Promote compress() from a recipe to being a regular itertool.

Doc/library/collections.rst

@@ -298,7 +298,7 @@ counts less than one::
       Section 4.6.3, Exercise 19*\.
 
     * To enumerate all distinct multisets of a given size over a given set of
-      elements, see the :func:`combinations_with_replacement` function in the
+      elements, see :func:`combinations_with_replacement` in the
       :ref:`itertools-recipes` for itertools::
 
           map(Counter, combinations_with_replacement('ABC', 2)) --> AA AB AC BB BC CC

Doc/library/itertools.rst

@@ -139,6 +139,20 @@ loops that truncate the stream.
 
    .. versionadded:: 2.6
 
+.. function:: compress(data, selectors)
+
+   Make an iterator that filters elements from *data* returning only those that
+   have a corresponding element in *selectors* that evaluates to ``True``.
+   Stops when either the *data* or *selectors* iterables have been exhausted.
+   Equivalent to::
+
+       def compress(data, selectors):
+           # compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F
+           return (d for d, s in izip(data, selectors) if s)
+
+   .. versionadded:: 2.7
+
+
 .. function:: count([n])
 
    Make an iterator that returns consecutive integers starting with *n*. If not
@@ -679,10 +693,6 @@ which incur interpreter overhead.
        for n in xrange(2**len(pairs)):
            yield set(x for m, x in pairs if m&n)
 
-   def compress(data, selectors):
-       "compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F"
-       return (d for d, s in izip(data, selectors) if s)
-
    def combinations_with_replacement(iterable, r):
        "combinations_with_replacement('ABC', 2) --> AA AB AC BB BC CC"
        # number items returned:  (n+r-1)! / r! / (n-1)!

Lib/test/test_itertools.py

@@ -191,6 +191,21 @@ def permutations2(iterable, r=None):
         self.assertEqual(len(set(map(id, permutations('abcde', 3)))), 1)
         self.assertNotEqual(len(set(map(id, list(permutations('abcde', 3))))), 1)
 
+    def test_compress(self):
+        self.assertEqual(list(compress('ABCDEF', [1,0,1,0,1,1])), list('ACEF'))
+        self.assertEqual(list(compress('ABCDEF', [0,0,0,0,0,0])), list(''))
+        self.assertEqual(list(compress('ABCDEF', [1,1,1,1,1,1])), list('ABCDEF'))
+        self.assertEqual(list(compress('ABCDEF', [1,0,1])), list('AC'))
+        self.assertEqual(list(compress('ABC', [0,1,1,1,1,1])), list('BC'))
+        n = 10000
+        data = chain.from_iterable(repeat(range(6), n))
+        selectors = chain.from_iterable(repeat((0, 1)))
+        self.assertEqual(list(compress(data, selectors)), [1,3,5] * n)
+        self.assertRaises(TypeError, compress, None, range(6))      # 1st arg not iterable
+        self.assertRaises(TypeError, compress, range(6), None)      # 2nd arg not iterable
+        self.assertRaises(TypeError, compress, range(6))            # too few args
+        self.assertRaises(TypeError, compress, range(6), None)      # too many args
+
     def test_count(self):
         self.assertEqual(zip('abc',count()), [('a', 0), ('b', 1), ('c', 2)])
         self.assertEqual(zip('abc',count(3)), [('a', 3), ('b', 4), ('c', 5)])
@@ -701,6 +716,9 @@ def test_combinations(self):
         self.assertEqual(list(combinations(range(4), 3)),
                          [(0,1,2), (0,1,3), (0,2,3), (1,2,3)])
 
+    def test_compress(self):
+        self.assertEqual(list(compress('ABCDEF', [1,0,1,0,1,1])), list('ACEF'))
+
     def test_count(self):
         self.assertEqual(list(islice(count(10), 5)), [10, 11, 12, 13, 14])
 
@@ -781,6 +799,10 @@ def test_combinations(self):
         a = []
         self.makecycle(combinations([1,2,a,3], 3), a)
 
