# incrmnt.py
import db
 
def increment():
  count = db.get_count()
 
  new_count = count + 1
  db.set_count(new_count)
 
  return new_count

# 线程1和线程2在不同的进程中同时执行
# 为了展示的目的，在这里并排放置
# 在垂直方向分开它们，以说明在每个时间点上执行什么代码
# Thread 1（线程1）         # Thread 2（线程2）
def increment():
                  def increment():
  # get_count returns 0
  count = db.get_count()
                    # get_count returns 0 again
                    count = db.get_count()
  new_count = count + 1
  # set_count called with 1
  db.set_count(new_count)
                    new_count = count + 1
                    # set_count called with 1 again
                    db.set_count(new_count)

# Thread 1             # Thread 2
def increment():
  # get_count returns 0
  count = db.get_count()
                  def increment():
                    # get_count returns 0 again
                    count = db.get_count()
 
                    # set_count called with 1
                    new_count = count + 1
                    db.set_count(new_count)
  # set_count called with 1 again
  new_count = count + 1
  db.set_count(new_count)

# test_incrmnt.py
 
import unittest
 
import before_after
 
import db
import incrmnt
 
class TestIncrmnt(unittest.TestCase):
  def setUp(self):
    db.reset_db()
 
  def test_increment_race(self):
    # after a call to get_count, call increment
    with before_after.after('incrmnt.db.get_count', incrmnt.increment):
      # start off the race with a call to increment
      incrmnt.increment()
 
    count = db.get_count()
    self.assertEqual(count, 2)

# incrmnt.py
 
def locking_increment():
  with db.get_lock():
    return increment()

# test_incrmnt.py
 
def test_locking_increment_race(self):
  def erroring_locking_increment():
    # Trying to get a lock when the other thread has it will cause a
    # CouldNotLock exception - catch it here or the test will fail
    with self.assertRaises(db.CouldNotLock):
      incrmnt.locking_increment()
 
  with before_after.after('incrmnt.db.get_count', erroring_locking_increment):
    incrmnt.locking_increment()
 
  count = db.get_count()
  self.assertEqual(count, 1)

# incrmnt.py
 
def retrying_locking_increment():
  @retry(tries=5, errors=db.CouldNotLock)
  def _increment():
    return locking_increment()
 
  return _increment()