Add DefaultValue.disallow

[mdickinson]

This PR adds a new value to the DefaultValue enumeration: DefaultValue.disallow. The intent is to use this for trait types that don't have a valid default. Right now that's Event and Disallow, but it could potentially be extended to other trait types in the future. The main feature of DefaultValue.disallow is that an attempt to get the default value of the corresponding CTrait instance using default_value_for will fail, raising ValueError.

Rationale: With the current situation, the Event trait has no way to communicate that its default shouldn't be used: the default_value_for method succeeds, and indicates that the default value is a constant, and that that constant value Undefined. That means that a trait like Tuple(Event(), Event()) has a default value of (Undefined, Undefined). We could potentially update the logic in traits to check for Undefined everywhere and interpret that as "no default", but it seems cleaner to introduce a new default value type for this.

This PR doesn't fix #1541, but it should enable an easy fix for that issue.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• [ ] Update User manual (docs/source/traits_user_manual) N/A
• Update type annotation hints in traits-stubs

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Update: we renamed from DefaultValue.unsupported to DefaultValue.disallow; I've edited the description and title accordingly.

[mdickinson]

The default_kind property is essentially unused; I'm including this only for completeness.

[mdickinson]

There's also potential to use this in the future for Instance (or other) traits where we want to enforce that the default of None is never used in practice.

[mdickinson]

I'm not wedded to the "unsupported" name here; alternative naming suggestions welcome. I was also considering "disallow", to match the Disallow trait type.

[mdickinson]

Related: the CantHaveDefaultValue constant:

traits/traits/has_traits.py

Lines 110 to 111 in eb62eb6

	# Trait types which cannot have default values
	CantHaveDefaultValue = ("event", "delegate", "constant")

[rahulporuri]

IIRC from the internal discussion, we decided to rename this to disallowed instead of unsupported.

[mdickinson]

IIRC from the internal discussion, we decided to rename this to disallowed instead of unsupported.

Yes, I'll rename (but I think to "disallow" rather than "disallowed").

[mdickinson]

Updated for the suggested renaming; I've also edited the PR title and description to match to make it easier to find in the future.