PyGObject: modify icon contest for ToolButton

[<c buhtz posteo jp>]

I read about Gtk.IconTheme - very nice.
https://lazka.github.io/pgi-docs/#Gtk-3.0/classes/IconTheme.html

I want to use some special icons on Gtk.ToolButton() Gtk.Toolbar().

But when using 'icon_name' in a ToolButton it looks like that only the
'Action' context of is used. When I try a name from another icon
context the icon is empty.

I checked the existence of a icon from another context via
Gtk.IconTheme.list_icons() and looking into the filesystem itself.

Is there a easy way to make Gtk.ToolButton() useing another icon
context?

I know the workaround with Gtk.ToolButton.icon-widget. This would be to
much code/work for such a simple task.