Use _Py_c_pow() to test implementation

Author: skirpichev

Lib/test/test_complex.py

@@ -6,10 +6,10 @@
 
 from random import random
 from math import atan2, isnan, copysign
-from cmath import log, exp, isclose, isnan as cisnan
 from functools import reduce
 from itertools import combinations
 import operator
+import _testcapi
 
 INF = float("inf")
 NAN = float("nan")
@@ -345,18 +345,32 @@ def test_pow_with_small_integer_exponents(self):
                 except OverflowError:
                     continue
                 r_pro = reduce(lambda x, y: x*y, [z]*e) if e else 1+0j
-                test = str(r_pow) == str(r_pro)
-                if not test:
-                    # We might fail here, because associativity of multiplication
-                    # is broken already for floats.
-                    # Consider z = 1-1j.  Then z*z*z*z = ((z*z)*z)*z = -4+0j,
-                    # while in the algorithm for pow() a diffenent grouping
-                    # of operations is used: z**4 = (z*z)*(z*z) = -4-0j.
-                    r_pro = exp(e*log(z))
-                self.assertTrue(test or isclose(r_pow, r_pro))
-                if not cisnan(r_pow):
-                    self.assertEqual(copysign(1, r_pow.real), copysign(1, r_pro.real))
-                    self.assertEqual(copysign(1, r_pow.imag), copysign(1, r_pro.imag))
+                if str(r_pow) == str(r_pro):
+                    continue
+
+                self.assertNotIn(z.real, {0.0, -0.0, INF, -INF, NAN})
+                self.assertNotIn(z.imag, {0.0, -0.0, INF, -INF, NAN})
+
+                # We might fail here, because associativity of multiplication
+                # is broken already for floats.
+                # Consider z = 1-1j.  Then z*z*z*z = ((z*z)*z)*z = -4+0j,
+                # while in the algorithm for pow() a diffenent grouping
+                # of operations is used: z**4 = (z*z)*(z*z) = -4-0j.
+                #
+                # Fallback to the generic complex power algorithm.
+                r_pro, r_pro_errno = _testcapi._py_c_pow(z, e)
+                self.assertEqual(r_pro_errno, 0)
+                self.assertClose(r_pow, r_pro)
+                if not isnan(r_pow.real):
+                    self.assertEqual(copysign(1, r_pow.real),
+                                     copysign(1, r_pro.real))
+                else:
+                    self.assertTrue(isnan(r_pro.real))
+                if not isnan(r_pow.imag):
+                    self.assertEqual(copysign(1, r_pow.imag),
+                                     copysign(1, r_pro.imag))
+                else:
+                    self.assertTrue(isnan(r_pro.imag))
 
     def test_boolcontext(self):
         for i in range(100):