Not working with ipywidgets_bokeh and Panel

[MarcSkovMadsen]

CROSS POST: https://github.com/bokeh/ipywidgets_bokeh/issues/44 CROSS POST: https://github.com/holoviz/panel/issues/3023

Panel: 0.12.6 Bokeh: 2.4.2 ipywidgets_bokeh: 1.2.1 ipycanvas: 0.10.2 ipykernel: 6.6.0

Raises expected a string or ArrayBuffer, got object.

# pip install ipycanvas panel ipywidgets_bokeh
import panel as pn
from ipycanvas import Canvas

pn.extension('ipywidgets')

canvas = Canvas(width=200, height=200)

# Cubic curves example
def draw():
    canvas.begin_path()
    canvas.move_to(75, 40)
    canvas.bezier_curve_to(75, 37, 70, 25, 50, 25)
    canvas.bezier_curve_to(20, 25, 20, 62.5, 20, 62.5)
    canvas.bezier_curve_to(20, 80, 40, 102, 75, 120)
    canvas.bezier_curve_to(110, 102, 130, 80, 130, 62.5)
    canvas.bezier_curve_to(130, 62.5, 130, 25, 100, 25)
    canvas.bezier_curve_to(85, 25, 75, 37, 75, 40)
    canvas.fill()
    print("drawn")

pn.state.onload(draw)

pn.panel(canvas, height=200, width=200).servable()

[MarcSkovMadsen]

Additional Context

Panel should work with ipywidgets via ipywidgets_bokeh. See https://panel.holoviz.org/reference/panes/IPyWidget.html#

[martinRenou]

It might be an issue with Bokeh itself. I see the error raises in ipywidgets_bokeh.js, which is not ipycanvas's JavaScript code nor ipywidgets's one.

[MarcSkovMadsen]

We had a similar issue with ipyleaflet. We investigated panel, ipywidgets_bokeh and ipyleaflet. It turned out it was an error in ipyleaflet https://github.com/jupyter-widgets/ipyleaflet/pull/873. So it can be anywhere and I've crossposted with Panel and ipywidgets_bokeh. You where actually involved in solving that one. 😄 Thanks.

[martinRenou]

Ok. Does the error show up also when not trying to draw anything? Are you able to show an empty canvas?

[MarcSkovMadsen]

Yes

# pip install ipycanvas panel ipywidgets_bokeh
import panel as pn
from ipycanvas import Canvas

pn.extension('ipywidgets')

canvas = Canvas(width=200, height=200)

pn.panel(canvas, height=200, width=200).servable()

panel serve script.py --autoreload

[MarcSkovMadsen]

With Ipyleaflet the below had to be added.

Could that be the problem here as well?

[martinRenou]

I don't think so. If the Canvas shows up, then the canvas widget model is less likely to be the problem.

I think it's an issue with binary buffers in custom comm messages in bokeh.

Can you click here? So we can see in which part of the code it fails?

[MarcSkovMadsen]

[martinRenou]

Would you be able to open the debugger at this point and show what e contains?

[MarcSkovMadsen]

This is an example. I don't believe this is the one failing.

I will try to debug and share.

[martinRenou]

Maybe a conditional breakpoint would help, with the condition:

!(e instanceof ArrayBuffer)

[MarcSkovMadsen]

It seems its when e is null

[martinRenou]

I don't know what this ipywidgets_bokeh message is. Is it some custom messaging logic that Bokeh uses for wrapping all Jupyter messages?

[martinRenou]

It might be worth looking what happens Python side with this ipywidges_bokeh, there might be a reason why the message is null (or None Python side?)

[MarcSkovMadsen]

Probably its some custom messaging logic. But I am not familiar with ipywidgets_bokeh either.

