__del__中的strftime导致importlib错误

Question

在测试以下python代码片段时，我得到了python2.7和python3.4的错误：

from datetime import datetime

class Test(object):
    def __del__(self):
        print(datetime.now().strftime('%Y'))

a = Test()

python2.7错误：

Exception RuntimeError: 'sys.meta_path must be a list of import hooks' in <bound method Test.__del__ of <__main__.Test object at 0x7f151163b290> ignored

python3.4错误：

Exception ignored in: <bound method Test.__del__ of <__main__.Test object at 0x7f9a140e1160>>
Traceback (most recent call last):
  File "test.py", line 5, in __del__
  File "<frozen importlib._bootstrap>", line 2237, in _find_and_load
  File "<frozen importlib._bootstrap>", line 2222, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 2155, in _find_spec
TypeError: 'NoneType' object is not iterable

有没有人知道这里发生了什么？ 删除strftime或用isoformat（）替换它似乎运行良好，这真的很奇怪。