Fix a flaky test in ApproximateQuantilesTest (#39132)

Author: shunping

sdks/python/apache_beam/transforms/stats_test.py

@@ -264,6 +264,10 @@ def _approx_quantile_generator(size, num_of_quantiles, absoluteError):
     quantiles.append(size - 1)
     return quantiles
 
+  @staticmethod
+  def _sum_and_second(x):
+    return (sum(x), x[1])
+
   def test_quantiles_globaly(self):
     with TestPipeline() as p:
       pc = p | Create(list(range(101)))
@@ -490,22 +494,32 @@ def test_batched_quantiles(self):
               3, input_batched=True))
       with_key = (
           pc | 'Globally with key' >> beam.ApproximateQuantiles.Globally(
-              3, key=sum, input_batched=True))
+              3,
+              key=ApproximateQuantilesTest._sum_and_second,
+              input_batched=True))
       key_with_reversed = (
           pc | 'Globally with key and reversed' >>
           beam.ApproximateQuantiles.Globally(
-              3, key=sum, reverse=True, input_batched=True))
+              3,
+              key=ApproximateQuantilesTest._sum_and_second,
+              reverse=True,
+              input_batched=True))
       assert_that(
           globally,
           equal_to([[(0.0, 500), (49.9, 1), (99.9, 499)]]),
           label='checkGlobally')
+      # When key is present, both (72.5, 225) and (22.5, 275) produce the exact same
+      # sum (297.5). If we just use key=sum, tie-breaking is sensitive to bundle merging
+      # order and shared class-level jitter state, leading to flaky test failures.
+      # With the secondary key (defined in _sum_and_second), we can break ties
+      # deterministically.
       assert_that(
           with_key,
           equal_to([[(50.0, 0), (72.5, 225), (99.9, 499)]]),
           label='checkGloballyWithKey')
       assert_that(
           key_with_reversed,
-          equal_to([[(99.9, 499), (72.5, 225), (50.0, 0)]]),
+          equal_to([[(99.9, 499), (22.5, 275), (50.0, 0)]]),
           label='checkGloballyWithKeyAndReversed')
 
   def test_batched_weighted_quantiles(self):