def getReportPeriod(request):
	"""
	Return: {string} 'filterReportPeriod' param value based on hierarchy: URL param value -> session (last used) -> none.
	"""
	filterVal = request.GET.get('reportperiod', None)
	filterByLastUsed = request.session.get('filterReportPeriod', 'last90')
	if not filterVal and filterByLastUsed:
		filterVal = filterByLastUsed
	
	return filterVal