Map.fit_bounds() has no effect on Xeus-Python kernel in JupyterLab 3

It seems like map.fit_bounds() does not have any effect when run on the Xeus-Python (Python 3.8 (XPython)), but works as expected on the “normal” Python 3 kernel. The same effect can be seen when running in the “normal” mode (not the “simple” one in which the screenshots below were made) and when running as a “classic” notebook (after pressing “Help -> Launch Classic Notebook”).

This took me a while to find out and I’m opening it here first, but I’ve opened one on Xeus kernel as a cross-reference: https://github.com/jupyter-xeus/xeus-python/issues/432. I’m using JupyterLab 3.0.10.

import ipyleaflet

print(ipyleaflet.__version__)
m = ipyleaflet.Map(center=[0, 0], zoom=1)

((s, w), (n, e)) = ((42, -6), (52, 9))
m += ipyleaflet.Polygon(locations=[(s, w), (n, w), (n, e), (s, e)])

# No effect under Xeus-Python (Python 3.8 (XPython)) kernel,
# but as expected under Python 3 kernel:
m.fit_bounds(((s, w), (n, e)))

m

[davidbrochart]

I think it is because xeus-python doesn’t have event loop integration as ipykernel has. We could also implement fit_bounds() using https://github.com/mapbox/mercantile, which would work with all python kernels, but that would only work for Mercator projections. Maybe this is fine.

[martinRenou]

Oh, I thought fit_bounds was using comm messages, but it actually doesn’t do that. Then what I said is not valid.