fix for string attributes default values

cmdx.py

@@ -1022,10 +1022,20 @@ def addAttr(self, attr):
 
         """
 
-        if isinstance(attr, _AbstractAttribute):
-            attr = attr.create()
+        attr_obj = attr.create() if isinstance(attr, _AbstractAttribute) else attr
+        self._fn.addAttribute(attr_obj)
+
+        # workaround for string attribute default value, see String.default() for details
+        def setAttr(attr):
+            if isinstance(attr, String):
+                default = _AbstractAttribute.default(attr)
+                if default:
+                    self.findPlug(attr["name"]).setString(default)
+            elif isinstance(attr, Compound):
+                for child in attr["children"]:
+                    setAttr(child)
+        setAttr(attr)
 
-        self._fn.addAttribute(attr)
 
     def hasAttr(self, attr):
         """Return whether or not `attr` exists
@@ -5120,8 +5130,43 @@ class String(_AbstractAttribute):
     Default = ""
 
     def default(self, cls=None):
-        default = str(super(String, self).default(cls))
-        return om.MFnStringData().create(default)
+        """
+        string attributes can't have default values (http://help.autodesk.com/cloudhelp/2020/ENU/Maya-Tech-Docs/Commands/addAttr.html#flagdefaultValue),
+        or at least it isn't saved in scenes (https://github.com/mottosso/cmdx/issues/34)
+        -> don't pass the default to OpenMaya, instead set it during Node.addAttr()
+
+        Test:
+            >>> import os
+            >>> os.environ["CMDX_SAFE_MODE"] = "True"  # disable caching (node/plug reuse)
+            >>> import cmdx
+            >>> # new scene with a sphere with "my_string" attribute
+            >>> _ = cmds.file(new=True, force=True)
+            >>> sphere_name = cmds.sphere()[0]
+            >>> sphere = cmdx.encode(sphere_name)
+            >>> sphere.add_attr(cmdx.String("my_string", default="foo"))
+            >>> sphere["my_string"].name()
+            'my_string'
+            >>> # save scene
+            >>> import tempfile
+            >>> scene_file = tempfile.NamedTemporaryFile(prefix="test_string_attribute_", suffix=".ma")
+            >>> scene_path = scene_file.name
+            >>> _ = cmds.file(rename=scene_path)
+            >>> _ = cmds.file(save=True, type="mayaAscii")
+            >>> # reload scene & check "my_string"
+            >>> _ = cmds.file(scene_path, open=True, force=True)
+            >>> sphere = cmdx.encode(sphere_name)
+            >>> str(sphere)
+            '|nurbsSphere1'
+            >>> if sphere["my_string"].name() != "my_string":
+            ...     print(scene_path)
+            ...     with open(scene_path) as f: print(f.read())
+            >>> sphere["my_string"].name()
+            'my_string'
+            >>> scene_file.close()  # deletes the temp file (supposedly)
+            >>> sphere["my_string"].read()
+            u'foo'
+        """
+        return None
 
     def read(self, data):
         return data.inputValue(self["mobject"]).asString()