How to sort list of lists according to length of sublists

Use key parameter available in sort and sorted. It specifies a function of one argument that is used to extract a comparison key from each list element

In [6]: a = [['a', 'b', 'c'], ['d', 'e'], ['f', 'g', 'h'], ['i', 'j', 'k', 'l'], ['m', 'n'], ['o']]

In [7]: a.sort(key=len)

In [8]: print a
[['o'], ['d', 'e'], ['m', 'n'], ['a', 'b', 'c'], ['f', 'g', 'h'], ['i', 'j', 'k', 'l']]

can be done by

sorted(a, key=len)