Allow printing a leading '-' and the maximum number of exponent digits

rather than raising RuntimeError (allocated space is sufficient for the
additional character).

Lib/test/test_decimal.py

@@ -4977,6 +4977,54 @@ class CWhitebox(unittest.TestCase):
             x = "1e%d" % (-sys.maxsize-1)
             self.assertRaises(InvalidOperation, Decimal, x)
 
+    def test_from_tuple(self):
+        Decimal = C.Decimal
+        localcontext = C.localcontext
+        InvalidOperation = C.InvalidOperation
+        Overflow = C.Overflow
+        Underflow = C.Underflow
+
+        with localcontext() as c:
+
+            c.traps[InvalidOperation] = True
+            c.traps[Overflow] = True
+            c.traps[Underflow] = True
+
+            # SSIZE_MAX
+            x = (1, (), sys.maxsize)
+            self.assertEqual(str(c.create_decimal(x)), '-0E+999999')
+            self.assertRaises(InvalidOperation, Decimal, x)
+
+            x = (1, (0, 1, 2), sys.maxsize)
+            self.assertRaises(Overflow, c.create_decimal, x)
+            self.assertRaises(InvalidOperation, Decimal, x)
+
+            # SSIZE_MIN
+            x = (1, (), -sys.maxsize-1)
+            self.assertEqual(str(c.create_decimal(x)), '-0E-1000026')
+            self.assertRaises(InvalidOperation, Decimal, x)
+
+            x = (1, (0, 1, 2), -sys.maxsize-1)
+            self.assertRaises(Underflow, c.create_decimal, x)
+            self.assertRaises(InvalidOperation, Decimal, x)
+
+            # OverflowError
+            x = (1, (), sys.maxsize+1)
+            self.assertRaises(OverflowError, c.create_decimal, x)
+            self.assertRaises(OverflowError, Decimal, x)
+
+            x = (1, (), -sys.maxsize-2)
+            self.assertRaises(OverflowError, c.create_decimal, x)
+            self.assertRaises(OverflowError, Decimal, x)
+
+            # Specials
+            x = (1, (), "N")
+            self.assertEqual(str(Decimal(x)), '-sNaN')
+            x = (1, (0,), "N")
+            self.assertEqual(str(Decimal(x)), '-sNaN')
+            x = (1, (0, 1), "N")
+            self.assertEqual(str(Decimal(x)), '-sNaN1')
+
 
 all_tests = [
   CExplicitConstructionTest, PyExplicitConstructionTest,

Modules/_decimal/_decimal.c

@@ -2435,8 +2435,8 @@ dectuple_as_str(PyObject *dectuple)
     if (sign_special[1] == '\0') {
         /* not a special number */
         *cp++ = 'E';
-        n = snprintf(cp, MPD_EXPDIGITS+1, "%" PRI_mpd_ssize_t, exp);
+        n = snprintf(cp, MPD_EXPDIGITS+2, "%" PRI_mpd_ssize_t, exp);
-        if (n < 0 || n >= MPD_EXPDIGITS+1) {
+        if (n < 0 || n >= MPD_EXPDIGITS+2) {
             PyErr_SetString(PyExc_RuntimeError,
                 "internal error in dec_sequence_as_str");
             goto error;