gh-113841: fix possible undefined division by 0 in _Py_c_pow() (GH-127211)

skirpichev · web-flow · commit f7bb658124ab · 2024-11-23T23:37:37.000-08:00
`x**y == 1/x**-y ` thus changing `/=` to `*=` by negating the exponent.
diff --git a/Lib/test/test_complex.py b/Lib/test/test_complex.py
@@ -338,6 +338,11 @@ def test_pow(self):
                     except OverflowError:
                         pass
 
+        # gh-113841: possible undefined division by 0 in _Py_c_pow()
+        x, y = 9j, 33j**3
+        with self.assertRaises(OverflowError):
+            x**y
+
     def test_pow_with_small_integer_exponents(self):
         # Check that small integer exponents are handled identically
         # regardless of their type.
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2024-11-24-07-01-28.gh-issue-113841.WFg-Bu.rst b/Misc/NEWS.d/next/Core_and_Builtins/2024-11-24-07-01-28.gh-issue-113841.WFg-Bu.rst
@@ -0,0 +1,2 @@
+Fix possible undefined behavior division by zero in :class:`complex`'s
+:c:func:`_Py_c_pow`.
diff --git a/Objects/complexobject.c b/Objects/complexobject.c
@@ -168,7 +168,7 @@ _Py_c_pow(Py_complex a, Py_complex b)
         at = atan2(a.imag, a.real);
         phase = at*b.real;
         if (b.imag != 0.0) {
-            len /= exp(at*b.imag);
+            len *= exp(-at*b.imag);
             phase += b.imag*log(vabs);
         }
         r.real = len*cos(phase);

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	+Fix possible undefined behavior division by zero in :class:`complex`'s
	`2`	+:c:func:`_Py_c_pow`.
Original file line number	Diff line number	Diff line change
`@@ -168,7 +168,7 @@ _Py_c_pow(Py_complex a, Py_complex b)`
`168`	`168`	`at = atan2(a.imag, a.real);`
`169`	`169`	`phase = at*b.real;`
`170`	`170`	`if (b.imag != 0.0) {`
`171`		`- len /= exp(at*b.imag);`
	`171`	`+ len = exp(-atb.imag);`
`172`	`172`	`phase += b.imag*log(vabs);`
`173`	`173`	`}`
`174`	`174`	`r.real = len*cos(phase);`