But the source js line is here I believe https://github.com/bokeh/ipywidgets_bokeh/blob/379308211dd81e67eeb792dab2c58d5e98598cc2/ipywidgets_bokeh/src/widget.ts#L73

[MarcSkovMadsen]

I've changed https://github.com/bokeh/ipywidgets_bokeh/blob/379308211dd81e67eeb792dab2c58d5e98598cc2/ipywidgets_bokeh/kernel.py#L44

to

class SessionWebsocket(session.Session):

    def send(self, stream, msg_type, content=None, parent=None, ident=None, buffers=None, track=False, header=None, metadata=None):
        print("\n---------SEND--------------")
        print("stream", stream)
        print("msg_type", msg_type)
        print("content", content)
        print("parent", parent)
        print("header", header)
        print("metadata", metadata)
        msg = self.msg(msg_type, content=content, parent=parent, header=header, metadata=metadata)
        msg['channel'] = stream.channel

        doc = self.document

        # Ensure document message handler is only added once
        try:
            doc.remove_on_message("ipywidgets_bokeh", self.receive)
        except Exception:
            pass
        finally:
            doc.on_message("ipywidgets_bokeh", self.receive)

        packed = self.pack(msg)

        data: Union[bytes, str]
        if buffers is not None and len(buffers) != 0:
            buffers = [packed] + buffers
            nbufs = len(buffers)

            start = 4*(1 + nbufs)
            offsets = [start]

            for buffer in buffers[:-1]:
                start += len(buffer)
                offsets.append(start)

            u32 = lambda n: n.to_bytes(4, "big")
            items = [u32(nbufs)] + [ u32(offset) for offset in offsets ] + buffers
            data = b"".join(items)
        else:
            data = packed.decode("utf-8")
        print("data", data)
        event = MessageSentEvent(doc, "ipywidgets_bokeh", data)
        self._trigger_change(event)

and it then shows

$ panel serve 'tests\script.py' --auto --show

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_open
content {'data': {'state': {'_model_module': '@jupyter-widgets/base', '_model_module_version': '1.2.0', '_model_name': 'LayoutModel', '_view_count': None, '_view_module': '@jupyter-widgets/base', '_view_module_version': '1.2.0', '_view_name': 'LayoutView', 'align_content': None, 'align_items': None, 'align_self': None, 'border': None, 'bottom': None, 'display': None, 'flex': None, 'flex_flow': None, 'grid_area': None, 'grid_auto_columns': None, 'grid_auto_flow': None, 'grid_auto_rows': None, 'grid_column': None, 'grid_gap': None, 'grid_row': None, 'grid_template_areas': None, 'grid_template_columns': None, 'grid_template_rows': None, 'height': None, 'justify_content': None, 'justify_items': None, 'left': None, 'margin': None, 'max_height': None, 'max_width': None, 'min_height': None, 'min_width': None, 'object_fit': None, 'object_position': None, 'order': None, 'overflow': None, 'overflow_x': None, 'overflow_y': None, 'padding': None, 'right': None, 'top': None, 'visibility': None, 'width': None}, 'buffer_paths': []}, 'comm_id': 
'1f5969f0fc1a48119290d0dc83806716', 'target_name': 'jupyter.widget', 'target_module': None}
parent {}
header None
metadata {'version': '2.0.0'}
data {"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_0", "msg_type": "comm_open", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.028109Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_0", "msg_type": "comm_open", "parent_header": {}, "content": {"data": {"state": {"_model_module": "@jupyter-widgets/base", "_model_module_version": "1.2.0", "_model_name": "LayoutModel", "_view_count": null, "_view_module": "@jupyter-widgets/base", "_view_module_version": "1.2.0", "_view_name": "LayoutView", "align_content": null, "align_items": null, "align_self": null, "border": null, "bottom": null, "display": null, "flex": null, "flex_flow": null, "grid_area": null, "grid_auto_columns": null, "grid_auto_flow": null, "grid_auto_rows": null, "grid_column": null, "grid_gap": null, "grid_row": null, "grid_template_areas": null, "grid_template_columns": null, "grid_template_rows": null, "height": null, "justify_content": null, "justify_items": null, "left": 
null, "margin": null, "max_height": null, "max_width": null, "min_height": null, "min_width": null, "object_fit": null, "object_position": null, "order": null, "overflow": null, "overflow_x": null, "overflow_y": null, "padding": null, "right": null, "top": null, "visibility": null, "width": null}, "buffer_paths": []}, "comm_id": "1f5969f0fc1a48119290d0dc83806716", "target_name": "jupyter.widget", "target_module": null}, "metadata": {"version": "2.0.0"}, "channel": "iopub"}

