[3.14] GH-132661: Add `string.templatelib.convert()` (GH-135217) (#136671)

GH-132661: Add ``string.templatelib.convert()`` (GH-135217)
(cherry picked from commit 5b969fd645)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Lib/string/templatelib.py

@@ -1,15 +1,22 @@
 """Support for template string literals (t-strings)."""
 
-__all__ = [
-    "Interpolation",
-    "Template",
-]
-
 t = t"{0}"
 Template = type(t)
 Interpolation = type(t.interpolations[0])
 del t
 
+def convert(obj, /, conversion):
+    """Convert *obj* using formatted string literal semantics."""
+    if conversion is None:
+        return obj
+    if conversion == 'r':
+        return repr(obj)
+    if conversion == 's':
+        return str(obj)
+    if conversion == 'a':
+        return ascii(obj)
+    raise ValueError(f'invalid conversion specifier: {conversion}')
+
 def _template_unpickle(*args):
     import itertools
 

Lib/test/test_string/test_templatelib.py

@@ -1,7 +1,7 @@
 import pickle
 import unittest
 from collections.abc import Iterator, Iterable
-from string.templatelib import Template, Interpolation
+from string.templatelib import Template, Interpolation, convert
 
 from test.test_string._support import TStringBaseCase, fstring
 
@@ -169,5 +169,25 @@ def test_exhausted(self):
         self.assertRaises(StopIteration, next, template_iter)
 
 
+class TestFunctions(unittest.TestCase):
+    def test_convert(self):
+        from fractions import Fraction
+
+        for obj in ('Café', None, 3.14, Fraction(1, 2)):
+            with self.subTest(f'{obj=}'):
+                self.assertEqual(convert(obj, None), obj)
+                self.assertEqual(convert(obj, 's'), str(obj))
+                self.assertEqual(convert(obj, 'r'), repr(obj))
+                self.assertEqual(convert(obj, 'a'), ascii(obj))
+
+                # Invalid conversion specifier
+                with self.assertRaises(ValueError):
+                    convert(obj, 'z')
+                with self.assertRaises(ValueError):
+                    convert(obj, 1)
+                with self.assertRaises(ValueError):
+                    convert(obj, object())
+
+
 if __name__ == '__main__':
     unittest.main()