Docserver: Update Future.Then() to be more Promise-like

chrome/common/extensions/docs/server2/future.py

Index: chrome/common/extensions/docs/server2/future.py
diff --git a/chrome/common/extensions/docs/server2/future.py b/chrome/common/extensions/docs/server2/future.py
index 4b09073d4c25c0812fcbb9494dd731333f4cedf9..05f4d4224cff380a81283818994b965260060ab7 100644
--- a/chrome/common/extensions/docs/server2/future.py
+++ b/chrome/common/extensions/docs/server2/future.py
@@ -7,6 +7,10 @@ import sys
 _no_value = object()
 
 
+def _DefaultErrorHandler(error):
+  raise error
+
+
 def All(futures, except_pass=None):
   '''Creates a Future which returns a list of results from each Future in
   |futures|.
@@ -61,12 +65,15 @@ class Future(object):
         self._exc_info is None):
       raise ValueError('Must have either a value, error, or callback.')
 
-  def Then(self, callback):
+  def Then(self, callback, error_handler=_DefaultErrorHandler):

[Yoyo Zhou, 2014/08/01 22:03:19]

This should come with tests.

P.S. You should make changes like this separate from the other changes, since
they don't inherently have anything to do with each other. (It would be
different if some of them used this.)

[ahernandez, 2014/08/04 18:51:19]

The other changes do use the new changes from future.py (see file_system.py for
example).

     '''Creates and returns a future that runs |callback| on the value of this
-    future.
+    future. Optional |error_handler| can be passed for custom error handling.
     '''
     def then():
-      return callback(self.Get())
+      try:
+        return callback(self.Get())

[Yoyo Zhou, 2014/08/01 22:03:19]

If I'm reading the Promise description correctly, it seems like the try/except
here should only cover self.Get. Otherwise, if you have a chain of callbacks,
and one of the later one throws an exception, it gets caught, and only the first
one's error handler gets called. But feel free to enlighten me on what the
correct behavior should be.

+      except Exception as e:
+        error_handler(e)
     return Future(callback=then)
 
   def Get(self):