def price_query_filter(price_query_dict):
    """
        Ensure that certain keys are available.
            - exchange
            - period
    """
    if not isinstance(price_query_dict, dict):
        raise TypeError('dict_list is not a list. Please try again')
    required = ['period', 'exchange']
    pkeys = price_query_dict.keys()
    for r in required:
        if r not in pkeys:
            raise KeyError('An important key is not available {}'.format(r))
    return True
