Fix ahi_hsd cleanup during interpreter shutdown

[sfinkens]

I get the following warning when reading bzipped AHI HSD files

Exception ignored in: <function AHIHSDFileHandler.__del__ at 0x7f5fff886200>
Traceback (most recent call last):
  File ".../satpy/readers/ahi_hsd.py", line 409, in __del__
AttributeError: 'NoneType' object has no attribute 'path'

This is because the os module is set to None during interpreter shutdown. From the docs:

del() can be executed during interpreter shutdown. As a consequence, the global variables it needs to access (including other modules) may already have been deleted or set to None

I followed the advice in the docs and registered the cleanup using weakref.finalize.

[mraspaud]

Looking at the documentation, I don't see it clearly written that weakref.finalize would solve the problem you mention. Could you verify yourself that it does indeed work?

[sfinkens]

Yes, with finalizer the warnings are gone. Did you see that section in the documentation?

A finalizer will never invoke its callback during the later part of the interpreter shutdown when module globals are liable to have been replaced by None.

[mraspaud]

no, but I'm probably looking in the wrong place :) https://docs.python.org/3/reference/datamodel.html#object.del

[sfinkens]

Oh, sorry that's from https://docs.python.org/3/library/weakref.html#weakref.finalize

[mraspaud]

Thanks for the link, I understand what finalize does here, so LGTM! I have to admit I'm a bit disappointed by __del__'s behaviour :) or I don't understand what it is for.