Changing a dict in a List(Dict) trait raises a TraitError

[enthought]

Changing a dict in a List(Dict) trait raises a TraitError

[Bouke]

I ran into the same issue today, did you manage to work around this issue?

[corranwebster]

Related to this (not raising an error, but surprising):

from traits.api import HasTraits, Dict, Str, List, Int

class TestList(HasTraits):
    x = List(List(Int))

class TestDict(HasTraits):
    x = Dict(Str, List)

class TestDict2(HasTraits):
    x = Dict(Str, Dict(Str, Int))

def listener(obj, name, old, new):
    print obj, name, old, new.added

t1 = TestList(x=[[1, 2, 3]])
t1.on_trait_change(listener, name='x_items')
t1.x[0].append(4)  # listener fires with data from sub-list as if it were main list

t2 = TestDict(x={'a': [1, 2, 3]})
t2.on_trait_change(listener, name='x_items')
t2.x['a'].append(4)  # exception, as noted above

t3 = TestDict2(x={'a': {'b': 1}})
t3.on_trait_change(listener, name='x_items')
t3.x['a']['c'] = 2  # listener fires with data from sub-dict as if it were main dict

The root issue is with recursive calls to List.validate or Dict.validate when building the TraitListObject or TraitDictObject instances. This is non-trivial to fix: it's not even clear what the "right" behaviour should be.

[jonathanrocher]

I ran into the same issue (in the Dict(Str, List) case). To work around it, @Bouke , I simplified my declaration to Dict(). @rkern also suggested offline to use Dict(Str, Instance(list)) if you need the typechecking.

[mdickinson]

This is essentially the same issue as #281. We should close one or the other.

[mdickinson]

Closed #281 as a duplicate of this issue; there's some useful discussion in comments on that issue.