In the tutorial https://www.quantopian.com/tutorials/pipeline#lesson10, provided an example of self defined custom factor as below:
def compute(self, today, asset_ids, out, values):
# Calculates the column-wise standard deviation, ignoring NaNs
out[:] = numpy.nanstd(values, axis=0)
My question: in above 'compute' function, there are four input parameters: self, today, asset_ids, out, values, besides out and values, today is not used, can we delete today?