Issue #28019: itertools.count() no longer rounds non-integer step in range

between 1.0 and 2.0 to 1.

Lib/test/test_itertools.py

@@ -511,12 +511,18 @@ class TestBasicOps(unittest.TestCase):
         self.assertEqual(take(2, zip('abc',count(-3))), [('a', -3), ('b', -2)])
         self.assertRaises(TypeError, count, 2, 3, 4)
         self.assertRaises(TypeError, count, 'a')
-        self.assertEqual(list(islice(count(maxsize-5), 10)),
+        self.assertEqual(take(10, count(maxsize-5)),
                          list(range(maxsize-5, maxsize+5)))
-        self.assertEqual(list(islice(count(-maxsize-5), 10)),
+        self.assertEqual(take(10, count(-maxsize-5)),
                          list(range(-maxsize-5, -maxsize+5)))
-        self.assertEqual(list(islice(count(10, maxsize+5), 3)),
-                         list(range(10, 10+3*(maxsize+5), maxsize+5)))
+        self.assertEqual(take(3, count(3.25)), [3.25, 4.25, 5.25])
+        self.assertEqual(take(3, count(3.25-4j)), [3.25-4j, 4.25-4j, 5.25-4j])
+        self.assertEqual(take(3, count(Decimal('1.1'))),
+                         [Decimal('1.1'), Decimal('2.1'), Decimal('3.1')])
+        self.assertEqual(take(3, count(Fraction(2, 3))),
+                         [Fraction(2, 3), Fraction(5, 3), Fraction(8, 3)])
+        BIGINT = 1<<1000
+        self.assertEqual(take(3, count(BIGINT)), [BIGINT, BIGINT+1, BIGINT+2])
         c = count(3)
         self.assertEqual(repr(c), 'count(3)')
         next(c)
@@ -524,8 +530,10 @@ class TestBasicOps(unittest.TestCase):
         c = count(-9)
         self.assertEqual(repr(c), 'count(-9)')
         next(c)
-        self.assertEqual(repr(count(10.25)), 'count(10.25)')
         self.assertEqual(next(c), -8)
+        self.assertEqual(repr(count(10.25)), 'count(10.25)')
+        self.assertEqual(repr(count(10.0)), 'count(10.0)')
+        self.assertEqual(type(next(count(10.0))), float)
         for i in (-sys.maxsize-5, -sys.maxsize+5 ,-10, -1, 0, 10, sys.maxsize-5, sys.maxsize+5):
             # Test repr
             r1 = repr(count(i))
@@ -549,16 +557,22 @@ class TestBasicOps(unittest.TestCase):
                          [('a', 2), ('b', 5), ('c', 8)])
         self.assertEqual(lzip('abc',count(step=-1)),
                          [('a', 0), ('b', -1), ('c', -2)])
+        self.assertRaises(TypeError, count, 'a', 'b')
         self.assertEqual(lzip('abc',count(2,0)), [('a', 2), ('b', 2), ('c', 2)])
         self.assertEqual(lzip('abc',count(2,1)), [('a', 2), ('b', 3), ('c', 4)])
         self.assertEqual(lzip('abc',count(2,3)), [('a', 2), ('b', 5), ('c', 8)])
         self.assertEqual(take(20, count(maxsize-15, 3)), take(20, range(maxsize-15, maxsize+100, 3)))
         self.assertEqual(take(20, count(-maxsize-15, 3)), take(20, range(-maxsize-15,-maxsize+100, 3)))
+        self.assertEqual(take(3, count(10, maxsize+5)),
+                         list(range(10, 10+3*(maxsize+5), maxsize+5)))
+        self.assertEqual(take(3, count(2, 1.25)), [2, 3.25, 4.5])
         self.assertEqual(take(3, count(2, 3.25-4j)), [2, 5.25-4j, 8.5-8j])
         self.assertEqual(take(3, count(Decimal('1.1'), Decimal('.1'))),
                          [Decimal('1.1'), Decimal('1.2'), Decimal('1.3')])
         self.assertEqual(take(3, count(Fraction(2,3), Fraction(1,7))),
                          [Fraction(2,3), Fraction(17,21), Fraction(20,21)])
+        BIGINT = 1<<1000
+        self.assertEqual(take(3, count(step=BIGINT)), [0, BIGINT, 2*BIGINT])
         self.assertEqual(repr(take(3, count(10, 2.5))), repr([10, 12.5, 15.0]))
         c = count(3, 5)
         self.assertEqual(repr(c), 'count(3, 5)')
@@ -576,6 +590,10 @@ class TestBasicOps(unittest.TestCase):
         self.assertEqual(repr(count(10.5, 1.25)), 'count(10.5, 1.25)')
         self.assertEqual(repr(count(10.5, 1)), 'count(10.5)')           # suppress step=1 when it's an int
         self.assertEqual(repr(count(10.5, 1.00)), 'count(10.5, 1.0)')   # do show float values lilke 1.0
+        self.assertEqual(repr(count(10, 1.00)), 'count(10, 1.0)')
+        c = count(10, 1.0)
+        self.assertEqual(type(next(c)), int)
+        self.assertEqual(type(next(c)), float)
         for i in (-sys.maxsize-5, -sys.maxsize+5 ,-10, -1, 0, 10, sys.maxsize-5, sys.maxsize+5):
             for j in  (-sys.maxsize-5, -sys.maxsize+5 ,-10, -1, 0, 1, 10, sys.maxsize-5, sys.maxsize+5):
                 # Test repr

