NaN can be stored in DecimalField but cannot be retrieved

[Xabier Bello]

Same as ticket https://code.djangoproject.com/ticket/33033, but I managed to trigger it anyway:

Steps to reproduce

• Create a brand new project using python 3.10 and django 4.1 with the default sqlite3 backend.

• Create a model with a DecimalField:

      class MyModel(models.Model):
          value = models.DecimalField(max_digits=10, decimal_places=5)
  

• Programmatically create a model instance with value="nan",

      obj = MyModel.objects.create(value="nan")
      obj.save()
  

• Then try to retrieve the object from the database (or refresh from database):

      MyModel.objects.get(pk=1)
  

Traceback

    Traceback (most recent call last):
      File "/sandbox/dj/bug/dec/views.py", line 9, in <module>
        MyModel.objects.get(pk=1)
      File "/lib64/python3.10/site-packages/django/db/models/manager.py", line 85, in manager_method
        return getattr(self.get_queryset(), name)(*args, **kwargs)
      File "/lib64/python3.10/site-packages/django/db/models/query.py", line 646, in get
        num = len(clone)
      File "/lib64/python3.10/site-packages/django/db/models/query.py", line 376, in __len__
        self._fetch_all()
      File "/lib64/python3.10/site-packages/django/db/models/query.py", line 1866, in _fetch_all
        self._result_cache = list(self._iterable_class(self))
      File "/lib64/python3.10/site-packages/django/db/models/query.py", line 117, in __iter__
        for row in compiler.results_iter(results):
      File "/lib64/python3.10/site-packages/django/db/models/sql/compiler.py", line 1333, in apply_converters
        value = converter(value, expression, connection)
      File "/lib64/python3.10/site-packages/django/db/backends/sqlite3/operations.py", line 344, in converter
        return create_decimal(value).quantize(
    TypeError: argument must be int or float

The value "nan" (and maybe "inf" also) skip the validation in DecimalField.to_python, because is not None, and is not instance of float. But decimal.Decimal("nan") works without triggering the exception, so NaN gets stored in the DB.