The function was defined to either take a space separated list of names or a sequence of names. The docs seem pretty clear to me: > The field_names are a seque

The function was defined to either take a space separated list of names or a sequence of names. The docs seem pretty clear to me: > The field_names are a sequence of strings such as ['x', 'y']. Alternatively, field_names can be a single string with each fieldname separated by whitespace and/or commas, for example 'x y' or 'x, y'.