def _get_sources(filepath):
    """
    Function to create a list of sources that will be used in the query to the
    MongoDB instance.

    Parameters
    ----------

    filepath: String.
                Path to file containing the source keys. File should have a
                single key on each line.


    Returns
    -------

    sources: List.
                List of sources, basically.

    """
    with open(filepath, 'r') as f:
        sources = [key.replace('\n', '') for key in f.readlines()]
    return sources