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

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

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 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|.
 
   If any Future raises an error other than those in |except_pass| the returned
   Future will raise as well.
   '''
   def resolve():
     resolved = []
     for f in futures:
@@ skipping 34 matching lines @@
   '''
   def __init__(self, value=_no_value, callback=None, exc_info=None):
     self._value = value
     self._callback = callback
     self._exc_info = exc_info
     if (self._value is _no_value and
         self._callback is None and
         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):
     '''Creates and returns a future that runs |callback| on the value of this
-    future.
+    future, or runs optional |error_handler| if resolving this future results in
+    an exception.
     '''
     def then():
-      return callback(self.Get())
+      try:
+        val = self.Get()
+      except Exception as e:
+        return error_handler(e)
+      return callback(val)
     return Future(callback=then)
 
   def Get(self):
     '''Gets the stored value, error, or callback contents.
     '''
     if self._value is not _no_value:
       return self._value
     if self._exc_info is not None:
       self._Raise()
     try:
       self._value = self._callback()
       return self._value
     except:
       self._exc_info = sys.exc_info()
       self._Raise()
 
   def _Raise(self):
     exc_info = self._exc_info
     raise exc_info[0], exc_info[1], exc_info[2]