bpo-35606: Implement math.prod (GH-11359)

Author: pablogsal

Doc/library/math.rst

@@ -178,6 +178,18 @@ Number-theoretic and representation functions
    of *x* and are floats.
 
 
+.. function:: prod(iterable, *, start=1)
+
+   Calculate the product of all the elements in the input *iterable*.
+   The default *start* value for the product is ``1``.
+
+   When the iterable is empty, return the start value.  This function is
+   intended specifically for use with numeric values and may reject
+   non-numeric types.
+
+   .. versionadded:: 3.8
+
+
 .. function:: remainder(x, y)
 
    Return the IEEE 754-style remainder of *x* with respect to *y*.  For

Doc/whatsnew/3.8.rst

@@ -171,6 +171,15 @@ json.tool
 Add option ``--json-lines`` to parse every input line as separate JSON object.
 (Contributed by Weipeng Hong in :issue:`31553`.)
 
+
+math
+----
+
+Added new function, :func:`math.prod`, as analogous function to :func:`sum`
+that returns the product of a 'start' value (default: 1) times an iterable of
+numbers. (Contributed by Pablo Galindo in :issue:`issue35606`)
+
+
 os.path
 -------
 

Lib/test/test_math.py

@@ -1724,6 +1724,37 @@ def test_fractions(self):
         self.assertAllClose(fraction_examples, rel_tol=1e-8)
         self.assertAllNotClose(fraction_examples, rel_tol=1e-9)
 
+    def test_prod(self):
+        prod = math.prod
+        self.assertEqual(prod([]), 1)
+        self.assertEqual(prod([], start=5), 5)
+        self.assertEqual(prod(list(range(2,8))), 5040)
+        self.assertEqual(prod(iter(list(range(2,8)))), 5040)
+        self.assertEqual(prod(range(1, 10), start=10), 3628800)
+
+        self.assertEqual(prod([1, 2, 3, 4, 5]), 120)
+        self.assertEqual(prod([1.0, 2.0, 3.0, 4.0, 5.0]), 120.0)
+        self.assertEqual(prod([1, 2, 3, 4.0, 5.0]), 120.0)
+        self.assertEqual(prod([1.0, 2.0, 3.0, 4, 5]), 120.0)
+
+        # Test overflow in fast-path for integers
+        self.assertEqual(prod([1, 1, 2**32, 1, 1]), 2**32)
+        # Test overflow in fast-path for floats
+        self.assertEqual(prod([1.0, 1.0, 2**32, 1, 1]), float(2**32))
+
+        self.assertRaises(TypeError, prod)
+        self.assertRaises(TypeError, prod, 42)
+        self.assertRaises(TypeError, prod, ['a', 'b', 'c'])
+        self.assertRaises(TypeError, prod, ['a', 'b', 'c'], '')
+        self.assertRaises(TypeError, prod, [b'a', b'c'], b'')
+        values = [bytearray(b'a'), bytearray(b'b')]
+        self.assertRaises(TypeError, prod, values, bytearray(b''))
+        self.assertRaises(TypeError, prod, [[1], [2], [3]])
+        self.assertRaises(TypeError, prod, [{2:3}])
+        self.assertRaises(TypeError, prod, [{2:3}]*2, {2:3})
+        self.assertRaises(TypeError, prod, [[1], [2], [3]], [])
+        with self.assertRaises(TypeError):
+            prod([10, 20], [30, 40])     # start is a keyword-only argument
 
 def test_main():
     from doctest import DocFileSuite

Misc/NEWS.d/next/Library/2018-12-29-21-59-03.bpo-35606.NjGjou.rst

@@ -0,0 +1,3 @@
+Implement :func:`math.prod` as analogous function to :func:`sum` that
+returns the product of a 'start' value (default: 1) times an iterable of
+numbers. Patch by Pablo Galindo.

Modules/clinic/mathmodule.c.h

@@ -2494,6 +2494,172 @@ math_isclose_impl(PyObject *module, double a, double b, double rel_tol,
 }
 
