missing_value doesn't work for Int fields

[tseaver]

In https://bugs.launchpad.net/zope.schema/+bug/98479, @webmaven reported:

The following creates an error:

page_ordinal = zope.schema.Int(
    title = _(u'Ordinal'),
    description=_(u'Order of the pages'),
    default=0,
    missing_value=0, #don't know why this doesn't work
    )

The error is as follows:

ConfigurationError: ('Invalid value for', 'class', "(None, (<type 'int'>, <type 'long'>))")

@mgedmin followed-up:

I've looked briefly at this error. The error is raised because the defaults for min_value and max_value are both None. This fails type checking, because None is not an int or a long. When missing_value is None, type checking is skipped.

I have to admit that I do not understand the semantics of missing_value.

I think that Int should allow min_value and max_value to be None regardless of the missing_value setting. Nones should be treated as minus infinity and plus infinity respectively.

@webmaven replied:

As I understand it, the value of missing_value is supposed to be used when the submitted form-field is empty. As opposed to default_value, which is supposed to pre-populate the form field.

@benji-york opined:

That's how it seems to me too. I looked briefly at the widget tests and couldn't figure out how to test it, so will have to look later (unless someone beats me to it, hint, hint).

[webmaven]

@tseaver, does this old bug need a new test case or anything to be considered confirmed?

[tseaver]

@webmaven the ideal would be a PR with a test case (based on your example) and a fix.