Misc/NEWS

@@ -65,6 +65,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #28019: itertools.count() no longer rounds non-integer step in range
+  between 1.0 and 2.0 to 1.
+
 - Issue #25969: Update the lib2to3 grammar to handle the unpacking
   generalizations added in 3.5.
 

Modules/itertoolsmodule.c

@@ -3905,7 +3905,7 @@ static PyObject *
 count_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 {
     countobject *lz;
-    int slow_mode = 0;
+    int fast_mode;
     Py_ssize_t cnt = 0;
     PyObject *long_cnt = NULL;
     PyObject *long_step = NULL;
@@ -3922,16 +3922,26 @@ count_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
                     return NULL;
     }
 
+    fast_mode = (long_cnt == NULL || PyLong_Check(long_cnt)) &&
+                (long_step == NULL || PyLong_Check(long_step));
+
+    /* If not specified, start defaults to 0 */
     if (long_cnt != NULL) {
-        cnt = PyLong_AsSsize_t(long_cnt);
-        if ((cnt == -1 && PyErr_Occurred()) || !PyLong_Check(long_cnt)) {
-            PyErr_Clear();
-            slow_mode = 1;
+        if (fast_mode) {
+            assert(PyLong_Check(long_cnt));
+            cnt = PyLong_AsSsize_t(long_cnt);
+            if (cnt == -1 && PyErr_Occurred()) {
+                PyErr_Clear();
+                fast_mode = 0;
+            }
         }
         Py_INCREF(long_cnt);
     } else {
         cnt = 0;
         long_cnt = PyLong_FromLong(0);
+        if (long_cnt == NULL) {
+            return NULL;
+        }
     }
 
     /* If not specified, step defaults to 1 */
@@ -3947,21 +3957,24 @@ count_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     assert(long_cnt != NULL && long_step != NULL);
 
     /* Fast mode only works when the step is 1 */
-    step = PyLong_AsLong(long_step);
-    if (step != 1) {
-        slow_mode = 1;
-        if (step == -1 && PyErr_Occurred())
-            PyErr_Clear();
+    if (fast_mode) {
+        assert(PyLong_Check(long_step));
+        step = PyLong_AsLong(long_step);
+        if (step != 1) {
+            fast_mode = 0;
+            if (step == -1 && PyErr_Occurred())
+                PyErr_Clear();
+        }
     }
 
-    if (slow_mode)
-        cnt = PY_SSIZE_T_MAX;
-    else
+    if (fast_mode)
         Py_CLEAR(long_cnt);
+    else
+        cnt = PY_SSIZE_T_MAX;
 
-    assert((cnt != PY_SSIZE_T_MAX && long_cnt == NULL && !slow_mode) ||
-           (cnt == PY_SSIZE_T_MAX && long_cnt != NULL && slow_mode));
-    assert(slow_mode ||
+    assert((cnt != PY_SSIZE_T_MAX && long_cnt == NULL && fast_mode) ||
+           (cnt == PY_SSIZE_T_MAX && long_cnt != NULL && !fast_mode));
+    assert(!fast_mode ||
            (PyLong_Check(long_step) && PyLong_AS_LONG(long_step) == 1));
 
     /* create countobject structure */