For the First you can do something like
>>> [a[i:i+4] for i in range(0,len(a),4)]
[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 14, 15, 16], [17, 18, 19, 20]]
For the Second you can simply read and generate the tuples with value
as the first item
>>> [('value',i) for i in a]
[('value', 1), ('value', 2), ('value', 3), ('value', 4), ('value', 5), ('value', 6), ('value', 7), ('value', 8), ('value', 9), ('value', 10), ('value', 11), ('value', 12), ('value', 13), ('value', 14), ('value', 15), ('value', 16), ('value', 17), ('value', 18), ('value', 19), ('value', 20)]
another version using itertools.izip_longest though the above is more redable
list(itertools.izip_longest([],a,fillvalue='value'))