EventEmitter loop in napari dock widget

Describe the bug

RuntimeError: EventEmitter loop detected! occurs, although events should be blocked (as far as I understand).

ERROR:root:Unhandled exception:
Traceback (most recent call last):
  File "/home/uwe/research/stardist/napari/magicgui/magicgui/widgets/_bases/value_widget.py", line 44, in <lambda>
    lambda *x: self.changed(value=x[0] if x else None)
  File "/home/uwe/research/stardist/napari/magicgui/magicgui/events.py", line 576, in __call__
    raise RuntimeError("EventEmitter loop detected!")
RuntimeError: EventEmitter loop detected!

To Reproduce

Code for simple napari dock widget (bug is triggered when entering lowercase characters for param):

from napari_plugin_engine import napari_hook_implementation
from magicgui import magic_factory

def to_upper(event):
    val = str(event.value)
    if val != val.upper():
        with event.source.changed.blocker():
            event.source.value = val.upper()

def widget_init(widget):
    widget.param.changed.connect(to_upper)

@magic_factory(widget_init=widget_init)
def widget(param: str):
    pass

@napari_hook_implementation
def napari_experimental_provide_dock_widget():
    return widget

Expected behavior No RuntimeError: EventEmitter loop detected!.

Environment (please complete the following information):

• OS: Ubuntu 20.04 LTS
• magicgui version 0.2.7

[uschmidt83]

FYI, it also happens with @magicgui and a wrapper function:

def widget_wrapper():
    @magicgui
    def widget(param: str):
        pass
    ...
    return widget

@napari_hook_implementation
def napari_experimental_provide_dock_widget():
    return widget_wrapper

[tlambert03]

Strange, the bug happens via both Python and IPython in my plugin.

as long as I can reproduce somewhere, I can probably fix it in both If I remember correctly, it had something to do with the order of connecting events… (though that shouldn’t matter).