Replace map(None, *iterables) with zip(*iterables).

Doc/library/functions.rst

@@ -648,17 +648,7 @@ available.  They are listed here in alphabetical order.
    Return an iterator that applies *function* to every item of *iterable*,
    yielding the results.  If additional *iterable* arguments are passed,
    *function* must take that many arguments and is applied to the items from all
-   iterables in parallel.  If one iterable is shorter than another it is assumed
-   to be extended with ``None`` items.  If *function* is ``None``, the identity
-   function is assumed; if there are multiple arguments, :func:`map` returns a
-   list consisting of tuples containing the corresponding items from all
-   iterables (a kind of transpose operation).  The *iterable* arguments may be a
-   sequence or any iterable object; the result is always a list.
-
-   Note that for only one *iterable* argument, ``map(function, iterable)`` is
-   equivalent to the generator expression ``(function(item) for item in
-   iterable)`` if *function* is not ``None``.
-
+   iterables in parallel.
 
 .. function:: max(iterable[, args...], *[, key])
 

Doc/library/itertools.rst

@@ -204,20 +204,12 @@ loops that truncate the stream.
 .. function:: imap(function, *iterables)
 
    Make an iterator that computes the function using arguments from each of the
-   iterables.  If *function* is set to ``None``, then :func:`imap` returns the
-   arguments as a tuple.  Like :func:`map` but stops when the shortest iterable is
-   exhausted instead of filling in ``None`` for shorter iterables.  The reason for
-   the difference is that infinite iterator arguments are typically an error for
-   :func:`map` (because the output is fully evaluated) but represent a common and
-   useful way of supplying arguments to :func:`imap`. Equivalent to::
+   iterables.  Equivalent to::
 
       def imap(function, *iterables):
-          iterables = map(iter, iterables)
+          iterables = [iter(it) for it in iterables)
           while True:
-              args = [next(i) for i in iterables]
-              if function is None:
-                  yield tuple(args)
-              else:
+              args = [next(it) for it in iterables]
               yield function(*args)
 
 

Lib/heapq.py

@@ -351,7 +351,8 @@ def nsmallest(n, iterable, key=None):
     Equivalent to:  sorted(iterable, key=key)[:n]
     """
     in1, in2 = tee(iterable)
-    it = izip(map(key, in1), count(), in2)                  # decorate
+    keys = in1 if key is None else map(key, in1)
+    it = izip(keys, count(), in2)                           # decorate
     result = _nsmallest(n, it)
     return list(map(itemgetter(2), result))                 # undecorate
 
@@ -362,7 +363,8 @@ def nlargest(n, iterable, key=None):
     Equivalent to:  sorted(iterable, key=key, reverse=True)[:n]
     """
     in1, in2 = tee(iterable)
-    it = izip(map(key, in1), map(neg, count()), in2)        # decorate
+    keys = in1 if key is None else map(key, in1)
+    it = izip(keys, map(neg, count()), in2)                 # decorate
     result = _nlargest(n, it)
     return list(map(itemgetter(2), result))                 # undecorate
 

Lib/test/test_builtin.py

@@ -1099,18 +1099,6 @@ class BuiltinTest(unittest.TestCase):
         # self.assertRaises(TypeError, long, Foo5())
 
     def test_map(self):
-        self.assertEqual(
-            list(map(None, 'hello')),
-            [('h',), ('e',), ('l',), ('l',), ('o',)]
-        )
-        self.assertEqual(
-            list(map(None, 'abcd', 'efg')),
-            [('a', 'e'), ('b', 'f'), ('c', 'g')]
-        )
-        self.assertEqual(
-            list(map(None, range(3))),
-            [(0,), (1,), (2,)]
-        )
         self.assertEqual(
             list(map(lambda x: x*x, range(1,4))),
             [1, 4, 9]
@@ -1145,18 +1133,10 @@ class BuiltinTest(unittest.TestCase):
             list(map(plus, [1, 3, 7], [4, 9, 2], [1, 1, 0])),
             [1+4+1, 3+9+1, 7+2+0]
         )
-        self.assertEqual(
-            list(map(None, Squares(10))),
-            [(0,), (1,), (4,), (9,), (16,), (25,), (36,), (49,), (64,), (81,)]
-        )
         self.assertEqual(
             list(map(int, Squares(10))),
             [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
         )
-        self.assertEqual(
-            list(map(None, Squares(3), Squares(2))),
-            [(0,0), (1,1)]
-        )
         def Max(a, b):
             if a is None:
                 return b
@@ -1169,7 +1149,6 @@ class BuiltinTest(unittest.TestCase):
         )
         self.assertRaises(TypeError, map)
         self.assertRaises(TypeError, map, lambda x: x, 42)
-        self.assertEqual(list(map(None, [42])), [(42,)])
         class BadSeq:
             def __iter__(self):
                 raise ValueError

Lib/test/test_iter.py

@@ -382,13 +382,10 @@ class TestCase(unittest.TestCase):
 
