[Zope3-dev] zope.schema Field... constraints: should they really constrain each other?

[Shaun Cutts]

I am a new user of the component system (currently using it with
Twisted, not using zope itself). I'm not sure I understand all the
philosophy behind its design. Nevertheless, I believe the following
behavior is not the best:
 
>>> a = Int( missing_value = -1 )
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "c:\python24\lib\site-packages\zope\schema\_bootstrapfields.py",
line 351, in __init__
    super(Int, self).__init__(*args, **kw)
  File "c:\python24\lib\site-packages\zope\schema\_bootstrapfields.py",
line 231, in __init__
    self.min = min
  File "c:\python24\lib\site-packages\zope\schema\_bootstrapfields.py",
line 42, in __set__
    inst.validate(value)
  File "c:\python24\lib\site-packages\zope\schema\_bootstrapfields.py",
line 138, in validate
    self._validate(value)
  File "c:\python24\lib\site-packages\zope\schema\_bootstrapfields.py",
line 240, in _validate
    super(Orderable, self)._validate(value)
  File "c:\python24\lib\site-packages\zope\schema\_bootstrapfields.py",
line 165, in _validate
    raise WrongType(value, self._type)
zope.schema._bootstrapinterfaces.WrongType: (None, (<type 'int'>, <type
'long'>))
 
 
What is happening is that "min" (and max) are ValidatedProperties of
Orderable. When a "missing_value" is supplied, min = None (by default)
is checked and fails validation.
 
One solution is to change "None" into missing_value in the Orderable
__init__. More exhaustively, min, max, and default should have defaults
as "__missing_value_marker" in the Orderable __init__ (using the trick
in Field.__init__), which would then be converted to the actual
missing_value, if specified, or None, if not.
 
This gives me the following __init__ in
zope.schema_bootstrapfields.Orderable:
 
    __missing_value_marker = object()
 
    def __init__(self, min=__missing_value_marker,
max=__missing_value_marker,
                 default=__missing_value_marker, **kw):
 
        # Set min and max to None so that we can validate if
        # one of the super methods invoke validation.
        self.min = None
        self.max = None
 
        super(Orderable, self).__init__(**kw)
 
        # make sure that unspecified min, max, default get correct
missing value
        try: missing = self.missing_value
        except AttributeError: missing = None
            
        if min is self.__missing_value_marker: min = missing
        if max is self.__missing_value_marker: max = missing
        if default is self.__missing_value_marker: default = missing
 
        # Now really set min and max
        self.min = min
        self.max = max
 
        # We've taken over setting default so it can be limited by min
        # and max.
        self.default = default
 
 
(But I don't know if constraints should (always?) apply to each other.
For instance, if "min" were actually defined as the largest value
smaller than the range, it wouldn't validate against itself.)
 
- Shaun
 
 
 
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