def combine_dicts(new_dict, old_dict):
    """
    returns a dictionary with all key, value pairs from new_dict.
    also returns key, value pairs from old_dict, if that key does not exist in new_dict.
    if a key is present in both new_dict and old_dict, the new_dict value will take precedence.
    """
    old_data_keys = list(old_dict.keys())
    new_data_keys = list(new_dict.keys())
    all_keys = set(old_data_keys).union(new_data_keys)
    combined_data_dict = {}
    for k in all_keys:
        try:
            combined_data_dict[k] = new_dict[k]
        except KeyError:
            combined_data_dict[k] = old_dict[k]
    return combined_data_dict