def _set_cadence(lkwargs):
    """ Select the cadence of the data to download
    
    Determines the extension to use later in the lookup of cached fits files,
    to be passed to LightKurve for online lookup.
    
    If no cadence argument is passed it will default to long cadence. 
    
    Parameters
    ----------
    lkwargs : dict
        Dictionary to be passed to LightKurve.
        
    Returns
    -------
    ext : str
        Fits file short/long cadence extension.
        
    """
    if not lkwargs['cadence']:
        lkwargs['cadence'] = 'long'
    
    if lkwargs['cadence'] == 'short':
        ext = '*_slc.fits'
    elif lkwargs['cadence'] == 'long':
        ext = '*_lc.fits'
    else:
        raise TypeError('Unrecognized cadence input for %s' % (id))
    return ext