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):

[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):
     '''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]