Deprecate non-essential TraitHandler subclasses for 6.0 release?

[mdickinson]

Here's the complete list of TraitHandler subclasses in current master.

>>> from traits.api import TraitHandler
>>> for cls in TraitHandler.__subclasses__(): print(cls.__name__)
... 
TraitCoerceType
ThisClass
TraitClass
TraitFunction
TraitEnum
TraitPrefixList
TraitMap
TraitExpression
TraitCompound
TraitTuple
TraitCallable
TraitList
TraitDict

Some of these are still central to Traits itself, like TraitCoerceType and TraitCompound. Others, like TraitList and TraitDict aren't needed by either Traits or by new code, and are largely unused in projects - the major uses are in other ETS libraries (apptools, pyface, mayavi).

I'm considering deprecating the less essential classes in Traits 6.0.0. A deprecation warning could be issued at instantiation time.

---

Update: TraitCallable is gone in #698. TraitExpression is removed in #699. TraitClass is removed in #710.

[mdickinson]

And for some of these classes, we may be able to simply remove them. TraitCallable appears to fit into this category.

[mdickinson]

TraitExpression can also go. TraitThis is used only in ListThis (which is itself deprecated), and nowhere else in ETS; I think both TraitThis and ListThis could safely be removed.

Everything else is either used by the general Trait machinery, or is used elsewhere in ETS.

[mdickinson]

TraitThis

Sorry; I meant ThisClass. And unfortunately, TraitInstance subclasses ThisClass (somewhat randomly), and TraitInstance is used fundamentally by the Traits machinery.

[mdickinson]

TraitClass is removed in #711

[corranwebster]

ListThis was modified so that it doesn't use ThisClass in #730. It should be fairly easy to make TraitInstance inherit directly from TraitHandler by copying any code that is used from ThisClass and then ThisClass can be deprecated.

[mdickinson]

After a slew of PRs, the current status, and likely the Traits 6.0 status is:

Subclasses still present:

TraitCoerceType
TraitInstance
TraitFunction
TraitEnum
TraitPrefixList
TraitMap
TraitPrefixMap (subclass of TraitMap)
TraitCompound

Subclasses deprecated:

TraitTuple
TraitList
TraitDict

Subclasses removed in the Traits 6.0 release:

TraitRange
TraitString
ThisClass
TraitClass
TraitExpression
TraitCallable