Members with min and or max values should not be allowed to be None

[claudiodsf]

Currently, a member specification like the following:

keyword = float(min=0, max=1, default=0.5)

will pass validation when setting keyword = None.

I have therefore to check this afterwards in my code.

I was wondering, whether this could be implemented within ConfigObj.

More generally, there could be an option for allowing/disallowing None values, e.g.,

keyword = float(allow_none=False)