use the inflection library for converting Table class names to database table names

Whilst investigating the bug with mixins, I realised that if a `Table` has a name like `ManagerA`, the database table was wrongly being called `manager` instead of `manager_a`. To avoid similar subtle bugs, I replaced the regular expression previously used with a popular library.

Author: dantownsend

piccolo/utils/naming.py

@@ -1,14 +1,8 @@
-import re
+import inflection
 
 
-_camel_words = re.compile(r"([A-Z][a-z0-9_]+)")
-
-
-def _camel_to_snake(s):
-    """ Convert CamelCase to snake_case.
+def _camel_to_snake(string: str):
+    """
+    Convert CamelCase to snake_case.
     """
-    return "_".join(
-        [
-            i.lower() for i in _camel_words.split(s)[1::2]
-        ]
-    )
+    return inflection.underscore(string)

requirements.txt

@@ -4,3 +4,4 @@ black
 colorama>=0.4.0
 Jinja2>=2.11.0
 targ>=0.1.0
+inflection>=0.5.1

tests/utils/test_naming.py

@@ -0,0 +1,15 @@
+from piccolo.utils.naming import _camel_to_snake
+from unittest import TestCase
+
+
+class TestCamelToSnake(TestCase):
+    def test_converting_tablenames(self):
+        """
+        Make sure Table names are converted correctly.
+        """
+        self.assertEqual(_camel_to_snake("HelloWorld"), "hello_world")
+        self.assertEqual(_camel_to_snake("Manager1"), "manager1")
+        self.assertEqual(_camel_to_snake("ManagerAbc"), "manager_abc")
+        self.assertEqual(_camel_to_snake("ManagerABC"), "manager_abc")
+        self.assertEqual(_camel_to_snake("ManagerABCFoo"), "manager_abc_foo")
+        self.assertEqual(_camel_to_snake("ManagerA"), "manager_a")