Unexpected failure mode for dispatch="ui" when there is no UI

[corranwebster]

When using dispatch="ui" and there is no UI (in particular, Pyface has not done toolkit selection), the dispatcher fails with an attempt to call None as the handler:

>>> from traits.api import HasTraits, Int, observe
>>> class Test(HasTraits):
...    x = Int()
... 
>>> t = Test()
>>> def handler(event):
...     print(event)
... 
>>> t.observe(handler, 'x', dispatch='ui')
>>> t.x = 1 
Exception occurred in traits notification handler for event object: TraitChangeEvent(object=<__main__.Test object at 0x10de8a270>, name='x', old=0, new=1)
Traceback (most recent call last):
  File "/Users/cwebster/.edm/envs/enclosure-38/lib/python3.8/site-packages/traits/observation/_trait_event_notifier.py", line 122, in __call__
    self.dispatcher(handler, event)
  File "/Users/cwebster/.edm/envs/enclosure-38/lib/python3.8/site-packages/traits/trait_notifiers.py", line 50, in ui_dispatch
    ui_handler(handler, *args, **kw)
TypeError: 'NoneType' object is not callable

This also happens when the Pyface toolkit is null (which may be a Pyface bug):

>>> from traits.etsconfig.api import ETSConfig
>>> ETSConfig.toolkit = 'null'
>>> import pyface.api
>>> t.x = 2     
Exception occurred in traits notification handler for event object: TraitChangeEvent(object=<__main__.Test object at 0x10de8a270>, name='x', old=1, new=2)
Traceback (most recent call last):
  File "/Users/cwebster/.edm/envs/enclosure-38/lib/python3.8/site-packages/traits/observation/_trait_event_notifier.py", line 122, in __call__
    self.dispatcher(handler, event)
  File "/Users/cwebster/.edm/envs/enclosure-38/lib/python3.8/site-packages/traits/trait_notifiers.py", line 50, in ui_dispatch
    ui_handler(handler, *args, **kw)
TypeError: 'NoneType' object is not callable

I'm not entirely sure what the correct behaviour is here.

I think I would expect dispatch to work if the current thread is the main thread, given the short-circuit in the code here:

traits/traits/trait_notifiers.py

Lines 46 to 50 in 92ecd7e

	def ui_dispatch(handler, *args, **kw):
	if threading.current_thread().ident == ui_thread:
	handler(*args, **kw)
	else:
	ui_handler(handler, *args, **kw)

But given there is no way to dispatch something back to the main thread if there is no event loop around, I'd expect at least a better failure message if the dispatch is not on the main thread.

[corranwebster]

Presumably there is a similar issue with on_trait_change-based dispatch:

traits/traits/trait_notifiers.py

Lines 617 to 621 in 92ecd7e

	def dispatch(self, handler, *args):
	if threading.current_thread().ident == ui_thread:
	handler(*args)
	else:
	ui_handler(handler, *args)

[mdickinson]

Drive-by note: threading.current_thread() == threading.main_thread() would be a better test here.

Agreed that there should be a clear failure message if we're not on the main thread and there's no ui. And yes, I suppose we should just ignore dispatch="ui" and execute the handler directly if we're on the main thread. I'm not sure that that's what I'd choose if we were implementing this for the first time, but we're not. (Having an observer that sometimes executes synchronously and sometimes asynchronously sounds a bit like a recipe for confusion.)

