Add a TypedReadOnly trait

[itziakos]

This PR adds a Typed version of the ReadOnly trait.

[itziakos]

sorry @corranwebster and @mdickinson I thought that I had the PR ready but it looks I am still missing something

[itziakos]

@mdickinson and @corranwebster this pr is ready for review.

[codecov-io]

Codecov Report

Merging #488 into master will increase coverage by 0.33%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #488      +/-   ##
==========================================
+ Coverage   65.15%   65.48%   +0.33%     
==========================================
  Files          44       44              
  Lines        7040     7061      +21     
  Branches     1413     1416       +3     
==========================================
+ Hits         4587     4624      +37     
+ Misses       2030     2014      -16     
  Partials      423      423

Impacted Files	Coverage Δ
traits/api.py	86.2% <ø> (ø)	⬆️
traits/trait_types.py	63.98% <100%> (+0.58%)	⬆️
traits/has_traits.py	66.32% <0%> (+0.07%)	⬆️
traits/trait_handlers.py	63.4% <0%> (+0.33%)	⬆️
traits/etsconfig/etsconfig.py	63.58% <0%> (+6.17%)	⬆️

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[mdickinson]

@itziakos Thank you! I'll look at this within the next couple of days.

[mdickinson]

@itziakos Thank you! I'll look at this within the next couple of days.

For a fairly geological interpretation of the word "days", obviously ... Sorry.

A couple of comments, below.

This trait doesn't behave quite the way I expect: I'd expect to get the Undefined object if I access before setting, but instead I get the default value. Perhaps there's no reasonable way to get the expected behaviour here.

>>> class A(HasTraits):
...     foo = TypedReadOnly(Int)
...     bar = ReadOnly()
... 
>>> a = A()
>>> a.foo  # expect undefined, or an error
0
>>> a.foo = 3
>>> a.foo
3
>>> a.foo = 5
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 1314, in set
    raise TraitError(message.format(name, object.__class__.__name__))
traits.trait_errors.TraitError: Cannot set 'foo' of 'A' more than once.
>>> a.bar 
<undefined>
>>> a.bar = 3
>>> a.bar
3
>>> a.bar = 5
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
traits.trait_errors.TraitError: Cannot modify the read only 'bar' attribute of a 'A' object.

[mdickinson]

Let's lose the "Author" and "Date" fields; we can track that information through version control.

[mdickinson]

Should inner_traits be a method (taking no additional arguments) rather than an attribute? I don't get the behaviour I expect on accessing inner_traits on the corresponding CTrait object:

>>> class A(HasTraits):
...     foo = TypedReadOnly(Int)
...     bar = List(Int)
... 
>>> a = A()
>>> a.traits()["foo"].inner_traits  # gives None
>>> a.traits()["bar"].inner_traits
(<traits.traits.CTrait object at 0x10f4d72c0>,)

[corranwebster]

I'm OK with the behaviour of returning a default before a value is set - I think that might be a more pragmatically useful behaviour - but perhaps the name of the trait is wrong: should it be SetOnce or something like that?