     # Test map()'s use of iterators.
     def test_builtin_map(self):
-        self.assertEqual(list(map(None, SequenceClass(5))),
-                         [(0,), (1,), (2,), (3,), (4,)])
         self.assertEqual(list(map(lambda x: x+1, SequenceClass(5))),
                          list(range(1, 6)))
 
         d = {"one": 1, "two": 2, "three": 3}
-        self.assertEqual(list(map(None, d)), [(k,) for k in d])
         self.assertEqual(list(map(lambda k, d=d: (k, d[k]), d)),
                          list(d.items()))
         dkeys = list(d.keys())
@@ -396,11 +393,6 @@ class TestCase(unittest.TestCase):
                      i,
                      i < len(d) and dkeys[i] or None)
                     for i in range(3)]
-        self.assertEqual(list(map(None,
-                                  d,
-                                  SequenceClass(5),
-                                  iter(d.keys()))),
-                         expected)
 
         f = open(TESTFN, "w")
         try:

Lib/test/test_itertools.py

@@ -236,7 +236,7 @@ class TestBasicOps(unittest.TestCase):
         self.assertEqual(list(izip_longest('abcdef')), list(zip('abcdef')))
 
         self.assertEqual(list(izip_longest('abc', 'defg', **{})),
-                         list(map(None, list('abc')+[None], 'defg'))) # empty keyword dict
+                         list(izip(list('abc')+[None], 'defg'))) # empty keyword dict
         self.assertRaises(TypeError, izip_longest, 3)
         self.assertRaises(TypeError, izip_longest, range(3), 3)
 
@@ -281,14 +281,17 @@ class TestBasicOps(unittest.TestCase):
     def test_imap(self):
         self.assertEqual(list(imap(operator.pow, range(3), range(1,7))),
                          [0**1, 1**2, 2**3])
-        self.assertEqual(list(imap(None, 'abc', range(5))),
+        def tupleize(*args):
+            return args
+        self.assertEqual(list(imap(tupleize, 'abc', range(5))),
                          [('a',0),('b',1),('c',2)])
-        self.assertEqual(list(imap(None, 'abc', count())),
+        self.assertEqual(list(imap(tupleize, 'abc', count())),
                          [('a',0),('b',1),('c',2)])
-        self.assertEqual(take(2,imap(None, 'abc', count())),
+        self.assertEqual(take(2,imap(tupleize, 'abc', count())),
                          [('a',0),('b',1)])
         self.assertEqual(list(imap(operator.pow, [])), [])
         self.assertRaises(TypeError, imap)
+        self.assertRaises(TypeError, list, imap(None, range(3), range(3)))
         self.assertRaises(TypeError, imap, operator.neg)
         self.assertRaises(TypeError, next, imap(10, range(5)))
         self.assertRaises(ValueError, next, imap(errfunc, [4], [5]))

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 3.0a3?
 Core and Builtins
 -----------------
 
+- map() and itertools.imap() no longer accept None for the first argument.
+  Use zip() instead.
+
 - Issue #1769: Now int("- 1") is not allowed any more.
 
 - Object/longobject.c: long(float('nan')) raises an OverflowError instead

Modules/itertoolsmodule.c

@@ -1490,31 +1490,6 @@ imap_traverse(imapobject *lz, visitproc visit, void *arg)
 	return 0;
 }
 
-/*	
-imap() is an iterator version of __builtins__.map() except that it does
-not have the None fill-in feature.  That was intentionally left out for
-the following reasons:
-
-  1) Itertools are designed to be easily combined and chained together.
-     Having all tools stop with the shortest input is a unifying principle
-     that makes it easier to combine finite iterators (supplying data) with
-     infinite iterators like count() and repeat() (for supplying sequential
-     or constant arguments to a function).
-
-  2) In typical use cases for combining itertools, having one finite data 
-     supplier run out before another is likely to be an error condition which
-     should not pass silently by automatically supplying None.
-
-  3) The use cases for automatic None fill-in are rare -- not many functions
-     do something useful when a parameter suddenly switches type and becomes
-     None.  
-
-  4) If a need does arise, it can be met by __builtins__.map() or by 
-     writing:  chain(iterable, repeat(None)).
-
-  5) Similar toolsets in Haskell and SML do not have automatic None fill-in.
-*/
-
 static PyObject *
 imap_next(imapobject *lz)
 {
@@ -1536,8 +1511,6 @@ imap_next(imapobject *lz)
 		}
 		PyTuple_SET_ITEM(argtuple, i, val);
 	}
-	if (lz->func == Py_None) 
-		return argtuple;
 	result = PyObject_Call(lz->func, argtuple, NULL);
 	Py_DECREF(argtuple);
 	return result;
@@ -1547,10 +1520,7 @@ PyDoc_STRVAR(imap_doc,
 "imap(func, *iterables) --> imap object\n\
 \n\
 Make an iterator that computes the function using arguments from\n\
-each of the iterables.	Like map() except that it returns\n\
-an iterator instead of a list and that it stops when the shortest\n\
-iterable is exhausted instead of filling in None for shorter\n\
-iterables.");
+each of the iterables.	Stops when the shortest iterable is exhausted.");
 
 static PyTypeObject imap_type = {
 	PyVarObject_HEAD_INIT(NULL, 0)

Python/bltinmodule.c

@@ -815,7 +815,6 @@ Return an iterator yielding the results of applying the function to the\n\
 items of the argument iterables(s).  If more than one iterable is given,\n\
 the function is called with an argument list consisting of the\n\
 corresponding item of each iterable, until an iterable is exhausted.\n\
-If the function is None, 'lambda *a: a' is assumed.\n\
 (This is identical to itertools.imap().)");