[homosapien-lcy]

Hi, I think the second exception may have already been solved by the latest pyface:

(py311) (base) cyliu@aus552cyliu traits % python3.11
Python 3.11.2 (main, Feb 16 2023, 03:15:23) [Clang 14.0.0 (clang-1400.0.29.202)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from traits.api import HasTraits, Int, observe
>>> class Test(HasTraits):
...     x = Int()
... 
>>> t = Test()
>>> from traits.etsconfig.api import ETSConfig
>>> ETSConfig.toolkit = 'null'
>>> import pyface.api
>>> t.x = 2     
>>> 
>>> 

But the first error remains:

Python 3.11.2 (main, Feb 16 2023, 03:15:23) [Clang 14.0.0 (clang-1400.0.29.202)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from traits.api import HasTraits, Int, observe
>>> class Test(HasTraits):
...     x = Int()
... 
>>> t = Test()
>>> from traits.etsconfig.api import ETSConfig
>>> ETSConfig.toolkit = 'null'
>>> import pyface.api
>>> t.x = 2     
>>> 
>>> def handler(event):
...     print(event)
... 
>>> t.observe(handler, 'x', dispatch='ui')
>>> t.x = 1 
Exception occurred in traits notification handler for event object: TraitChangeEvent(object=<__main__.Test object at 0x1015a8590>, name='x', old=2, new=1)
Traceback (most recent call last):
  File "/Users/cyliu/Documents/3.11_test/traits/traits/observation/_trait_event_notifier.py", line 122, in __call__
    self.dispatcher(handler, event)
  File "/Users/cyliu/Documents/3.11_test/traits/traits/trait_notifiers.py", line 50, in ui_dispatch
    ui_handler(handler, *args, **kw)
TypeError: 'NoneType' object is not callable

Environment: Python3.11, MacOS
Packages:
Package Version Editable project location

---

attrdict 2.0.1
attrdict3 2.0.2
chaco 5.1.0 /Users/cyliu/Documents/3.11_test/chaco
configobj 5.0.8
enable 5.4.0.dev28 /Users/cyliu/Documents/3.11_test/enable
fonttools 4.39.2
joblib 1.2.0
numpy 1.24.2
pandas 1.5.3
Pillow 9.4.0
pip 22.3.1
PyAudio 0.2.13
pyface 8.0.0
pygarrayimage 1.0
pyglet 2.0.5 /Users/cyliu/Documents/3.11_test/pyglet
Pygments 2.14.0
pyparsing 3.0.9
PySide6 6.4.3
PySide6-Addons 6.4.3
PySide6-Essentials 6.4.3
python-dateutil 2.8.2
pytz 2023.2
reportlab 3.6.12
scikit-learn 1.2.2
scipy 1.10.1
setuptools 65.6.3
shiboken6 6.4.3
six 1.16.0
threadpoolctl 3.1.0
traits 7.0.0.dev1840 /Users/cyliu/Documents/3.11_test/traits
traitsui 7.4.3
wxPython 4.2.0

[notice] A new release of pip available: 22.3.1 -> 23.1
[notice] To update, run: pip install --upgrade pip

[homosapien-lcy]

After some testing, I found the first error from Corran will resolve if the main thread check in the ui_dispatch is changed to the following:

def ui_dispatch(handler, *args, **kw):
    if threading.current_thread().ident == threading.main_thread().ident:
        handler(*args, **kw)
    else:
        ui_handler(handler, *args, **kw)

Then the code will run successfully:

Python 3.11.2 (main, Feb 16 2023, 03:15:23) [Clang 14.0.0 (clang-1400.0.29.202)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from traits.api import HasTraits, Int, observe
^[[A
>>> 
>>> 
>>> class Test(HasTraits):
...     x = Int()
... 
>>> t = Test()
>>> def handler(event):
...     print(event)
... 
>>> t.observe(handler, 'x', dispatch='ui')
>>> t.x = 1 
TraitChangeEvent(object=<__main__.Test object at 0x104ec0720>, name='x', old=0, new=1)

So is the dispatch supposed to run when there is no ui but on the main thread?

[mdickinson]

So is the dispatch supposed to run when there is no ui but on the main thread?

Yes, I think that's probably the right behaviour here.

[homosapien-lcy]

So is the dispatch supposed to run when there is no ui but on the main thread?

Yes, I think that's probably the right behaviour here.

OK, I will create a PR for this change then