Calling `hasattr`/`getattr` creates trait if missing.

[felixkol]

Calling `hasattr`/`getattr` creates trait if missing.

[mdickinson]

Thanks for the report. It's not exactly intended behaviour (it's clearly not desirable), but it is behaviour that's fairly deeply baked in, and hard to change without breaking existing code (and unfortunately there's a lot of existing Traits-using code out there). Existing issues #358 and #58 are related.

Slightly simpler reproducer:

>>> class A(HasStrictTraits):
...     bar = Int()
... 
>>> a = A()
>>> "foo" in a.__class_traits__
False
>>> "foo" in a.__class_traits__
False
>>> a.foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'A' object has no attribute 'foo'
>>> "foo" in a.__class_traits__
True

[felixkol]

Wouldn't it be possible to split getting and setting prefix traits?

On getting, a value would be returned, when it has been set or the prefix trait has a default value. On the latter case, the prefixed trait could be created.
Setting would work just as it is currently.

Then no trait would exist where getting raises an exception.

Would this break the current functionality of prefix traits?