ReactiveHTML Param Watcher: Coroutine never awaited.

[emcd]

Probably a case not covered by #3262 or holoviz/param#913.

ALL software version info

$ hatch run python -V
Python 3.10.14

$ hatch run pip freeze | grep -E '^(bokeh|panel|param|tornado)'
bokeh==3.4.3
panel==1.4.5
param==2.1.1
tornado==6.4.1

Description of expected behavior and the observed behavior

Expected behavior (which is seen with synchronous event handler):

$ hatch run python bug-reports/panel-rehtml-async-bug.py
...
Event(what='value', name='clicked', obj=Div(clicked=True), cls=Div(clicked=True), old=False, new=True, type='changed')

Observed behavior (which is seen with coroutine event handler):

$ hatch run python bug-reports/panel-rehtml-async-bug.py
...
/home/me/.local/share/hatch/env/virtual/aiwb/WoaaKFKL/aiwb/lib/python3.10/site-packages/param/parameterized.py:2506: RuntimeWarning: coroutine 'on_div_clicked' was never awaited
  watcher.fn(*args, **kwargs)

Complete, minimal, self-contained example code that reproduces the issue

import param

from panel.reactive import ReactiveHTML


class Div( ReactiveHTML ):

    clicked = param.Event( default = False )

    _template = '''<div id="Div" onclick="${_div_click}">Testing...</div>'''

    def _div_click( self, event ):
        self.clicked = True


#def on_div_clicked( event ): # Works
async def on_div_clicked( event ): # Broken
    print( event )


div = Div( )
div.param.watch( lambda event: on_div_clicked( event ), 'clicked' )


div.show( )

[philippjfr]

This is a bug in your code, not Panel itself. Specifically you cannot call an asynchronous function in your lambda without awaiting it. It works just fine if you simply do:

div.param.watch(on_div_clicked, 'clicked' )

[emcd]

Oops... forgot to replace the lambda when converting from sync to async. Not obvious from the above reproducer, but I use lambdas in my actual code to drop the event argument, since I usually don't care about it, and to access a closure cell instead, which carries the information that I do care about. Sorry for the noise. I'll switch to using partial functions.