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# Copyright 2010 the Melange authors.
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# See the License for the specific language governing permissions and
# limitations under the License.
"""Helper class for deadline-based batch processing using the Task API"""
import datetime
from google.appengine.runtime import DeadlineExceededError
class Timekeeper:
"""Raise a DeadlineExceededError on your schedule
Python raises this exception when your request handler is about to
exceed its 30 second deadline. But you have less than a second to handle
the exception, which is probably not enough time to reliably requeue a
task in the Task API, for example.
You can get some breathing room by setting your own artificial deadline
and leaving sufficient time to, e.g., interrupt and requeue your task
def __init__(self, timelimit, starttime=None):
# Allow override for testing
if not starttime:
starttime =
# Calculate the deadline as an offset from starttime
self.deadline = starttime + datetime.timedelta(milliseconds=timelimit)
def ping(self, currtime=None):
"""Enforce the deadline, return time remaining"""
# Allow override for testing
if not currtime:
currtime =
# Raise artifical deadline
if currtime >= self.deadline:
raise DeadlineExceededError()
# Return time remaining
return self.deadline - currtime
def iterate(self, items):
"""Iterate a sequence, pinging for each"""
for item in items:
yield, item