bpo-44698: Restore complex pow behaviour for small integral exponents (GH-27772)

Author: mdickinson

Lib/test/test_complex.py

@@ -269,6 +269,34 @@ def test_pow(self):
  except OverflowError:
  pass
 
+ def test_pow_with_small_integer_exponents(self):
+ # Check that small integer exponents are handled identically
+ # regardless of their type.
+ values = [
+ complex(5.0, 12.0),
+ complex(5.0e100, 12.0e100),
+ complex(-4.0, INF),
+ complex(INF, 0.0),
+ ]
+ exponents = [-19, -5, -3, -2, -1, 0, 1, 2, 3, 5, 19]
+ for value in values:
+ for exponent in exponents:
+ with self.subTest(value=value, exponent=exponent):
+ try:
+ int_pow = value**exponent
+ except OverflowError:
+ int_pow = "overflow"
+ try:
+ float_pow = value**float(exponent)
+ except OverflowError:
+ float_pow = "overflow"
+ try:
+ complex_pow = value**complex(exponent)
+ except OverflowError:
+ complex_pow = "overflow"
+ self.assertEqual(str(float_pow), str(int_pow))
+ self.assertEqual(str(complex_pow), str(int_pow))
+
  def test_boolcontext(self):
  for i in range(100):
  self.assertTrue(complex(random() + 1e-6, random() + 1e-6))

Misc/NEWS.d/next/Core and Builtins/2021-08-15-10-39-06.bpo-44698.lITKNc.rst

@@ -0,0 +1,2 @@
+Restore behaviour of complex exponentiation with integer-valued exponent of
+type :class:`float` or :class:`complex`.

Objects/complexobject.c

@@ -172,14 +172,7 @@ c_powu(Py_complex x, long n)
 static Py_complex
 c_powi(Py_complex x, long n)
 {
- Py_complex cn;
-
- if (n > 100 || n < -100) {
- cn.real = (double) n;
- cn.imag = 0.;
- return _Py_c_pow(x,cn);
- }
- else if (n > 0)
+ if (n > 0)
  return c_powu(x,n);
  else
  return _Py_c_quot(c_1, c_powu(x,-n));
@@ -523,19 +516,12 @@ complex_pow(PyObject *v, PyObject *w, PyObject *z)
  return NULL;
  }
  errno = 0;
- // Check if w is an integer value that fits inside a C long, so we can
- // use a faster algorithm. TO_COMPLEX(w, b), above, already handled the
- // conversion from larger longs, as well as other types.
- if (PyLong_Check(w)) {
- int overflow = 0;
- long int_exponent = PyLong_AsLongAndOverflow(w, &overflow);
- if (int_exponent == -1 && PyErr_Occurred())
- return NULL;
- if (overflow == 0)
- p = c_powi(a, int_exponent);
- else
- p = _Py_c_pow(a, b);
- } else {
+ // Check whether the exponent has a small integer value, and if so use
+ // a faster and more accurate algorithm.
+ if (b.imag == 0.0 && b.real == floor(b.real) && fabs(b.real) <= 100.0) {
+ p = c_powi(a, (long)b.real);
+ }
+ else {
  p = _Py_c_pow(a, b);
  }