Commits on Oct 12, 2019

1. Improve handling of file resources …

   Follow Python's file object semantics. User code is responsible for
   closing resources (usually through a context manager) in a deterministic
   way.
   
   To achieve this, remove __del__ functions. These functions used to
   closed open file handlers in an attempt to silence Python
   ResourceWarnings. However, using __del__ has the following drawbacks:
   
   - __del__ isn't called until the object's reference count reaches 0.
     Therefore, resource handlers remain open or in use longer than
     necessary.
   
   - The __del__ method isn't guaranteed to execute on system exit. See the
     Python documentation:
   
     https://docs.python.org/3/reference/datamodel.html#object.__del__
   
     > It is not guaranteed that __del__() methods are called for objects
     > that still exist when the interpreter exits.
   
   - Exceptions that occur inside __del__ are ignored instead of raised.
     This has the potential of hiding bugs. This is also in the Python
     documentation:
   
     > Warning: Due to the precarious circumstances under which __del__()
     > methods are invoked, exceptions that occur during their execution
     > are ignored, and a warning is printed to sys.stderr instead.
   
   Instead, always close resource handlers when they are no longer in use.
   This will close the file handler at a specified point in the user's code
   and not wait until the interpreter chooses to. It is always guaranteed
   to run. And, if an exception occurs while closing the file handler, the
   bug will not be ignored.
   
   Now, when code receives a ResourceWarning, it will highlight an area
   that is mishandling resources. It should not simply be silenced, but
   fixed by closing resources with a context manager.
   
   All warnings that were emitted during tests have been cleaned up. To
   enable warnings, I passed the `-Wa` CLI option to Python. This exposed
   some mishandling of resources in ImageFile.__init__() and
   SpiderImagePlugin.loadImageSeries(), they too were fixed.

   

   jdufresne committed Oct 12, 2019
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