Fix short acronym boundaries in camel_to_underscore

_python_utils_tests/test_formatters.py

@@ -73,6 +73,24 @@ def test_camel_to_underscore_keeps_acronyms_whole(
     assert formatters.camel_to_underscore(name) == expected
 
 
+@pytest.mark.parametrize(
+    ('name', 'expected'),
+    [
+        ('ABc', 'a_bc'),
+        ('ABCd', 'ab_cd'),
+        ('IPad', 'i_pad'),
+        ('AName', 'a_name'),
+        ('__ABc__', '__a_bc__'),
+        ('XMLParser', 'xml_parser'),
+    ],
+)
+def test_camel_to_underscore_splits_short_acronym_boundary(
+    name: str, expected: str
+) -> None:
+    """Keep the final capital with the word following even a short acronym."""
+    assert formatters.camel_to_underscore(name) == expected
+
+
 def test_timesince_date_raises_type_error() -> None:
     """Keep raising ``TypeError`` for a date, which has no time."""
     today: typing.Any = datetime.date.today()

python_utils/formatters.py

@@ -42,11 +42,12 @@ def camel_to_underscore(name: str) -> str:
                 # Uppercase and the previous character isn't upper/underscore?
                 # Add the underscore
                 output.append('_')
-            elif i > 3 and c.islower():
-                # Will return the last 3 letters to check if we are changing
-                # case. Only a lowercase letter ends an acronym, an underscore
-                # or a digit after it leaves the acronym whole.
-                previous = name[i - 3 : i]
+            elif i > 1 and c.islower():
+                # Two preceding capitals are enough to mark the boundary:
+                # the final capital belongs to the following word. Only a
+                # lowercase letter ends an acronym; an underscore or digit
+                # after it leaves the acronym whole.
+                previous = name[i - 2 : i]
                 if previous.isalpha() and previous.isupper():
                     output.insert(len(output) - 1, '_')