>>> s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
>>> s.bind(('', 0))
>>> s.sendto(u'hellé', s.getsockname())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sendto() takes exactly 3 arguments (2 given)
>>> s.sendto(3, s.getsockname())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sendto() takes exactly 3 arguments (2 given)
>>> s.sendto('hello', s.getsockname())
5

The TypeError gives the wrong explanation for what's wrong.

The original code was trying to call PyArg_ParseTuple assuming 2 args and in case of failure (*any* failure) was starting over assuming 3 args.
The attached patch makes PyArg_ParseTuple accept 2 or 3 args and re-arranges the last two if 'flags' is passed.
The tests pass, but a few more tests should be added.

Added tests and fixed all the problems they found.

New changeset 7c3a20b5943a by Ezio Melotti in branch '2.7':
#10169: Fix argument parsing in socket.sendto() to avoid error masking.
http://hg.python.org/cpython/rev/7c3a20b5943a

This turned out to be a duplicate of #5421 already fixed in 3.x, so I backported the patch and added the tests to 3.x too.