Convert Sparse Feature to Dense Feature

sparse_to_dense.py

import sys

class Data:
        def __init__(self, line):
                data = line.split()
                self.label = data[0]
                self.feats = dict()
                for i in range(len(data)-1):
                        a,b = data[i+1].split(':')
                        self.feats[int(a)] = b
        def max_idx(self):
                return max(self.feats.keys())

        def print_line(self, f, maxi):
                f.write(self.label)
                for i in range(maxi+1):
                        if i in self.feats:
                                f.write(" %s" % self.feats[i])
                        else:
                                f.write(" 0")
                f.write("\n")


data = [Data(x) for x in open(sys.argv[1]).readlines()]
max_idx = max([x.max_idx() for x in data])
f = open(sys.argv[1]+'.cvt', 'w')
for d in data:
        d.print_line(f, max_idx)

what is input format here? If you can provide an example it will be very helpful. thanks in advance.