def _non_present_values_to_zero(test_data):
    """
    Adds keys to the test data of one dataframe, if key was not present in that one but in one of the other datasets.

    :param test_data:
    :return:
    """
    for dataset1 in test_data:
        for dataset2 in test_data:
            for key in dataset1.keys():
                if key not in dataset2:
                    dataset2[key] = 0
    return test_data
