                  returns None.
+
+        :rtype: :class:`gcloud.datastore.transaction.Transaction` or None
+        """
+        top = Batch.current()
+        if isinstance(top, Transaction):
+            return top
+
     def begin(self):
         """Begins a transaction.