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_open
content {'data': {'state': {'_dom_classes': (), '_model_module': 'ipycanvas', '_model_module_version': '^0.10.2', '_model_name': 'CanvasModel', '_send_client_ready_event': True, '_view_count': None, '_view_module': 'ipycanvas', '_view_module_version': '^0.10.2', '_view_name': 'CanvasView', 'height': 200, 'image_data': None, 'layout': 'IPY_MODEL_1f5969f0fc1a48119290d0dc83806716', 'sync_image_data': False, 'width': 200}, 'buffer_paths': []}, 'comm_id': '6a92a12f2c574fd7a8fd485dc5bb7441', 'target_name': 'jupyter.widget', 'target_module': None}
parent {}
header None
metadata {'version': '2.0.0'}
data {"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_1", "msg_type": "comm_open", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.036153Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_1", "msg_type": "comm_open", "parent_header": {}, "content": {"data": {"state": {"_dom_classes": [], "_model_module": "ipycanvas", "_model_module_version": "^0.10.2", "_model_name": "CanvasModel", "_send_client_ready_event": true, "_view_count": null, "_view_module": "ipycanvas", "_view_module_version": "^0.10.2", "_view_name": "CanvasView", "height": 200, "image_data": null, "layout": "IPY_MODEL_1f5969f0fc1a48119290d0dc83806716", "sync_image_data": false, "width": 200}, "buffer_paths": []}, "comm_id": "6a92a12f2c574fd7a8fd485dc5bb7441", "target_name": "jupyter.widget", "target_module": null}, "metadata": {"version": "2.0.0"}, "channel": "iopub"}

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_msg
content {'data': {'method': 'custom', 'content': {'shape': (11,), 'dtype': 'uint8'}}, 'comm_id': '6a92a12f2c574fd7a8fd485dc5bb7441'}
parent {}
header None
metadata {}
data b'\x00\x00\x00\x02\x00\x00\x00\x0c\x00\x00\x01\xf8{"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_2", "msg_type": "comm_msg", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.042142Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_2", "msg_type": "comm_msg", "parent_header": {}, "content": {"data": {"method": "custom", "content": {"shape": [11], "dtype": "uint8"}}, "comm_id": "6a92a12f2c574fd7a8fd485dc5bb7441"}, "metadata": {}, "channel": "iopub"}[14, [], 0]'

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_msg
content {'data': {'method': 'custom', 'content': {'shape': (17,), 'dtype': 'uint8'}}, 'comm_id': '6a92a12f2c574fd7a8fd485dc5bb7441'}
parent {}
header None
metadata {}
data b'\x00\x00\x00\x02\x00\x00\x00\x0c\x00\x00\x01\xf8{"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_3", "msg_type": "comm_msg", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.046142Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_3", "msg_type": "comm_msg", "parent_header": {}, "content": {"data": {"method": "custom", "content": {"shape": [17], "dtype": "uint8"}}, "comm_id": "6a92a12f2c574fd7a8fd485dc5bb7441"}, "metadata": {}, "channel": "iopub"}[19, [75, 40], 0]'

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_msg
content {'data': {'method': 'custom', 'content': {'shape': (33,), 'dtype': 'uint8'}}, 'comm_id': '6a92a12f2c574fd7a8fd485dc5bb7441'}
parent {}
header None
metadata {}
data b'\x00\x00\x00\x02\x00\x00\x00\x0c\x00\x00\x01\xf8{"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_4", "msg_type": "comm_msg", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.050142Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_4", "msg_type": "comm_msg", "parent_header": {}, "content": {"data": {"method": "custom", "content": {"shape": [33], "dtype": "uint8"}}, "comm_id": "6a92a12f2c574fd7a8fd485dc5bb7441"}, "metadata": {}, "channel": "iopub"}[26, [75, 37, 70, 25, 50, 25], 0]'

