walker_class attribute for filesystem (#119)

* added walker_class attribute to FS; overrode walk property in WrapFS to use same walker as wrapped fs uses

* used assertion from six module to run tets both on python 2 and 3

fs/base.py

@@ -43,6 +43,9 @@ class FS(object):
 
     # This is the "standard" meta namespace.
     _meta = {}
+
+    # most FS will use default walking algorithms
+    walker_class=Walker
 
     def __init__(self):
         """Create a filesystem. See help(type(self)) for accurate signature.
@@ -65,7 +68,7 @@ def __exit__(self, exc_type, exc_value, traceback):
     def walk(self):
         """`~fs.walk.BoundWalker`: a walker bound to this filesystem.
         """
-        return Walker.bind(self)
+        return self.walker_class.bind(self)
 
     # ---------------------------------------------------------------- #
     # Required methods                                                 #

fs/wrapfs.py

@@ -397,3 +397,8 @@ def validatepath(self, path):
             _fs.validatepath(_path)
         path = abspath(normpath(path))
         return path
+
+    @property
+    def walk(self):
+        return self._wrap_fs.walker_class.bind(self)
+

tests/test_walk.py

@@ -5,6 +5,8 @@
 from fs.errors import FSError
 from fs.memoryfs import MemoryFS
 from fs import walk
+from fs.wrap import read_only
+import six
 
 
 class TestWalkerBase(unittest.TestCase):
@@ -196,3 +198,46 @@ def broken_scandir(path, namespaces=None):
     def test_on_error_invalid(self):
         with self.assertRaises(TypeError):
             walk.Walker(on_error='nope')
+
+    def test_subdir_uses_same_walker(self):
+        class CustomWalker(walk.Walker):
+
+            @classmethod
+            def bind(cls, fs):
+                return walk.BoundWalker(fs, walker_class=CustomWalker)
+
+        class CustomizedMemoryFS(MemoryFS):
+            walker_class=CustomWalker
+
+        base_fs=CustomizedMemoryFS()
+        base_fs.settext("a", "a")
+        base_fs.makedirs("b")
+        base_fs.settext("b/c", "c")
+        base_fs.settext("b/d", "d")
+        base_walker=base_fs.walk
+        self.assertEqual(base_walker.walker_class, CustomWalker)
+        six.assertCountEqual(self, ["/a", "/b/c", "/b/d"], base_walker.files())
+
+        sub_fs=base_fs.opendir("b")
+        sub_walker=sub_fs.walk
+        self.assertEqual(sub_walker.walker_class, CustomWalker)
+        six.assertCountEqual(self, ["/c", "/d"], sub_walker.files())
+
+    def test_readonly_wrapper_uses_same_walker(self):
+        class CustomWalker(walk.Walker):
+
+            @classmethod
+            def bind(cls, fs):
+                return walk.BoundWalker(fs, walker_class=CustomWalker)
+
+        class CustomizedMemoryFS(MemoryFS):
+            walker_class=CustomWalker
+
+        base_fs=CustomizedMemoryFS()
+        base_walker=base_fs.walk
+        self.assertEqual(base_walker.walker_class, CustomWalker)
+
+        readonly_fs=read_only(CustomizedMemoryFS())
+        readonly_walker=readonly_fs.walk
+        self.assertEqual(readonly_walker.walker_class, CustomWalker)
+