round_trace: don't trace ndigits if it's the default value

my2to3/fixes/fix_round_trace.py

@@ -18,7 +18,7 @@ class FixRoundTrace(BaseDynamicTraceFix):
   def _dynamic_trace(number, ndigits=0):
     return (
       round(number, ndigits),
-      (parse_type(type(number)), parse_type(type(ndigits)))
+      (parse_type(type(number)), None if ndigits == 0 else parse_type(type(ndigits)))
     )
 
   # Inspired by https://github.com/python/cpython/blob/e42b705188271da108de42b55d9344642170aa2b/Lib/lib2to3/fixes/fix_xrange.py#L14

my2to3/tests/testFixRoundTrace.py

+import unittest
+
+from . import FixerTestCase
+
+
+class testFixRoundTrace(FixerTestCase):
+  fixer = "round_trace"
+
+  def test_single_parameter(self):
+    b = """round(1.23)"""
+    a = """round_traced("<string>",1,0,1.23)"""
+    self.check(b, a)
+
+    self.exec_code(a)
+    self.assertDataEqual("round_modified", [(u'<string>', 1, 0)])
+    self.assertDataEqual("round_trace", [(u'<string>', 1, 0, u'float', None)])
+
+  def test_both_parameters(self):
+    b = """round(1.23, 4)"""
+    a = """round_traced("<string>",1,0,1.23, 4)"""
+    self.check(b, a)
+
+    self.exec_code(a)
+    self.assertDataEqual("round_modified", [(u'<string>', 1, 0)])
+    self.assertDataEqual("round_trace", [(u'<string>', 1, 0, u'float', u'int')])
+
+
+if __name__ == '__main__':
+  unittest.main()