is it safe to use ModelAdmin variables like this

Is it safe to set a modeladmin variable and use it like this; it is set in the changelist_view based on the url search criteria and is used to limit the choices of a foreign key when adding new item

class MyModelAdmin(reversion.VersionAdmin):
...
    selected_period = None

    def formfield_for_foreignkey(self, db_field, request, **kwargs):
        if db_field.name == "period":
            kwargs["queryset"] = Period.objects.filter(id=self.selected_period)
        return super(MyModelAdmin, self).formfield_for_foreignkey(db_field, request, **kwargs)

    def changelist_view(self, request, extra_context=None):
       ..
        if request.GET.has_key('period__id__exact'):
            self.selected_period = request.GET['period__id__exact']
        return super(MyModelAdmin, self).changelist_view(request, extra_context=extra_context)
admin.site.register(MyClassroomAbsenceLog, MyClassroomAbsenceLogAdmin)

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