Use a class constant for the default path based backend path

When no path is provided to a path based backend via configuration
use a class constant to provide the default, and override this in
backends that support providing defaults.

Change-Id: I0a6c88398403a162b113e34abe7e56821d1f02bc

taskflow/persistence/backends/impl_memory.py

@@ -309,10 +309,12 @@ class MemoryBackend(path_based.PathBasedBackend):
     guarantee that there will be no inter-thread race conditions when
     writing and reading by using a read/write locks.
     """
+
+    #: Default path used when none is provided.
+    DEFAULT_PATH = pp.sep
+
     def __init__(self, conf=None):
         super(MemoryBackend, self).__init__(conf)
-        if self._path is None:
-            self._path = pp.sep
         self.memory = FakeFilesystem(deep_copy=self._conf.get('deep_copy',
                                                               True))
         self.lock = fasteners.ReaderWriterLock()

taskflow/persistence/backends/impl_zookeeper.py

@@ -40,10 +40,12 @@ class ZkBackend(path_based.PathBasedBackend):
             "path": "/taskflow",
         }
     """
+
+    #: Default path used when none is provided.
+    DEFAULT_PATH = '/taskflow'
+
     def __init__(self, conf, client=None):
         super(ZkBackend, self).__init__(conf)
-        if not self._path:
-            self._path = '/taskflow'
         if not paths.isabs(self._path):
             raise ValueError("Zookeeper path must be absolute")
         if client is not None:

taskflow/persistence/path_based.py

@@ -34,9 +34,14 @@ class PathBasedBackend(base.Backend):
     the contents of those objects for later reading and writing.
     """
 
+    #: Default path used when none is provided.
+    DEFAULT_PATH = None
+
     def __init__(self, conf):
         super(PathBasedBackend, self).__init__(conf)
         self._path = self._conf.get('path', None)
+        if not self._path:
+            self._path = self.DEFAULT_PATH
 
     @property
     def path(self):