Conver APIListDataSource to use Future.Then() rather than Get().

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):
@@ skipping 51 matching lines @@
     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, error_handler=_DefaultErrorHandler):
     '''Creates and returns a future that runs |callback| on the value of this
     future, or runs optional |error_handler| if resolving this future results in
     an exception.
+
+    If |callback| returns a non-Future value then the returned Future will
+    resolve to that value.
+
+    If |callback| returns a Future then it gets chained to the current Future.
+    This means that the returned Future will resolve to *that* Future's value.
+    This behaviour is transitive.
+
+    For example,
+
+      def fortytwo():
+        return Future(value=42)
+
+      def inc(x):
+        return x + 1
+
+      def inc_future(x):
+        return Future(value=x + 1)
+
+    fortytwo().Then(inc).Get()                         ==> 43
+    fortytwo().Then(inc_future).Get()                  ==> 43
+    fortytwo().Then(inc_future).Then(inc_future).Get() ==> 44
     '''
     def then():
+      val = None
       try:
         val = self.Get()
       except Exception as e:
-        return error_handler(e)
-      return callback(val)
+        val = error_handler(e)
+      else:
+        val = callback(val)
+      return val.Get() if isinstance(val, Future) else 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]