Traits List not reporting items added or removed

问题

Given,

 from enthought.traits.api import HasTraits, Tuple, Delegate, Trait, Float,Dict,List

 class Foo(HasTraits):
     def __init__(self):
         super(Foo,self).__init__()
         self.add_trait('node',List)
     def _node_items_changed(self,name,old,new):    
         print name
         print old
         print new

Why do I get:

>>> f = Foo()
>>> f.node.append(0)
node_items
<undefined>
<traits.trait_handlers.TraitListEvent object at 0x05BA8CF0>

The documentation says I should get a list of items added/removed.

What am I missing here? This is traits 4.3 on windows 8.

Thanks!

回答1:

There seems to be a distinction between changing the value of the collection as a whole (the List) and changing a member within the collection. Appending appears to change a member within (or at least results in the same notification). If you change the value of the container as a whole, you do indeed get the changed list as the new value:

from enthought.traits.api import HasTraits, Tuple, Delegate, Trait, Float,Dict,List

class Foo(HasTraits):
    def __init__(self):
        super(Foo,self).__init__()
        self.add_trait('node',List)
    def _node_changed(self,name,old,new):
        print("_node_changed: %s %s %s" % (name, str(old), str(new)))
    def _node_items_changed(self,name,old,new):
        print("_node_items_changed: %s %s %s" % (name, str(old), str(new)))

f = Foo()

# change the List membership with append:
f.node.append(0)
# _node_items_changed: node_items <undefined> <traits.trait_handlers.TraitListEvent object at 0x10128af50>

# change the List itself:
f.node = [1,2,3]
# _node_changed: node [0] [1, 2, 3]

# change a member (same result as append):
f.node[1] = 4
# _node_items_changed: node_items <undefined> <traits.trait_handlers.TraitListEvent object at 0x10128af50>

There's more info here, if you haven't seen this section already. See this answer too.

来源：https://stackoverflow.com/questions/19041426/traits-list-not-reporting-items-added-or-removed

标签

python

event-handling

traits

enthought