This is a "Heisenbug", a race condition. You might not be able to reproduce it, but I'll describe it anyway.

We're running this as an automated test, as a user we cannot control, so use export PLOTLY_DIR=whatever

re: #1076, addressed in #1195

This is still a problem with plotly-3.7.1. In plotly/files.py:

 27 def _permissions():
 28 try:
 29 if not os.path.exists(PLOTLY_DIR):
 30 os.mkdir(PLOTLY_DIR)
 31 with open(TEST_FILE, 'w') as f:
 32 f.write('testing\n')
 33 os.remove(TEST_FILE)
 34 return True
 35 except:
 36 return False

Lines 30 and 33 can fail, wrongly, if two processes run this at the same time. (f.write() is fine here, IMO.)

Note that I want to make warnings into errors in my tests. (pytest -W error), so this is a big problem for me even if you simply trap and warn().

I'm not sure if there is still a warning in 3.7.1, but like I said, it's a Heisenbug. I can only spend so much time on this. For me, it was happening via pytest-xdist, in a case where I lacked permissions to create ~/.plotly. PLOTLY_DIR lets me reduce the likelihood of error, but the race conditions remain.