+    def test_compress(self):
+        a = []
+        self.makecycle(compress('ABCDEF', [1,0,1,0,1,0]), a)
+
     def test_cycle(self):
         a = []
         self.makecycle(cycle([a]*2), a)
@@ -934,6 +956,15 @@ def test_chain(self):
             self.assertRaises(TypeError, list, chain(N(s)))
             self.assertRaises(ZeroDivisionError, list, chain(E(s)))
 
+    def test_compress(self):
+        for s in ("123", "", range(1000), ('do', 1.2), xrange(2000,2200,5)):
+            n = len(s)
+            for g in (G, I, Ig, S, L, R):
+                self.assertEqual(list(compress(g(s), repeat(1))), list(g(s)))
+            self.assertRaises(TypeError, compress, X(s), repeat(1))
+            self.assertRaises(TypeError, list, compress(N(s), repeat(1)))
+            self.assertRaises(ZeroDivisionError, list, compress(E(s), repeat(1)))
+
     def test_product(self):
         for s in ("123", "", range(1000), ('do', 1.2), xrange(2000,2200,5)):
             self.assertRaises(TypeError, product, X(s))
@@ -1125,7 +1156,7 @@ class SubclassWithKwargsTest(unittest.TestCase):
     def test_keywords_in_subclass(self):
         # count is not subclassable...
         for cls in (repeat, izip, ifilter, ifilterfalse, chain, imap,
-                    starmap, islice, takewhile, dropwhile, cycle):
+                    starmap, islice, takewhile, dropwhile, cycle, compress):
             class Subclass(cls):
                 def __init__(self, newarg=None, *args):
                     cls.__init__(self, *args)
@@ -1262,10 +1293,6 @@ def __init__(self, newarg=None, *args):
 ...     for n in xrange(2**len(pairs)):
 ...         yield set(x for m, x in pairs if m&n)
 
->>> def compress(data, selectors):
-...     "compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F"
-...     return (d for d, s in izip(data, selectors) if s)
-
 >>> def combinations_with_replacement(iterable, r):
 ...     "combinations_with_replacement('ABC', 3) --> AA AB AC BB BC CC"
 ...     pool = tuple(iterable)
@@ -1361,9 +1388,6 @@ def __init__(self, newarg=None, *args):
 >>> map(sorted, powerset('ab'))
 [[], ['a'], ['b'], ['a', 'b']]
 
->>> list(compress('abcdef', [1,0,1,0,1,1]))
-['a', 'c', 'e', 'f']
-
 >>> list(combinations_with_replacement('abc', 2))
 [('a', 'a'), ('a', 'b'), ('a', 'c'), ('b', 'b'), ('b', 'c'), ('c', 'c')]
 

Misc/NEWS

@@ -147,6 +147,8 @@ Library
 
 - Issue #4863: distutils.mwerkscompiler has been removed.
 
+- Added a new function:  itertools.compress().
+
 - Fix and properly document the multiprocessing module's logging
   support, expose the internal levels and provide proper usage
   examples.

Modules/itertoolsmodule.c

@@ -2506,6 +2506,162 @@ static PyTypeObject permutations_type = {
 };
 
