Refs #28643 -- Added LPad and RPad database functions.

Thanks Tim Graham for the review.

tests/db_functions/test_pad.py

@@ -0,0 +1,34 @@
+from django.db.models import Value
+from django.db.models.functions import LPad, RPad
+from django.test import TestCase
+
+from .models import Author
+
+
+class PadTests(TestCase):
+    def test_pad(self):
+        Author.objects.create(name='John', alias='j')
+        tests = (
+            (LPad('name', 7, Value('xy')), 'xyxJohn'),
+            (RPad('name', 7, Value('xy')), 'Johnxyx'),
+            (LPad('name', 6, Value('x')), 'xxJohn'),
+            (RPad('name', 6, Value('x')), 'Johnxx'),
+            # The default pad string is a space.
+            (LPad('name', 6), '  John'),
+            (RPad('name', 6), 'John  '),
+            # If string is longer than length it is truncated.
+            (LPad('name', 2), 'Jo'),
+            (RPad('name', 2), 'Jo'),
+            (LPad('name', 0), ''),
+            (RPad('name', 0), ''),
+        )
+        for function, padded_name in tests:
+            with self.subTest(function=function):
+                authors = Author.objects.annotate(padded_name=function)
+                self.assertQuerysetEqual(authors, [padded_name], lambda a: a.padded_name, ordered=False)
+
+    def test_pad_negative_length(self):
+        for function in (LPad, RPad):
+            with self.subTest(function=function):
+                with self.assertRaisesMessage(ValueError, "'length' must be greater or equal to 0."):
+                    function('name', -1)