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_msg
content {'data': {'method': 'custom', 'content': {'shape': (37,), 'dtype': 'uint8'}}, 'comm_id': '6a92a12f2c574fd7a8fd485dc5bb7441'}
parent {}
header None
metadata {}
data b'\x00\x00\x00\x02\x00\x00\x00\x0c\x00\x00\x01\xf8{"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_5", "msg_type": "comm_msg", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.053143Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_5", "msg_type": "comm_msg", "parent_header": {}, "content": {"data": {"method": "custom", "content": {"shape": [37], "dtype": "uint8"}}, "comm_id": "6a92a12f2c574fd7a8fd485dc5bb7441"}, "metadata": {}, "channel": "iopub"}[26, [20, 25, 20, 62.5, 20, 62.5], 0]'

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_msg
content {'data': {'method': 'custom', 'content': {'shape': (35,), 'dtype': 'uint8'}}, 'comm_id': '6a92a12f2c574fd7a8fd485dc5bb7441'}
parent {}
header None
metadata {}
data b'\x00\x00\x00\x02\x00\x00\x00\x0c\x00\x00\x01\xf8{"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_6", "msg_type": "comm_msg", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.055143Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_6", "msg_type": "comm_msg", "parent_header": {}, "content": {"data": {"method": "custom", "content": {"shape": [35], "dtype": "uint8"}}, "comm_id": "6a92a12f2c574fd7a8fd485dc5bb7441"}, "metadata": {}, "channel": "iopub"}[26, [20, 80, 40, 102, 75, 120], 0]'

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_msg
content {'data': {'method': 'custom', 'content': {'shape': (39,), 'dtype': 'uint8'}}, 'comm_id': '6a92a12f2c574fd7a8fd485dc5bb7441'}
parent {}
header None
metadata {}
data b'\x00\x00\x00\x02\x00\x00\x00\x0c\x00\x00\x01\xf8{"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_7", "msg_type": "comm_msg", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.060144Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_7", "msg_type": "comm_msg", "parent_header": {}, "content": {"data": {"method": "custom", "content": {"shape": [39], "dtype": "uint8"}}, "comm_id": "6a92a12f2c574fd7a8fd485dc5bb7441"}, "metadata": {}, "channel": "iopub"}[26, [110, 102, 130, 80, 130, 62.5], 0]'

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_msg
content {'data': {'method': 'custom', 'content': {'shape': (38,), 'dtype': 'uint8'}}, 'comm_id': '6a92a12f2c574fd7a8fd485dc5bb7441'}
parent {}
header None
metadata {}
data b'\x00\x00\x00\x02\x00\x00\x00\x0c\x00\x00\x01\xf8{"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_8", "msg_type": "comm_msg", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.064145Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_8", "msg_type": "comm_msg", "parent_header": {}, "content": {"data": {"method": "custom", "content": {"shape": [38], "dtype": "uint8"}}, "comm_id": "6a92a12f2c574fd7a8fd485dc5bb7441"}, "metadata": {}, "channel": "iopub"}[26, [130, 62.5, 130, 25, 100, 25], 0]'

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_msg
content {'data': {'method': 'custom', 'content': {'shape': (33,), 'dtype': 'uint8'}}, 'comm_id': '6a92a12f2c574fd7a8fd485dc5bb7441'}
parent {}
header None
metadata {}
data b'\x00\x00\x00\x02\x00\x00\x00\x0c\x00\x00\x01\xf8{"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_9", "msg_type": "comm_msg", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.068146Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_9", "msg_type": "comm_msg", "parent_header": {}, "content": {"data": {"method": "custom", "content": {"shape": [33], "dtype": "uint8"}}, "comm_id": "6a92a12f2c574fd7a8fd485dc5bb7441"}, "metadata": {}, "channel": "iopub"}[26, [85, 25, 75, 37, 75, 40], 0]'

