rewrite of the tuple unpacking code

according to unladen swallows' bm_unpack_sequence benchmark:
- ~25% faster for tuples (60% faster than CPython 2.7)
- ~3x faster for lists (40% faster than CPython 2.7)
also, apparently using less memory during gcc compilation

--HG--
extra : rebase_source : 1069d7a0c9c338c16e96cc289114e0d0052a7588

tests/run/unpack.pyx

@@ -7,9 +7,21 @@ def _it(N):
     for i in range(N):
         yield i
 
-def f(obj1, obj2, obj3, obj4, obj5):
+cdef class ItCount(object):
+    cdef object values
+    cdef readonly count
+    def __init__(self, values):
+        self.values = iter(values)
+        self.count = 0
+    def __iter__(self):
+        return self
+    def __next__(self):
+        self.count += 1
+        return next(self.values)
+
+def kunterbunt(obj1, obj2, obj3, obj4, obj5):
     """
-    >>> f(1, (2,), (3,4,5), (6,(7,(8,9))), 0)
+    >>> kunterbunt(1, (2,), (3,4,5), (6,(7,(8,9))), 0)
     (8, 9, (8, 9), (6, (7, (8, 9))), 0)
     """
     obj1, = obj2
@@ -19,16 +31,177 @@ def f(obj1, obj2, obj3, obj4, obj5):
     [obj1, obj2] = obj3
     return obj1, obj2, obj3, obj4, obj5
 
+def unpack_tuple(tuple it):
+    """
+    >>> unpack_tuple((1,2,3))
+    (1, 2, 3)
+
+    >>> a,b,c = None
+    Traceback (most recent call last):
+    TypeError: 'NoneType' object is not iterable
+    >>> unpack_tuple(None)
+    Traceback (most recent call last):
+    TypeError: 'NoneType' object is not iterable
+    """
+    a,b,c = it
+    return a,b,c
+
+def unpack_list(list it):
+    """
+    >>> unpack_list([1,2,3])
+    (1, 2, 3)
+
+    >>> a,b,c = None
+    Traceback (most recent call last):
+    TypeError: 'NoneType' object is not iterable
+    >>> unpack_list(None)
+    Traceback (most recent call last):
+    TypeError: 'NoneType' object is not iterable
+    """
+    a,b,c = it
+    return a,b,c
+
+def unpack_to_itself(it):
+    """
+    >>> it = _it(2)
+    >>> it, it = it
+    >>> it
+    1
+    >>> unpack_to_itself([1,2])
+    2
+    >>> unpack_to_itself((1,2))
+    2
+    >>> unpack_to_itself(_it(2))
+    1
+    >>> unpack_to_itself((1,2,3))
+    Traceback (most recent call last):
+    ValueError: too many values to unpack (expected 2)
+    >>> unpack_to_itself(_it(3))
+    Traceback (most recent call last):
+    ValueError: too many values to unpack (expected 2)
+    """
+    it, it = it
+    return it
+
+def unpack_partial(it):
+    """
+    >>> it = _it(2)
+    >>> a = b = c = 0
+    >>> a,b,c = it
+    Traceback (most recent call last):
+    ValueError: need more than 2 values to unpack
+    >>> a, b, c
+    (0, 0, 0)
+    >>> unpack_partial([1,2])
+    (0, 0, 0)
+    >>> unpack_partial((1,2))
+    (0, 0, 0)
+    >>> unpack_partial(_it(2))
+    (0, 0, 0)
+
+    >>> it = ItCount([1,2])
+    >>> a = b = c = 0
+    >>> a,b,c = it
+    Traceback (most recent call last):
+    ValueError: need more than 2 values to unpack
+    >>> a, b, c
+    (0, 0, 0)
+    >>> it.count
+    3
+    >>> it = ItCount([1,2])
+    >>> unpack_partial(it)
+    (0, 0, 0)
+    >>> it.count
+    3
+    """
+    a = b = c = 0
+    try:
+        a, b, c = it
+    except ValueError:
+        pass
+    return a, b, c
+
+def unpack_fail_assignment(it):
+    """
+    >>> it = ItCount([1, 2, 3])
+    >>> a = b = c = 0
+    >>> try: a, b[0], c = it
+    ... except TypeError: pass
+    >>> a,b,c
+    (1, 0, 0)
+    >>> it.count
+    4
+    >>> it = ItCount([1, 2, 3])
+    >>> unpack_fail_assignment(it)
+    (1, 0, 0)
+    >>> it.count
+    4
+    """
+    cdef object a,b,c
+    a = b = c = 0
+    try:
+        a, b[0], c = it
+    except TypeError:
+        pass
+    return a, b, c
+
+def unpack_partial_typed(it):
+    """
+    >>> unpack_partial_typed([1, 2, 'abc'])
+    (0, 0, 0)
+    >>> unpack_partial_typed((1, 'abc', 3))
+    (0, 0, 0)
+    >>> unpack_partial_typed(_set([1, 'abc', 3]))
+    (0, 0, 0)
+
+    >>> it = ItCount([1, 'abc', 3])
+    >>> unpack_partial_typed(it)
+    (0, 0, 0)
+    >>> it.count
+    4
+    """
+    cdef int a,b,c
+    a = b = c = 0
+    try:
+        a, b, c = it
+    except TypeError:
+        pass
+    return a, b, c
+
 def unpack_typed(it):
     """
     >>> unpack_typed((1, 2.0, [1]))
     (1, 2.0, [1])
+    >>> unpack_typed([1, 2.0, [1]])
+    (1, 2.0, [1])
+    >>> it = ItCount([1, 2.0, [1]])
+    >>> unpack_typed(it)
+    (1, 2.0, [1])
+    >>> it.count
+    4
+
     >>> unpack_typed((1, None, [1]))
     Traceback (most recent call last):
     TypeError: a float is required
+    >>> unpack_typed([1, None, [1]])
+    Traceback (most recent call last):
+    TypeError: a float is required
+    >>> it = ItCount([1, None, [1]])
+    >>> unpack_typed(it)
+    Traceback (most recent call last):
+    TypeError: a float is required
+    >>> it.count
+    4
+
     >>> unpack_typed((1, 2.0, (1,)))
     Traceback (most recent call last):
     TypeError: Expected list, got tuple
+    >>> it = ItCount([1, 2.0, (1,)])
+    >>> unpack_typed(it)
+    Traceback (most recent call last):
+    TypeError: Expected list, got tuple
+    >>> it.count
+    4
     """
     cdef int a
     cdef float b
@@ -67,6 +240,7 @@ def failure_too_many(it):
     ValueError: too many values to unpack (expected 3)
     """
     a,b,c = it
+    return a,b,c
 
 def failure_too_few(it):
     """
@@ -99,6 +273,7 @@ def failure_too_few(it):
     ValueError: need more than 2 values to unpack
     """
     a,b,c = it
+    return a,b,c
 
 def _it_failure(N):
     for i in range(N):
@@ -143,3 +318,4 @@ def failure_while_unpacking(it):
     ValueError: too many values to unpack (expected 3)
     """
     a,b,c = it
+    return a,b,c