Question

How to sort a list of tuples based on the first value i.e, in a dictionary we can use sorted(a.keys()).

How to do it for a list of tuples?

If these are the tuple values

t = [('2010-09-11', 'somedata', somedata),
     ('2010-06-11', 'somedata', somedata),
     ('2010-09-12', 'somedata', somedata)]

tuples should be sorted based on dates in the first field.

Answer 1

Usually, just sorted(t) works, as tuples are sorted by lexicographical order. If you really want to ignore everything after the first item (instead of sorting tuples with the same first element by the following elements), you can supply a key that picks out the first element. The simplest way would be operator.itemgetter:

import operator
...
for item in sorted(t, key=operator.itemgetter(0)):
    ...

Of course if you want to sort the list in-place, you can use t.sort(key=operator.itemgetter(0)) instead.