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_msg
content {'data': {'method': 'custom', 'content': {'shape': (20,), 'dtype': 'uint8'}}, 'comm_id': '6a92a12f2c574fd7a8fd485dc5bb7441'}
parent {}
header None
metadata {}
data b'\x00\x00\x00\x02\x00\x00\x00\x0c\x00\x00\x01\xfa{"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_10", "msg_type": "comm_msg", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.073146Z", "version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_10", "msg_type": "comm_msg", "parent_header": {}, "content": {"data": {"method": "custom", "content": {"shape": [20], "dtype": "uint8"}}, "comm_id": "6a92a12f2c574fd7a8fd485dc5bb7441"}, "metadata": {}, "channel": "iopub"}[18, ["nonzero"], 0]'
drawn
2021-12-20 11:21:42,123 Starting Bokeh server version 2.4.2 (running on Tornado 6.1)
2021-12-20 11:21:42,125 User authentication hooks NOT provided (default user enabled)
2021-12-20 11:21:42,127 Bokeh app running at: http://localhost:5006/script
2021-12-20 11:21:42,127 Starting Bokeh server with process id: 13704

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_open
content {'data': {'state': {'_model_module': '@jupyter-widgets/base', '_model_module_version': '1.2.0', '_model_name': 'LayoutModel', '_view_count': None, '_view_module': '@jupyter-widgets/base', '_view_module_version': '1.2.0', '_view_name': 'LayoutView', 'align_content': None, 'align_items': None, 'align_self': None, 'border': None, 'bottom': None, 'display': None, 'flex': None, 'flex_flow': None, 'grid_area': None, 'grid_auto_columns': None, 'grid_auto_flow': None, 'grid_auto_rows': None, 'grid_column': None, 'grid_gap': None, 'grid_row': None, 'grid_template_areas': None, 'grid_template_columns': None, 'grid_template_rows': None, 'height': None, 'justify_content': None, 'justify_items': None, 'left': None, 'margin': None, 'max_height': None, 'max_width': None, 'min_height': None, 'min_width': None, 'object_fit': None, 'object_position': None, 'order': None, 'overflow': None, 'overflow_x': None, 'overflow_y': None, 'padding': None, 'right': None, 'top': None, 'visibility': None, 'width': None}, 'buffer_paths': []}, 'comm_id': 
'6cc9f28254ed44b8aa8a49ccff2202e1', 'target_name': 'jupyter.widget', 'target_module': None}
parent {}
header None
metadata {'version': '2.0.0'}
data {"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_11", "msg_type": "comm_open", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.395050Z", 
"version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_11", "msg_type": "comm_open", "parent_header": {}, "content": {"data": {"state": {"_model_module": "@jupyter-widgets/base", "_model_module_version": "1.2.0", "_model_name": "LayoutModel", "_view_count": null, "_view_module": "@jupyter-widgets/base", "_view_module_version": "1.2.0", "_view_name": "LayoutView", "align_content": null, "align_items": null, "align_self": null, "border": null, "bottom": null, "display": null, "flex": null, "flex_flow": null, "grid_area": null, "grid_auto_columns": null, "grid_auto_flow": null, "grid_auto_rows": null, "grid_column": null, "grid_gap": null, "grid_row": null, "grid_template_areas": null, "grid_template_columns": null, "grid_template_rows": null, "height": null, "justify_content": null, "justify_items": null, "left": null, "margin": null, "max_height": null, "max_width": null, "min_height": null, "min_width": null, "object_fit": null, "object_position": null, "order": null, "overflow": null, "overflow_x": null, "overflow_y": null, "padding": null, "right": null, "top": null, "visibility": null, "width": null}, "buffer_paths": []}, "comm_id": "6cc9f28254ed44b8aa8a49ccff2202e1", "target_name": "jupyter.widget", "target_module": null}, "metadata": {"version": "2.0.0"}, "channel": "iopub"}

