better exceptions for .first, .last

src/aligned_textgrid/mixins/mixins.py

@@ -11,24 +11,26 @@ class PrecedenceMixins:
 
     Attributes:
         first (SequenceInterval): The first interval in the subset list
-        last  (SequenceInterval): The last interval in the subset list
+        last (SequenceInterval): The last interval in the subset list
     """
 
     @property
     def first(self):
-        if hasattr(self, "subset_list"):
+        if hasattr(self, "subset_list") and len(self.subset_list) > 0:
             return self.subset_list[0]
-        else:
-            raise Exception(f"{type(self).__name__} does not\
-                             have the attribute .first")
-
+        if hasattr(self, "subset_list"):
+            raise IndexError(f"{type(self).__name__} with label "\
+                             f"'{self.label}' subset list is empty.")
+        raise AttributeError(f"{type(self).__name__} is not indexable.")
+
     @property
     def last(self):
-        if hasattr(self, "subset_list"):
+        if hasattr(self, "subset_list") and len(self.subset_list) > 0:
             return self.subset_list[-1]
-        else:
-            raise Exception(f"{type(self).__name__} does not\
-                             have the attribute .first")        
+        if hasattr(self, "subset_list"):
+            raise IndexError(f"{type(self).__name__} with label "\
+                             f"'{self.label}' subset list is empty.")
+        raise AttributeError(f"{type(self).__name__} is not indexable.")        
 
     def set_fol(
             self, next_int):

tests/test_sequences/test_sequences.py

@@ -1,8 +1,10 @@
 import pytest
 from aligned_textgrid.sequences.sequences import *
+from aligned_textgrid.points.points import SequencePoint
 from aligned_textgrid.sequences.tiers import *
 import numpy as np
 from praatio.utilities.constants import Interval
+from praatio.utilities.constants import Point
 
 class TestSequenceIntervalDefault:
     """_Test default behavior of SequenceInterval_
@@ -295,6 +297,23 @@ def test_first_last(self):
         last_interval = upper1[-1]
         assert upper1.last is last_interval
 
+    def test_first_last_errors(self):
+        upper1 = self.UpperClass(Interval(0,10,"upper"))
+        point1 = SequencePoint(Point(0, "point"))
+
+        with pytest.raises(IndexError):
+            upper1.first
+
+        with pytest.raises(IndexError):
+            upper1.last
+
+        with pytest.raises(AttributeError):
+            point1.first
+
+        with pytest.raises(AttributeError):
+            point1.last            
+
+
     def test_subset_pop(self):
         upper1 = self.UpperClass(Interval(0,10,"upper"))
         lower1 = self.LowerClass(Interval(0,5,"lower1"))