Working on this PR (https://github.com/maxpoint/spylon/pull/49), I encountered an unexpected SystemError in Python 3.6 on my Mac and on Travis Linux. 

Exception in thread Thread-4:
Traceback (most recent call last):
  File "/Users/parente/miniconda3/envs/spylon-dev/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/Users/parente/miniconda3/envs/spylon-dev/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/parente/projects/spylon/spylon/spark/progress.py", line 99, in _spark_progress_thread_worker
    td = datetime.datetime.now() - start_times[stage_id]
SystemError: <built-in method __missing__ of collections.defaultdict object at 0x106013098> returned NULL without setting an error

The exception only occurs in Python 3.6, not in Python 3.5 or 3.4: https://travis-ci.org/maxpoint/spylon/builds/235992988

The defaultdict is both created and used as a local variable in a threading.Thread run() function. It's never accessed outside that context (https://github.com/maxpoint/spylon/pull/49/commits/2bd47dc32f6129f5f6a4824be1eaed568351df11#diff-1ba10d1cd92a35a380442cb586e310e2R179)

I didn't see a related issue on the tracker here nor did I see anything in the 3.6 changelog about a behavior change.