Add a "create" method for the client

Adds a "create" method to the client, which verifies
that the key did not exists in the key-value store,
before assigning the new value to the key

etcd3gw/client.py

@@ -122,6 +122,42 @@ def lock(self, id=str(uuid.uuid4()), ttl=DEFAULT_TIMEOUT):
         """
         return Lock(id, ttl=ttl, client=self)
 
+    def create(self, key, value):
+        """Atomically create the given key only if the key doesn't exist.
+
+        This verifies that the create_revision of a key equales to 0, then
+        creates the key with the value.
+        This operation takes place in a transaction.
+
+        :param key: key in etcd to create
+        :param value: value of the key
+        :type value: bytes or string
+        :returns: status of transaction, ``True`` if the create was
+                  successful, ``False`` otherwise
+        :rtype: bool
+        """
+        base64_key = _encode(key)
+        base64_value = _encode(value)
+        txn = {
+            'compare': [{
+                'key': base64_key,
+                'result': 'EQUAL',
+                'target': 'CREATE',
+                'create_revision': 0
+            }],
+            'success': [{
+                'request_put': {
+                    'key': base64_key,
+                    'value': base64_value,
+                }
+            }],
+            'failure': []
+        }
+        result = self.transaction(txn)
+        if 'succeeded' in result:
+            return result['succeeded']
+        return False
+
     def put(self, key, value, lease=None):
         """Put puts the given key into the key-value store.
 

etcd3gw/tests/test_etcd3gw.py

@@ -302,3 +302,28 @@ def test_client_locks(self):
         self.assertTrue(lock.release())
         self.assertFalse(lock.release())
         self.assertFalse(lock.is_acquired())
+
+    @unittest.skipUnless(
+        _is_etcd3_running(), "etcd3 is not available")
+    def test_create_success(self):
+        key = '/foo/unique' + str(uuid.uuid4())
+        # Verify that key is empty
+        self.assertEqual([], self.client.get(key))
+
+        status = self.client.create(key, 'bar')
+        # Verify that key is 'bar'
+        self.assertEqual(['bar'], self.client.get(key))
+        self.assertTrue(status)
+
+    @unittest.skipUnless(
+        _is_etcd3_running(), "etcd3 is not available")
+    def test_create_fail(self):
+        key = '/foo/' + str(uuid.uuid4())
+        # Assign value to the key
+        self.client.put(key, 'bar')
+        self.assertEqual(['bar'], self.client.get(key))
+
+        status = self.client.create(key, 'goo')
+        # Verify that key is still 'bar'
+        self.assertEqual(['bar'], self.client.get(key))
+        self.assertFalse(status)