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

   1 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 # Use of this source code is governed by a BSD-style license that can be
   3 # found in the LICENSE file.
   4
   5 import logging
   6 import sys
   7 import traceback
   8
   9 _no_value = object()
  10
  11
  12 def _DefaultErrorHandler(error):
  13   raise error
  14
  15
  16 def All(futures, except_pass=None, except_pass_log=False):
  17   '''Creates a Future which returns a list of results from each Future in
  18   |futures|.
  19
  20   If any Future raises an error other than those in |except_pass| the returned
  21   Future will raise as well.
  22
  23   If any Future raises an error in |except_pass| then None will be inserted as
  24   its result. If |except_pass_log| is True then the exception will be logged.
  25   '''
  26   def resolve():
  27     resolved = []
  28     for f in futures:
  29       try:
  30         resolved.append(f.Get())
  31       # "except None" will simply not catch any errors.
  32       except except_pass:
  33         if except_pass_log:
  34           logging.error(traceback.format_exc())
  35         resolved.append(None)
  36         pass
  37     return resolved
  38   return Future(callback=resolve)
  39
  40
  41 def Race(futures, except_pass=None, default=_no_value):
  42   '''Returns a Future which resolves to the first Future in |futures| that
  43   either succeeds or throws an error apart from those in |except_pass|.
  44
  45   If all Futures throw errors in |except_pass| then |default| is returned,
  46   if specified. If |default| is not specified then one of the passed errors
  47   will be re-thrown, for a nice stack trace.
  48   '''
  49   def resolve():
  50     first_future = None
  51     for future in futures:
  52       if first_future is None:
  53         first_future = future
  54       try:
  55         return future.Get()
  56       # "except None" will simply not catch any errors.
  57       except except_pass:
  58         pass
  59     if default is not _no_value:
  60       return default
  61     # Everything failed and there is no default value, propagate the first
  62     # error even though it was caught by |except_pass|.
  63     return first_future.Get()
  64   return Future(callback=resolve)
  65
  66
  67 class Future(object):
  68   '''Stores a value, error, or callback to be used later.
  69   '''
  70   def __init__(self, value=_no_value, callback=None, exc_info=None):
  71     self._value = value
  72     self._callback = callback
  73     self._exc_info = exc_info
  74     if (self._value is _no_value and
  75         self._callback is None and
  76         self._exc_info is None):
  77       raise ValueError('Must have either a value, error, or callback.')
  78
  79   def Then(self, callback, error_handler=_DefaultErrorHandler):
  80     '''Creates and returns a future that runs |callback| on the value of this
  81     future, or runs optional |error_handler| if resolving this future results in
  82     an exception.
  83
  84     If |callback| returns a non-Future value then the returned Future will
  85     resolve to that value.
  86
  87     If |callback| returns a Future then it gets chained to the current Future.
  88     This means that the returned Future will resolve to *that* Future's value.
  89     This behaviour is transitive.
  90
  91     For example,
  92
  93       def fortytwo():
  94         return Future(value=42)
  95
  96       def inc(x):
  97         return x + 1
  98
  99       def inc_future(x):
 100         return Future(value=x + 1)
 101
 102     fortytwo().Then(inc).Get()                         ==> 43
 103     fortytwo().Then(inc_future).Get()                  ==> 43
 104     fortytwo().Then(inc_future).Then(inc_future).Get() ==> 44
 105     '''
 106     def then():
 107       val = None
 108       try:
 109         val = self.Get()
 110       except Exception as e:
 111         val = error_handler(e)
 112       else:
 113         val = callback(val)
 114       return val.Get() if isinstance(val, Future) else val
 115     return Future(callback=then)
 116
 117   def Get(self):
 118     '''Gets the stored value, error, or callback contents.
 119     '''
 120     if self._value is not _no_value:
 121       return self._value
 122     if self._exc_info is not None:
 123       self._Raise()
 124     try:
 125       self._value = self._callback()
 126       return self._value
 127     except:
 128       self._exc_info = sys.exc_info()
 129       self._Raise()
 130
 131   def _Raise(self):
 132     exc_info = self._exc_info
 133     raise exc_info[0], exc_info[1], exc_info[2]