Fun with inspect and ast. I do not recommend using this code in production.

unpack.py

import inspect, ast
from itertools import islice, chain, cycle

def iter_n(iterator, n, default=None):
    return islice(chain(iterator, cycle([default])), n)

def unpack(sequence, default=None):
    stack = inspect.stack()
    try:
        frame = stack[1][0]
        source = inspect.getsource(inspect.getmodule(frame)).splitlines()
        line = source[frame.f_lineno-1].strip()
        try:
            tree = ast.parse(line, 'whatever', 'exec')
        except SyntaxError:
            return tuple(sequence)
        exp = tree.body[0]
        if not isinstance(exp, ast.Assign):
            return tuple(sequence)
        exp = exp.targets[0]
        if not isinstance(exp, ast.Tuple):
            return tuple(sequence)
        n_items = len(exp.elts)
        return tuple(iter_n(sequence, n_items, default))
    finally:
        del stack

unpack_test.py

# Extra items are discarded
x, y = unpack([1,2,3,4,5])
assert (x,y) == (1,2)

# Missing items become None
x, y, z = unpack([9])
assert (x, y, z) == (9, None, None)

# Or the default you provide
x, y, z = unpack([1], 'foo')
assert (x, y, z) == (1, 'foo', 'foo')

# unpack() is equivalent to tuple() if it's not part of an assignment
assert unpack('abc') == ('a', 'b', 'c')

# Or if it's part of an assignment that isn't sequence-unpacking
x = unpack([1,2,3])
assert x == (1,2,3)

# Add a comma to force tuple assignment:
x, = unpack([1,2,3])
assert x == 1

# unpack only uses the first assignment target
# So in this case, unpack('foobar') returns tuple('foo')
(x, y, z) = t = unpack('foobar')
assert (x, y, z) == t == ('f', 'o', 'o')

# But in this case, it returns tuple('foobar')
try:
    t = (x, y, z) = unpack('foobar')
except ValueError as e:
    assert str(e) == 'too many values to unpack'
else:
    raise Exception("That should have failed.")

# Also, it won't work if the call spans multiple lines, because it only
# inspects the actual line where the call happens:
try:
    (x, y, z) = unpack([
        1, 2, 3, 4])
except ValueError as e:
    assert str(e) == 'too many values to unpack'
else:
    raise Exception("That should have failed.")