Issue #9920: Skip tests for cmath.atan and cmath.atanh applied to

complex zeros on systems where the log1p function fails to respect
the sign of zero.  This fixes a test failure on AIX.

Lib/test/cmath_testcases.txt

@@ -733,10 +733,11 @@ asinh1036 asinh -inf inf -> -inf 0.78539816339744828
 ---------------------------
 
 -- zeros
-atan0000 atan 0.0 0.0 -> 0.0 0.0
-atan0001 atan 0.0 -0.0 -> 0.0 -0.0
-atan0002 atan -0.0 0.0 -> -0.0 0.0
-atan0003 atan -0.0 -0.0 -> -0.0 -0.0
+-- These are tested in testAtanSign in test_cmath.py
+-- atan0000 atan 0.0 0.0 -> 0.0 0.0
+-- atan0001 atan 0.0 -0.0 -> 0.0 -0.0
+-- atan0002 atan -0.0 0.0 -> -0.0 0.0
+-- atan0003 atan -0.0 -0.0 -> -0.0 -0.0
 
 -- values along both sides of imaginary axis
 atan0010 atan 0.0 -9.8813129168249309e-324 -> 0.0 -9.8813129168249309e-324
@@ -896,10 +897,11 @@ atan1040 atan -inf -inf -> -1.5707963267948966 -0.0
 ---------------------------------------
 
 -- zeros
-atanh0000 atanh 0.0 0.0 -> 0.0 0.0
-atanh0001 atanh 0.0 -0.0 -> 0.0 -0.0
-atanh0002 atanh -0.0 0.0 -> -0.0 0.0
-atanh0003 atanh -0.0 -0.0 -> -0.0 -0.0
+-- These are tested in testAtanhSign in test_cmath.py
+-- atanh0000 atanh 0.0 0.0 -> 0.0 0.0
+-- atanh0001 atanh 0.0 -0.0 -> 0.0 -0.0
+-- atanh0002 atanh -0.0 0.0 -> -0.0 0.0
+-- atanh0003 atanh -0.0 -0.0 -> -0.0 -0.0
 
 -- values along both sides of real axis
 atanh0010 atanh -9.8813129168249309e-324 0.0 -> -9.8813129168249309e-324 0.0

Lib/test/test_cmath.py

@@ -514,6 +514,24 @@ class CMathTests(unittest.TestCase):
         for z in complex_zeros:
             self.assertComplexIdentical(cmath.tanh(z), z)
 
+    # The algorithm used for atan and atanh makes use of the system
+    # log1p function; If that system function doesn't respect the sign
+    # of zero, then atan and atanh will also have difficulties with
+    # the sign of complex zeros.
+    @requires_IEEE_754
+    @unittest.skipIf(sysconfig.get_config_var('LOG1P_DROPS_ZERO_SIGN'),
+                     "system log1p() function doesn't preserve the sign")
+    def testAtanSign(self):
+        for z in complex_zeros:
+            self.assertComplexIdentical(cmath.atan(z), z)
+
+    @requires_IEEE_754
+    @unittest.skipIf(sysconfig.get_config_var('LOG1P_DROPS_ZERO_SIGN'),
+                     "system log1p() function doesn't preserve the sign")
+    def testAtanhSign(self):
+        for z in complex_zeros:
+            self.assertComplexIdentical(cmath.atanh(z), z)
+
 
 def test_main():
     run_unittest(CMathTests)

Misc/NEWS

@@ -21,6 +21,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #9920: Skip tests for cmath.atan and cmath.atanh applied to
+  complex zeros on systems where the log1p function fails to respect
+  the sign of zero.  This fixes a test failure on AIX.
+
 - Issue #10446: Module documentation generated by pydoc now links to a
   version-specific online reference manual.