Traits incorrectly added by depends_on with Property

[serwy]

Here's code that shows the behavior:

import traits.api as tr
class X(tr.HasStrictTraits):
    pass
class Y(tr.HasStrictTraits):
    x = tr.Instance(X)
    z = tr.Property(tr.Int, depends_on="x.jjj")
x = X()
print('jjj' in dir(x))  # False
y = Y(x=x)
print('jjj' in dir(x))  # True

print(x.trait_names())  # ['trait_added', 'trait_modified', 'jjj']

[mdickinson]

Confirmed on current Traits master.

[mdickinson]

Note: this doesn't happen on Traits 4.6.0, but it's still true that jjj gets added to x's trait_names. It's the dir change in recent Traits that exposed this bug.

[rkern]

Ultimately, this happens because _on_trait_change() will force the creation of a trait here: https://github.com/enthought/traits/blob/master/traits/has_traits.py#L2525

(see here for the interpretation of that 2: https://github.com/enthought/traits/blob/master/traits/ctraits.c#L1172)

The trait that is added is a Disallow trait. Likely __dir__() should omit Disallow traits from its list. There are some cases where they are explicitly part of the class definition, not just HasStrictTraits.

We probably could explicitly check for the Disallow trait in _on_trait_change(), clean up the created trait, then raise a TraitError so that this is discovered when the change handler is being added.

[FedeMiorelli]

This issue has the side effect that pickling is broken in case there are references to nonexisting traits in depends_on.

In the example below, if I have a typo in depends_on, I get a pickling error related to Disallow.

import pickle

from traits.api import HasStrictTraits, Str, Property


class MyTrait(HasStrictTraits):
    MyString = Str
    MyProperty = Property(depends_on='MyStringWITHTYPO') # 'MyString' would work
    
    def save(self):
        with open('test.dump', 'wb') as fid:
            pickle.dump(self.__getstate__(), fid, protocol=2)
        
    def load(self):
        with open('test.dump', 'rb') as fid:
            self.__setstate__(pickle.load(fid))
        
    
    def _get_MyProperty(self):
        return self.MyString + ' **** '
    
m1 = MyTrait(MyString='test')
m1.save()


m2 = MyTrait()
m2.load()

print(m2.MyString)
print(m2.MyProperty)

Error:

  File "testpickle.py", line 19, in save
    pickle.dump(self.__getstate__(), fid, protocol=2)
_pickle.PicklingError: Can't pickle <class 'traits.trait_types.Disallow'>: it's not the same object as traits.trait_types.Disallow