 
+/*[clinic input]
+math.prod
+
+    iterable: object
+    /
+    *
+    start: object(c_default="NULL") = 1
+
+Calculate the product of all the elements in the input iterable.
+
+The default start value for the product is 1.
+
+When the iterable is empty, return the start value.  This function is
+intended specifically for use with numeric values and may reject
+non-numeric types.
+[clinic start generated code]*/
+
+static PyObject *
+math_prod_impl(PyObject *module, PyObject *iterable, PyObject *start)
+/*[clinic end generated code: output=36153bedac74a198 input=4c5ab0682782ed54]*/
+{
+    PyObject *result = start;
+    PyObject *temp, *item, *iter;
+
+    iter = PyObject_GetIter(iterable);
+    if (iter == NULL) {
+        return NULL;
+    }
+
+    if (result == NULL) {
+        result = PyLong_FromLong(1);
+        if (result == NULL) {
+            Py_DECREF(iter);
+            return NULL;
+        }
+    } else {
+        Py_INCREF(result);
+    }
+#ifndef SLOW_PROD
+    /* Fast paths for integers keeping temporary products in C.
+     * Assumes all inputs are the same type.
+     * If the assumption fails, default to use PyObjects instead.
+    */
+    if (PyLong_CheckExact(result)) {
+        int overflow;
+        long i_result = PyLong_AsLongAndOverflow(result, &overflow);
+        /* If this already overflowed, don't even enter the loop. */
+        if (overflow == 0) {
+            Py_DECREF(result);
+            result = NULL;
+        }
+        /* Loop over all the items in the iterable until we finish, we overflow
+         * or we found a non integer element */
+        while(result == NULL) {
+            item = PyIter_Next(iter);
+            if (item == NULL) {
+                Py_DECREF(iter);
+                if (PyErr_Occurred()) {
+                    return NULL;
+                }
+                return PyLong_FromLong(i_result);
+            }
+            if (PyLong_CheckExact(item)) {
+                long b = PyLong_AsLongAndOverflow(item, &overflow);
+                long x = i_result * b;
+                /* Continue if there is no overflow */
+                if (overflow == 0
+                    && x < INT_MAX && x > INT_MIN
+                    && !(b != 0 && x / i_result != b)) {
+                    i_result = x;
+                    Py_DECREF(item);
+                    continue;
+                }
+            }
+            /* Either overflowed or is not an int.
+             * Restore real objects and process normally */
+            result = PyLong_FromLong(i_result);
+            if (result == NULL) {
+                Py_DECREF(item);
+                Py_DECREF(iter);
+                return NULL;
+            }
+            temp = PyNumber_Multiply(result, item);
+            Py_DECREF(result);
+            Py_DECREF(item);
+            result = temp;
+            if (result == NULL) {
+                Py_DECREF(iter);
+                return NULL;
+            }
+        }
+    }
+
+    /* Fast paths for floats keeping temporary products in C.
+     * Assumes all inputs are the same type.
+     * If the assumption fails, default to use PyObjects instead.
+    */
+    if (PyFloat_CheckExact(result)) {
+        double f_result = PyFloat_AS_DOUBLE(result);
+        Py_DECREF(result);
+        result = NULL;
+        while(result == NULL) {
+            item = PyIter_Next(iter);
+            if (item == NULL) {
+                Py_DECREF(iter);
+                if (PyErr_Occurred()) {
+                    return NULL;
+                }
+                return PyFloat_FromDouble(f_result);
+            }
+            if (PyFloat_CheckExact(item)) {
+                f_result *= PyFloat_AS_DOUBLE(item);
+                Py_DECREF(item);
+                continue;
+            }
+            if (PyLong_CheckExact(item)) {
+                long value;
+                int overflow;
+                value = PyLong_AsLongAndOverflow(item, &overflow);
+                if (!overflow) {
+                    f_result *= (double)value;
+                    Py_DECREF(item);
+                    continue;
+                }
+            }
+            result = PyFloat_FromDouble(f_result);
+            if (result == NULL) {
+                Py_DECREF(item);
+                Py_DECREF(iter);
+                return NULL;
+            }
+            temp = PyNumber_Multiply(result, item);
+            Py_DECREF(result);
+            Py_DECREF(item);
+            result = temp;
+            if (result == NULL) {
+                Py_DECREF(iter);
+                return NULL;
+            }
+        }
+    }
+#endif
+    /* Consume rest of the iterable (if any) that could not be handled
+     * by specialized functions above.*/
+    for(;;) {
+        item = PyIter_Next(iter);
+        if (item == NULL) {
+            /* error, or end-of-sequence */
+            if (PyErr_Occurred()) {
+                Py_DECREF(result);
+                result = NULL;
+            }
+            break;
+        }
+        temp = PyNumber_Multiply(result, item);
+        Py_DECREF(result);
+        Py_DECREF(item);
+        result = temp;
+        if (result == NULL)
+            break;
+    }
+    Py_DECREF(iter);
+    return result;
+}
+
+
 static PyMethodDef math_methods[] = {
     {"acos",            math_acos,      METH_O,         math_acos_doc},
     {"acosh",           math_acosh,     METH_O,         math_acosh_doc},
@@ -2541,6 +2707,7 @@ static PyMethodDef math_methods[] = {
     {"tan",             math_tan,       METH_O,         math_tan_doc},
     {"tanh",            math_tanh,      METH_O,         math_tanh_doc},
     MATH_TRUNC_METHODDEF
+    MATH_PROD_METHODDEF
     {NULL,              NULL}           /* sentinel */
 };