Progress tracking cleanup.

* use clearer names for the two status strings passed to js
* add functions to do canonical conversion of progress to js string
* fix updating bug in sequence.coffee
* add some todo comments and other docs to make future expansion easier

common/lib/xmodule/progress.py

@@ -1,6 +1,17 @@
 '''
 Progress class for modules.  Represents where a student is in a module.
-'''
+
+Useful things to know:
+ - Use Progress.to_js_status_str() to convert a progress into a simple
+   status string to pass to js.  
+ - Use Progress.to_js_detail_str() to convert a progress into a more detailed
+   string to pass to js.
+
+In particular, these functions have a canonical handing of None.
+
+For most subclassing needs, you should only need to reimplement
+frac() and __str__().
+''' 
 
 from collections import namedtuple
 import numbers
@@ -124,3 +135,26 @@ class Progress(object):
         (n, d) = a.frac()
         (n2, d2) = b.frac()
         return Progress(n + n2, d + d2)
+
+    @staticmethod
+    def to_js_status_str(progress):
+        '''
+        Return the "status string" version of the passed Progress
+        object that should be passed to js.  Use this function when
+        sending Progress objects to js to limit dependencies.
+        '''
+        if progress is None:
+            return "NA"
+        return progress.ternary_str()
+
+
+    @staticmethod
+    def to_js_detail_str(progress):
+        '''
+        Return the "detail string" version of the passed Progress
+        object that should be passed to js.  Use this function when
+        passing Progress objects to js to limit dependencies.
+        '''
+        if progress is None:
+            return "NA"
+        return str(progress)