I'm using custom basemap's defined as a WebMaps gallery_basemaps (https://developers.arcgis.com/python/api-reference/arcgis.mapping.toc.html#arcgis.mapping.WebMap.gal...)
According to the example under a WebMaps property, basemap, I should be able to set my webmap to a gallery_basemap item. If I'm creating a new webmap, this works perfectly. However, if I'm updating an existing basemap, it fails saying there is not a gallery-basemaps
wm = WebMap(webmap_item)
wm.basemap = <my_unique_basemap_name_from_the_gallery>
Exception: 'WebMap' object has no attribute '_gallery_basemaps'