Allow type checking on elements of List,Tuple,Set traits

[minrk]

This commit allows checking types of elements in a Container

Ref: #400 (comment)

So you can do:

class C(HasTraits):
    a = List([Int, Str])
    b = List(Int)
    c = List([0,'a'], types=[Int, Str])

• a enforces length 2 with Int and Str on elements 0 and 1. Default value is None.
• b enforces that all elements are Ints, but does not enforce length
• c is the same as a, but with a default value of [0,'a']

The check is only made on assignment, (so index-assignment, or calls to append/extend are not checked), but should cover enough cases to be useful.

Obviously only style b makes sense for Sets since they are unordered.

It also fixes an incorrect error message if validation fails in a ClassTypeTrait

[rkern]

Please follow Traits semantics or use different names. In Traits, List() only takes one trait for type-checking.

http://github.enthought.com/traits/traits_user_manual/defining.html#other-predefined-traits

[minrk]

Okay, now they look more similar to Traits, thanks. List/Set take single trait as first arg, and Tuple takes *traits e.g.:

a = List(Int, range(5))
b = Set(CStr, range(5))
c = Tuple(Int,Str,Unicode)

For traitlets consistency, however, I do allow them to be used as they have been - passing a single argument as the default value:

a = List(['abc',1,2,3])
b = Set([1,'a'])
c = Tuple((1,2,'a'))

In which case, no type checking will be performed, as no trait has been specified.

And they all keep the default_value kwarg that all traitlets have:

a = List(Int, default_value=range(5))
b = Tuple(Int,Int, default_value=(1,2))

[ellisonbg]

Min, this look great. I think it is a good idea to follow the traits api as closely as possible, because eventually we will likely be using it.

[fperez]

Yes, we should definitely try to keep api compatibility with the real traits everywhere. I don't lose hope we'll be able to reunite the two completely in the future, and api differences will just make it trickier.

[minrk]

minlen/maxlen being added to List, I think List,Tuple,Set should match the Traits api.