I use the following block of code to read lines out of a file 'f' into a nested list: for data in f: clean_data = data.rstrip() data = clean_data.split('\t') t += [data[0]] strmat += [data[1:]] Sometimes, however, the data is incomplete and a row may look like this: ['955.159', '62.8168', '', '', '', '', '', '', '', '', '', '', '', '', '', '29', '30', '0', '0'] It puts a spanner in the works because I would like Python to implicitly cast my list as floats but the empty fields '' cause it to be cast as an array of strings (dtype: s12). I could start a second 'if' statement and convert all empty fields into NULL (since 0 is wrong in this instance) but I was unsure whether this was best. Is this the best strategy of dealing with incomplete data? Should I edit the stream or do it post-hoc?