Fix test logic that depended on rounding errors

Corrects unit tests that only used to work because of their anticipation
of rounding errors when converting between floating point dates that
interpret decimal values as fractions of days in a year vs. fractions of
months in a year.

tests/test_frequencies.py

@@ -61,9 +61,15 @@ def test_get_pivots_from_tree_only(tree):
     pivots = get_pivots(observations, pivot_frequency)
     assert isinstance(pivots, np.ndarray)
 
-    # Floating point pivot values should be separated by the given number of
-    # months divided by number of months in a year.
-    assert pivots[1] - pivots[0] == pivot_frequency / 12.0
+    # Floating point pivot values should be roughly separated by the given
+    # number of months divided by number of months in a year. Numeric dates from
+    # pivots calculate decimal fractions as the proportion of days in the year,
+    # instead of months in the year (e.g., the fraction for the first three
+    # months of a non-leap year is (31 + 28 + 31) / 365 or 0.246575 instead of 3
+    # / 12 or 0.25). As a result of this difference in how we calculate
+    # fractions, we need to round to pivots to 2 decimal places which is the
+    # precision of month-based fractions.
+    assert np.round(pivots[1] - pivots[0], 2) == (pivot_frequency / 12.0)
 
 def test_get_pivots_from_start_and_end_date():
     """
@@ -163,7 +169,7 @@ def test_estimate_with_time_interval(self, tree):
         )
         frequencies = kde_frequencies.estimate(tree)
         assert hasattr(kde_frequencies, "pivots")
-        assert kde_frequencies.pivots[0] == 2015.5833
+        assert kde_frequencies.pivots[0] == 2015.5
         assert hasattr(kde_frequencies, "frequencies")
         assert list(frequencies.values())[0].shape == kde_frequencies.pivots.shape
 
@@ -437,9 +443,16 @@ def test_estimate(self, alignment):
         frequencies = kde_frequencies.estimate(alignment, observations)
 
         assert hasattr(kde_frequencies, "pivots")
-        assert np.around(kde_frequencies.pivots[1] - kde_frequencies.pivots[0], 2) == np.around(1 / 12.0, 2)
+        pivots = kde_frequencies.pivots
+
+        # Pivot decimal fractions represent fractions of a year by days, so
+        # comparisons with fractions of a year by months need to take into
+        # account rounding error between these calculations. In this test, we
+        # allow pivots to be spaced apart by a value that is "close" to 1 / 12
+        # months by the rounding error of the month-based precision.
+        assert np.isclose((pivots[-1] - pivots[-2]), 1 / 12.0, atol=0.005)
         assert hasattr(kde_frequencies, "frequencies")
-        assert list(frequencies.values())[0].shape == kde_frequencies.pivots.shape
+        assert list(frequencies.values())[0].shape == pivots.shape
 
         # Find a position with frequencies estimated for multiple bases.
         position = list(frequencies.keys())[0].split(":")[0]