Merge pull request #22149 from edx/feanil/update_rounding

BOM-738 - Review and update rounding for python 3

openedx/core/lib/grade_utils.py

@@ -1,6 +1,7 @@
 """
 Helpers functions for grades and scores.
 """
+import math
 
 
 def compare_scores(earned1, possible1, earned2, possible2, treat_undefined_as_zero=False):
@@ -42,3 +43,26 @@ def is_score_higher_or_equal(earned1, possible1, earned2, possible2, treat_undef
     """
     is_higher_or_equal, _, _ = compare_scores(earned1, possible1, earned2, possible2, treat_undefined_as_zero)
     return is_higher_or_equal
+
+
+def round_away_from_zero(number, digits=0):
+    """
+    Round numbers using the 'away from zero' strategy as opposed to the
+    'Banker's rounding strategy.'  The strategy refers to how we round when
+    a number is half way between two numbers.  eg. 0.5, 1.5, etc. In python 2
+    positive numbers in this category would be rounded up and negative numbers
+    would be rounded down. ie. away from zero.  In python 3 numbers round
+    towards even.  So 0.5 would round to 0 but 1.5 would round to 2.
+
+    See here for more on floating point rounding strategies:
+    https://en.wikipedia.org/wiki/IEEE_754#Rounding_rules
+
+    We want to continue to round away from zero so that student grades remain
+    consistent and don't suddenly change.
+    """
+    p = 10.0 ** digits
+
+    if number >= 0:
+        return float(math.floor((number * p) + 0.5)) / p
+    else:
+        return float(math.ceil((number * p) - 0.5)) / p

openedx/core/lib/tests/test_grade_utils.py

@@ -7,7 +7,7 @@ from unittest import TestCase
 
 import ddt
 
-from ..grade_utils import compare_scores
+from ..grade_utils import compare_scores, round_away_from_zero
 
 
 @ddt.ddt
@@ -45,3 +45,15 @@ class TestGradeUtils(TestCase):
         assert is_higher is True
         assert 0 == percentage_1
         assert 0 == percentage_2
+
+    @ddt.data(
+        (0.5, 1),
+        (1.45, 1.5, 1),
+        (-0.5, -1.0),
+        (-0.1, -0.0),
+        (0.1, 0.0),
+        (0.0, 0.0)
+    )
+    @ddt.unpack
+    def test_round_away_from_zero(self, precise, expected_rounded_number, rounding_precision=0):
+        assert round_away_from_zero(precise, rounding_precision) == expected_rounded_number