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type comm_open
content {'data': {'state': {'_dom_classes': (), '_model_module': 'ipycanvas', '_model_module_version': '^0.10.2', '_model_name': 'CanvasModel', '_send_client_ready_event': True, '_view_count': None, '_view_module': 'ipycanvas', '_view_module_version': '^0.10.2', '_view_name': 'CanvasView', 'height': 200, 'image_data': None, 'layout': 'IPY_MODEL_6cc9f28254ed44b8aa8a49ccff2202e1', 'sync_image_data': False, 'width': 200}, 'buffer_paths': []}, 'comm_id': '4e7e038267584664b625c77f3b861e9d', 'target_name': 'jupyter.widget', 'target_module': None}
parent {}
header None
metadata {'version': '2.0.0'}
data {"header": {"msg_id": "27462c87-846cc21583fe92c2c8421932_13704_12", "msg_type": "comm_open", "username": "username", "session": "27462c87-846cc21583fe92c2c8421932", "date": "2021-12-20T10:21:42.399046Z", 
"version": "5.3"}, "msg_id": "27462c87-846cc21583fe92c2c8421932_13704_12", "msg_type": "comm_open", "parent_header": {}, "content": {"data": {"state": {"_dom_classes": [], "_model_module": "ipycanvas", "_model_module_version": "^0.10.2", "_model_name": "CanvasModel", "_send_client_ready_event": true, "_view_count": null, "_view_module": "ipycanvas", "_view_module_version": "^0.10.2", "_view_name": "CanvasView", "height": 200, "image_data": null, "layout": "IPY_MODEL_6cc9f28254ed44b8aa8a49ccff2202e1", "sync_image_data": false, "width": 200}, "buffer_paths": []}, "comm_id": "4e7e038267584664b625c77f3b861e9d", "target_name": "jupyter.widget", "target_module": null}, "metadata": {"version": "2.0.0"}, "channel": "iopub"}
2021-12-20 11:21:42,651 WebSocket connection opened
2021-12-20 11:21:42,652 ServerConnection created

---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type status
content {'execution_state': 'busy'}
parent {'header': {'date': datetime.datetime(2021, 12, 20, 10, 21, 42, 730000, tzinfo=tzutc()), 'msg_id': '844de131-2be6-4111-9602-dc851ae47494', 'msg_type': 'comm_msg', 'session': '2ab7dfd6-30ac-43e4-8102-ddbb2fb76c82', 'username': '', 'version': '5.2'}, 'msg_id': '844de131-2be6-4111-9602-dc851ae47494', 'msg_type': 'comm_msg', 'parent_header': {}, 'metadata': {}, 'content': {'comm_id': '4e7e038267584664b625c77f3b861e9d', 'data': {'method': 'custom', 'content': {'event': 'client_ready'}}}, 'buffers': []}
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---------SEND--------------
stream <ipywidgets_bokeh.kernel.WebsocketStream object at 0x0000025C9909FF40>
msg_type status
content {'execution_state': 'idle'}
parent {'header': {'date': datetime.datetime(2021, 12, 20, 10, 21, 42, 730000, tzinfo=tzutc()), 'msg_id': '844de131-2be6-4111-9602-dc851ae47494', 'msg_type': 'comm_msg', 'session': '2ab7dfd6-30ac-43e4-8102-ddbb2fb76c82', 'username': '', 'version': '5.2'}, 'msg_id': '844de131-2be6-4111-9602-dc851ae47494', 'msg_type': 'comm_msg', 'parent_header': {}, 'metadata': {}, 'content': {'comm_id': '4e7e038267584664b625c77f3b861e9d', 'data': {'method': 'custom', 'content': {'event': 'client_ready'}}}, 'buffers': []}
header None
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[MarcSkovMadsen]

Refactoring my example to

# pip install ipycanvas panel ipywidgets_bokeh
import panel as pn
from ipycanvas import Canvas

pn.extension('ipywidgets')

canvas = Canvas(width=200, height=200)

# Cubic curves example
button = pn.widgets.Button(name="run")
@pn.depends(button.param.clicks, watch=True)
def draw(clicks):
    print("draw begin")
    canvas.begin_path()
    canvas.move_to(75, 40)
    canvas.bezier_curve_to(75, 37, 70, 25, 50, 25)
    canvas.fill()
    print("draw end")

pn.Column(
    button,
    pn.panel(canvas, height=200, width=200)
).servable()

I can see that the message is not send from that function as it does not print anything if I run the draw function. But the error on client side appears.

https://user-images.githubusercontent.com/42288570/146754367-77e84009-9dff-4954-b2bc-a16b3adee67e.mp4