gistfile1.txt

#!/usr/bin/env python

from itertools import chain, groupby
from csv import reader, writer

with open('foo.csv') as infile:
	r = reader(infile)
	groups = groupby(r, lambda x:x[0]) # Group by the first column
	lines = []
	for group in groups:
		gchain = chain(g[1:] for g in group[1]) # [[400, 1, 2, 3], [400, 4,5,6] -> [1,2,3,4,5,6]
		lines.append([group[0]] + list(gchain)) # [[400, 1,2,3,4,5,6], [410, a,b,c,d,e,f,g], …]

with open('bar.csv') as outfile:
	w = writer(outfile)
	w.writelines(zip(*lines)) # Transpose all the lines and return an iterable to write