Issue #2487. math.ldexp(x, n) raised OverflowError when n was large and

mark.dickinson · mark.dickinson · commit 822e129ef53c · 2008-05-09T17:54:23.000Z
negative; fix to return an (appropriately signed) zero instead. git-svn-id: http://svn.python.org/projects/python/trunk@62948 6015fed2-1504-0410-9fe1-9d1591cc4771
diff --git a/Lib/test/test_math.py b/Lib/test/test_math.py
@@ -374,6 +374,24 @@ def testLdexp(self):
         self.assertEquals(math.ldexp(NINF, -213), NINF)
         self.assert_(math.isnan(math.ldexp(NAN, 0)))
 
+        # large second argument
+        for n in [10**5, 10L**5, 10**10, 10L**10, 10**20, 10**40]:
+            self.assertEquals(math.ldexp(INF, -n), INF)
+            self.assertEquals(math.ldexp(NINF, -n), NINF)
+            self.assertEquals(math.ldexp(1., -n), 0.)
+            self.assertEquals(math.ldexp(-1., -n), -0.)
+            self.assertEquals(math.ldexp(0., -n), 0.)
+            self.assertEquals(math.ldexp(-0., -n), -0.)
+            self.assert_(math.isnan(math.ldexp(NAN, -n)))
+
+            self.assertRaises(OverflowError, math.ldexp, 1., n)
+            self.assertRaises(OverflowError, math.ldexp, -1., n)
+            self.assertEquals(math.ldexp(0., n), 0.)
+            self.assertEquals(math.ldexp(-0., n), -0.)
+            self.assertEquals(math.ldexp(INF, n), INF)
+            self.assertEquals(math.ldexp(NINF, n), NINF)
+            self.assert_(math.isnan(math.ldexp(NAN, n)))
+
     def testLog(self):
         self.assertRaises(TypeError, math.log)
         self.ftest('log(1/e)', math.log(1/math.e), -1)
diff --git a/Misc/NEWS b/Misc/NEWS
@@ -23,6 +23,11 @@ Extension Modules
 Library
 -------
 
+- Issue #2487: change the semantics of math.ldexp(x, n) when n is too
+  large to fit in a C long.  ldexp(x, n) now returns a zero (with
+  suitable sign) if n is large and negative; previously, it raised
+  OverflowError.
+
 - The toaiff module has been deprecated for removal in Python 3.0.
 
 - The test.testall module has been deprecated for removal in Python 3.0.
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
@@ -349,23 +349,65 @@ static PyObject *
 math_ldexp(PyObject *self, PyObject *args)
 {
 	double x, r;
-	int exp;
-	if (! PyArg_ParseTuple(args, "di:ldexp", &x, &exp))
+	PyObject *oexp;
+	long exp;
+	if (! PyArg_ParseTuple(args, "dO:ldexp", &x, &oexp))
 		return NULL;
-	errno = 0;
-	PyFPE_START_PROTECT("in math_ldexp", return 0)
-	r = ldexp(x, exp);
-	PyFPE_END_PROTECT(r)
-	if (Py_IS_FINITE(x) && Py_IS_INFINITY(r))
+
+	if (PyLong_Check(oexp)) {
+		/* on overflow, replace exponent with either LONG_MAX
+		   or LONG_MIN, depending on the sign. */
+		exp = PyLong_AsLong(oexp);
+		if (exp == -1 && PyErr_Occurred()) {
+			if (PyErr_ExceptionMatches(PyExc_OverflowError)) {
+				if (Py_SIZE(oexp) < 0) {
+					exp = LONG_MIN;
+				}
+				else {
+					exp = LONG_MAX;
+				}
+				PyErr_Clear();
+			}
+			else {
+				/* propagate any unexpected exception */
+				return NULL;
+			}
+		}
+	}
+	else if (PyInt_Check(oexp)) {
+		exp = PyInt_AS_LONG(oexp);
+	}
+	else {
+		PyErr_SetString(PyExc_TypeError,
+				"Expected an int or long as second argument "
+				"to ldexp.");
+		return NULL;
+	}
+
+	if (x == 0. || !Py_IS_FINITE(x)) {
+		/* NaNs, zeros and infinities are returned unchanged */
+		r = x;
+		errno = 0;
+	} else if (exp > INT_MAX) {
+		/* overflow */
+		r = copysign(Py_HUGE_VAL, x);
 		errno = ERANGE;
-	/* Windows MSVC8 sets errno = EDOM on ldexp(NaN, i);
-	   we unset it to avoid raising a ValueError here. */
-	if (errno == EDOM)
+	} else if (exp < INT_MIN) {
+		/* underflow to +-0 */
+		r = copysign(0., x);
 		errno = 0;
+	} else {
+		errno = 0;
+		PyFPE_START_PROTECT("in math_ldexp", return 0);
+		r = ldexp(x, (int)exp);
+		PyFPE_END_PROTECT(r);
+		if (Py_IS_INFINITY(r))
+			errno = ERANGE;
+	}
+
 	if (errno && is_error(r))
 		return NULL;
-	else
-		return PyFloat_FromDouble(r);
+	return PyFloat_FromDouble(r);
 }
 
 PyDoc_STRVAR(math_ldexp_doc,
