gh-132661: Implement PEP 750 (#132662)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Wingy <git@wingysam.xyz>
Co-authored-by: Koudai Aono <koxudaxi@gmail.com>
Co-authored-by: Dave Peck <davepeck@gmail.com>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
Co-authored-by: Paul Everitt <pauleveritt@me.com>
Co-authored-by: sobolevn <mail@sobolevn.me>

Lib/test/test_string/_support.py

@@ -0,0 +1,55 @@
+import unittest
+from string.templatelib import Interpolation
+
+
+class TStringBaseCase:
+    def assertTStringEqual(self, t, strings, interpolations):
+        """Test template string literal equality.
+
+        The *strings* argument must be a tuple of strings equal to *t.strings*.
+
+        The *interpolations* argument must be a sequence of tuples which are
+        compared against *t.interpolations*. Each tuple consists of
+        (value, expression, conversion, format_spec), though the final two
+        items may be omitted, and are assumed to be None and '' respectively.
+        """
+        self.assertEqual(t.strings, strings)
+        self.assertEqual(len(t.interpolations), len(interpolations))
+
+        for i, exp in zip(t.interpolations, interpolations, strict=True):
+            if len(exp) == 4:
+                actual = (i.value, i.expression, i.conversion, i.format_spec)
+                self.assertEqual(actual, exp)
+                continue
+
+            if len(exp) == 3:
+                self.assertEqual((i.value, i.expression, i.conversion), exp)
+                self.assertEqual(i.format_spec, '')
+                continue
+
+            self.assertEqual((i.value, i.expression), exp)
+            self.assertEqual(i.format_spec, '')
+            self.assertIsNone(i.conversion)
+
+
+def convert(value, conversion):
+    if conversion == "a":
+        return ascii(value)
+    elif conversion == "r":
+        return repr(value)
+    elif conversion == "s":
+        return str(value)
+    return value
+
+
+def fstring(template):
+    parts = []
+    for item in template:
+        match item:
+            case str() as s:
+                parts.append(s)
+            case Interpolation(value, _, conversion, format_spec):
+                value = convert(value, conversion)
+                value = format(value, format_spec)
+                parts.append(value)
+    return "".join(parts)