Reading javascript and converting to python is tricky.
Taking the example from the original blog post and combining it with this example from the map box docs.
def form_show(self, **event_arg):
self.mapbox = mapboxgl.Map(
{
"container": self.dom,
#'style': 'mapbox://styles/mapbox/streets-v11',
"style": "mapbox://styles/brookemyers/cklk04z7x1f5d17pedafupa3e",
"center": [0.1218, 52.2053], # center on Cambridge
"zoom": 12,
}
)
self.mapbox.on("load", self.on_load)
...
def on_load(self, *args):
url = "https://docs.mapbox.com/mapbox-gl-js/assets/cat.png"
self.mapbox.loadImage(url, self.on_load_image)
def on_load_image(self, error, image):
if error:
raise error
self.mapbox.addImage("cat", image)
self.mapbox.addSource(
"point",
{
"type": "geojson",
"data": {
"type": "FeatureCollection",
"features": [{"type": "Feature", "geometry": {"type": "Point", "coordinates": [0.1218, 52.2053]}}],
},
},
)
self.mapbox.addLayer(
{
"id": "points",
"type": "symbol",
"source": "point", # // reference the data source
"layout": {"icon-image": "cat", "icon-size": 0.25}, # // reference the image
}
)
This is a case of a library using callback as part of their api.
Whenever the library requires a callback I provide it with a method on the form.
Side note: If using a python function as a javsacript callback it’s often necessary to use *args since the javascript library may provide you with additional args you don’t need (and that are not documented 
)