def transform(entries, key, transformation):
    """Transform a column in a DataEntry

    Args:
        entries: list of `DataEntry`
        key: a string that identifies a column in the pandas dataframes
        transformation: a function that takes a value and returns some of it
    Returns:
        a list of `DataEntry` where the one given column has been transformed in all entries
    """
    new_entries = []
    for entry in entries:
        values = entry.values
        values[key] = values[key].map(transformation)
        new_entries.append(entry._replace(values=values))
    return new_entries