fix: remove mutable object as default for method/function values

use immutable tuples as default value markers for parameters where a
None value is valid.

It does seem that the _marker objects are used in places where None
would not be considered a valid value.

Author: rouilj

roundup/cgi/templating.py

@@ -1503,7 +1503,7 @@ def copy_url(self, exclude=("messages", "files")):
 class _HTMLUser(_HTMLItem):
     """Add ability to check for permissions on users.
     """
-    _marker = []
+    _marker = ('_HTMLUserMarker')
 
     def hasPermission(self, permission, classname=_marker,
                       property=None, itemid=None):
@@ -2200,7 +2200,7 @@ def field(self, labelfirst=False, y_label=None, n_label=None,
 
 class DateHTMLProperty(HTMLProperty):
 
-    _marker = []
+    _marker = ('HTMLPropertyMarker')
 
     def __init__(self, client, classname, nodeid, prop, name, value,
                  anonymous=0, offset=None):