Add unit tests for functions in math_utils.py

tools/auto_bisect/math_utils_test.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import math
 import unittest
 
 import math_utils
 
 
 class MathUtilsTest(unittest.TestCase):
   """Tests for mathematical utility functions."""
 
-  def testTruncatedMeanRaisesError(self):
-    """TruncatedMean should raise an error when passed an empty list."""
-    with self.assertRaises(TypeError):
-      math_utils.TruncatedMean([], 0)
+  def testTruncatedMean_EmptyList(self):
+    # TruncatedMean raises an error when passed an empty list.
+    self.assertRaises(TypeError, math_utils.TruncatedMean, [], 0)
 
-  def testMeanSingleNum(self):
-    """Tests the Mean function with a single number."""
+  def testTruncatedMean_TruncateTooMuch(self):
+    # An exception is raised if 50% or more is truncated from both sides.
+    self.assertRaises(TypeError, math_utils.TruncatedMean, [1, 2, 3], 1.0)
+    self.assertRaises(
+        ZeroDivisionError, math_utils.TruncatedMean, [1, 2, 3], 0.5)
+
+  def testTruncatedMean_AlwaysKeepsAtLeastTwoValues(self):
+    # If the length of the input is 1 or 2, nothing is truncated and
+    # the average is returned.
+    self.assertEqual(5.0, math_utils.TruncatedMean([5.0], 0.0))
+    self.assertEqual(5.0, math_utils.TruncatedMean([5.0], 0.25))
+    self.assertEqual(5.0, math_utils.TruncatedMean([5.0], 0.5))
+    self.assertEqual(5.5, math_utils.TruncatedMean([5.0, 6.0], 0.0))
+    self.assertEqual(5.5, math_utils.TruncatedMean([5.0, 6.0], 0.25))
+    self.assertEqual(5.5, math_utils.TruncatedMean([5.0, 6.0], 0.5))
+
+  def testTruncatedMean_Interquartile_NumValuesDivisibleByFour(self):
+    self.assertEqual(5.0, math_utils.TruncatedMean([1, 4, 6, 100], 0.25))
+    self.assertEqual(
+        6.5, math_utils.TruncatedMean([1, 2, 5, 6, 7, 8, 40, 50], 0.25))
+
+  def testTruncatedMean_Weighting(self):
+    # In the list [0, 1, 4, 5, 20, 100], when 25% of the list at the start
+    # and end are discarded, the part that's left is [1, 4, 5, 20], but
+    # first and last values are weighted so that they only count for half
+    # as much. So the truncated mean is (1/2 + 4 + 5 + 20/2) / 5.0.
+    self.assertEqual(6.5, (0.5 + 4 + 5 + 10) / 3.0)
+    self.assertEqual(6.5, math_utils.TruncatedMean([0, 1, 4, 5, 20, 100], 0.25))
+
+  def testMean_OneValue(self):
     self.assertEqual(3.0, math_utils.Mean([3]))
 
-  def testMeanShortList(self):
-    """Tests the Mean function with a short list."""
+  def testMean_ShortList(self):
     self.assertEqual(0.5, math_utils.Mean([-3, 0, 1, 4]))
 
-  def testMeanCompareAlternateImplementation(self):
+  def testMean_CompareAlternateImplementation(self):
     """Tests Mean by comparing against an alternate implementation."""
-    def AlternateMeanFunction(values):
-      """Simple arithmetic mean function."""
+    def AlternateMean(values):
       return sum(values) / float(len(values))
-    test_values_lists = [[1], [5, 6.5, 1.2, 3], [-3, 0, 1, 4],
-                         [-3, -1, 0.12, 0.752, 3.33, 8, 16, 32, 439]]
-    for values in test_values_lists:
-      self.assertEqual(
-          AlternateMeanFunction(values),
-          math_utils.Mean(values))
+    test_value_lists = [
+        [1],
+        [5, 6.5, 1.2, 3],
+        [-3, 0, 1, 4],
+        [-3, -1, 0.12, 0.752, 3.33, 8, 16, 32, 439],
+    ]
+    for value_list in test_value_lists:
+      self.assertEqual(AlternateMean(value_list), math_utils.Mean(value_list))
 
-  def testRelativeChange(self):
-    """Tests the common cases for calculating relative change."""
+  def testRelativeChange_NonZero(self):
     # The change is relative to the first value, regardless of which is bigger.
     self.assertEqual(0.5, math_utils.RelativeChange(1.0, 1.5))
     self.assertEqual(0.5, math_utils.RelativeChange(2.0, 1.0))
 
-  def testRelativeChangeFromZero(self):
-    """Tests what happens when relative change from zero is calculated."""
+  def testRelativeChange_FromZero(self):
     # If the first number is zero, then the result is not a number.
     self.assertEqual(0, math_utils.RelativeChange(0, 0))
-    self.assertTrue(
-        math.isnan(math_utils.RelativeChange(0, 1)))
-    self.assertTrue(
-        math.isnan(math_utils.RelativeChange(0, -1)))
+    self.assertTrue(math.isnan(math_utils.RelativeChange(0, 1)))
+    self.assertTrue(math.isnan(math_utils.RelativeChange(0, -1)))
 
-  def testRelativeChangeWithNegatives(self):
-    """Tests that relative change given is always positive."""
+  def testRelativeChange_Negative(self):
+    # Note that the return value of RelativeChange is always positive.
     self.assertEqual(3.0, math_utils.RelativeChange(-1, 2))
     self.assertEqual(3.0, math_utils.RelativeChange(1, -2))
     self.assertEqual(1.0, math_utils.RelativeChange(-1, -2))
 
+  def testVariance_EmptyList(self):
+    self.assertRaises(TypeError, math_utils.Variance, [])
+
+  def testVariance_OneValue(self):
+    self.assertEqual(0, math_utils.Variance([0]))
+    self.assertEqual(0, math_utils.Variance([4.3]))
+
+  def testVariance_ShortList(self):
+    # Population variance is the average of squared deviations from the mean.
+    # The deviations from the mean in this example are [3.5, 0.5, -0.5, -3.5],
+    # and the squared deviations are [12.25, 0.25, 0.25, 12.25].
+    # With sample variance, however, 1 is subtracted from the sample size.
+    # So the sample variance is sum([12.25, 0.25, 0.25, 12.25]) / 3.0.
+    self.assertAlmostEqual(8.333333334, sum([12.25, 0.25, 0.25, 12.25]) / 3.0)
+    self.assertAlmostEqual(8.333333334, math_utils.Variance([-3, 0, 1, 4]))
+
+  def testStandardDeviation(self):
+    # Standard deviation is the square root of variance.
+    self.assertRaises(TypeError, math_utils.StandardDeviation, [])
+    self.assertEqual(0.0, math_utils.StandardDeviation([4.3]))
+    self.assertAlmostEqual(2.88675135, math.sqrt(8.33333333333333))
+    self.assertAlmostEqual(2.88675135,
+                           math_utils.StandardDeviation([-3, 0, 1, 4]))
+
+  def testStandardError(self):
+    # Standard error is std. dev. divided by square root of sample size.
+    self.assertEqual(0.0, math_utils.StandardError([]))
+    self.assertEqual(0.0, math_utils.StandardError([4.3]))
+    self.assertAlmostEqual(1.44337567, 2.88675135 / math.sqrt(4))
+    self.assertAlmostEqual(1.44337567, math_utils.StandardError([-3, 0, 1, 4]))
 
 if __name__ == '__main__':
   unittest.main()