[Django]-Django how to pass custom variables to context to use in custom admin template?

class MyModelAdmin(admin.ModelAdmin):
    ...
    def changelist_view(self, request, extra_context=None):
        extra_context = extra_context or {}
        extra_context['some_var'] = 'This is what I want to show'
        return super(MyModelAdmin, self).changelist_view(request, extra_context=extra_context)

See: https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.changelist_view