 
+/* compress object ************************************************************/
+
+/* Equivalent to:
+
+	def compress(data, selectors):
+		"compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F"
+		return (d for d, s in izip(data, selectors) if s)
+*/
+
+typedef struct {
+	PyObject_HEAD
+	PyObject *data;
+	PyObject *selectors;
+} compressobject;
+
+static PyTypeObject compress_type;
+
+static PyObject *
+compress_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+	PyObject *seq1, *seq2;
+	PyObject *data=NULL, *selectors=NULL;
+	compressobject *lz;
+
+	if (type == &compress_type && !_PyArg_NoKeywords("compress()", kwds))
+		return NULL;
+
+	if (!PyArg_UnpackTuple(args, "compress", 2, 2, &seq1, &seq2))
+		return NULL;
+
+	data = PyObject_GetIter(seq1);
+	if (data == NULL)
+		goto fail;
+	selectors = PyObject_GetIter(seq2);
+	if (selectors == NULL)
+		goto fail;
+
+	/* create compressobject structure */
+	lz = (compressobject *)type->tp_alloc(type, 0);
+	if (lz == NULL)
+		goto fail;
+	lz->data = data;
+	lz->selectors = selectors;
+	return (PyObject *)lz;
+
+fail:
+	Py_XDECREF(data);
+	Py_XDECREF(selectors);
+	return NULL;
+}
+
+static void
+compress_dealloc(compressobject *lz)
+{
+	PyObject_GC_UnTrack(lz);
+	Py_XDECREF(lz->data);
+	Py_XDECREF(lz->selectors);
+	Py_TYPE(lz)->tp_free(lz);
+}
+
+static int
+compress_traverse(compressobject *lz, visitproc visit, void *arg)
+{
+	Py_VISIT(lz->data);
+	Py_VISIT(lz->selectors);
+	return 0;
+}
+
+static PyObject *
+compress_next(compressobject *lz)
+{
+	PyObject *data = lz->data, *selectors = lz->selectors;
+	PyObject *datum, *selector;
+	PyObject *(*datanext)(PyObject *) = *Py_TYPE(data)->tp_iternext;
+	PyObject *(*selectornext)(PyObject *) = *Py_TYPE(selectors)->tp_iternext;
+	int ok;
+
+	while (1) {
+		/* Steps:  get datum, get selector, evaluate selector.
+		   Order is important (to match the pure python version
+		   in terms of which input gets a chance to raise an
+		   exception first).
+		*/
+
+		datum = datanext(data);
+		if (datum == NULL)
+			return NULL;
+
+		selector = selectornext(selectors);
+		if (selector == NULL) {
+			Py_DECREF(datum);
+			return NULL;
+		}
+
+		ok = PyObject_IsTrue(selector);
+		Py_DECREF(selector);
+		if (ok == 1)
+			return datum;
+		Py_DECREF(datum);
+		if (ok == -1)
+			return NULL;
+	}
+}
+
+PyDoc_STRVAR(compress_doc,
+"compress(data sequence, selector sequence) --> iterator over selected data\n\
+\n\
+Return data elements corresponding to true selector elements.\n\
+Forms a shorter iterator from selected data elements using the\n\
+selectors to choose the data elements.");
+
+static PyTypeObject compress_type = {
+	PyVarObject_HEAD_INIT(NULL, 0)
+	"itertools.compress",		/* tp_name */
+	sizeof(compressobject),		/* tp_basicsize */
+	0,							/* tp_itemsize */
+	/* methods */
+	(destructor)compress_dealloc,	/* tp_dealloc */
+	0,								/* tp_print */
+	0,								/* tp_getattr */
+	0,								/* tp_setattr */
+	0,								/* tp_compare */
+	0,								/* tp_repr */
+	0,								/* tp_as_number */
+	0,								/* tp_as_sequence */
+	0,								/* tp_as_mapping */
+	0,								/* tp_hash */
+	0,								/* tp_call */
+	0,								/* tp_str */
+	PyObject_GenericGetAttr,		/* tp_getattro */
+	0,								/* tp_setattro */
+	0,								/* tp_as_buffer */
+	Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
+		Py_TPFLAGS_BASETYPE,		/* tp_flags */
+	compress_doc,					/* tp_doc */
+	(traverseproc)compress_traverse,	/* tp_traverse */
+	0,								/* tp_clear */
+	0,								/* tp_richcompare */
+	0,								/* tp_weaklistoffset */
+	PyObject_SelfIter,				/* tp_iter */
+	(iternextfunc)compress_next,	/* tp_iternext */
+	0,								/* tp_methods */
+	0,								/* tp_members */
+	0,								/* tp_getset */
+	0,								/* tp_base */
+	0,								/* tp_dict */
+	0,								/* tp_descr_get */
+	0,								/* tp_descr_set */
+	0,								/* tp_dictoffset */
+	0,								/* tp_init */
+	0,								/* tp_alloc */
+	compress_new,					/* tp_new */
+	PyObject_GC_Del,				/* tp_free */
+};
+
+
 /* ifilter object ************************************************************/
 
 typedef struct {
@@ -3552,6 +3708,7 @@ inititertools(void)
 		&starmap_type,
 		&imap_type,
 		&chain_type,
+		&compress_type,
 		&ifilter_type,
 		&ifilterfalse_type,
 		&count_type,