def check_column_existence(col_name, df, presence=True):
    """
    Checks whether or not `col_name` is in `df` and raises a helpful error msg
    if the desired condition is not met.

    Parameters
    ----------
    col_name : str.
        Should represent a column whose presence in `df` is to be checked.
    df : pandas DataFrame.
        The dataframe that will be checked for the presence of `col_name`.
    presence : bool, optional.
        If True, then this function checks for the PRESENCE of `col_name` from
        `df`. If False, then this function checks for the ABSENCE of
        `col_name` in `df`. Default == True.

    Returns
    -------
    None.
    """
    if presence:
        if col_name not in df.columns:
            msg = 'Ensure that `{}` is in `df.columns`.'
            raise ValueError(msg.format(col_name))
    elif col_name in df.columns:
        msg = 'Ensure that `{}` is not in `df.columns`.'
        raise ValueError(msg.format(col_name))
    return None
