def get_identifiers(args_list, valid_keys):
    """Find string identifiers for each experiment. Used by pop-ups.

    Args:
        args_list (list of dictionary): dictionary consists of keys and values
            that uniquely identify the plot.

    Returns:
        identifiers (list of strings)

    """
    # ignore keys which have no variation among results
    identifiers = []
    for args in args_list:
        identifier = ''
        for key in valid_keys:
            if key in args:
                identifier += '{}={},'.format(key, args[key])
        identifiers.append(identifier)
    return identifiers