bpo-46307: Add string.Template.get_identifiers() method

Lib/string.py

@@ -141,6 +141,16 @@ def convert(mo):
                              self.pattern)
         return self.pattern.sub(convert, self.template)
 
+    def get_identifiers(self, *, raise_on_invalid=True):
+        ids = []
+        for mo in self.pattern.finditer(self.template):
+            named = mo.group('named') or mo.group('braced')
+            if named is not None and named not in ids:
+                ids.append(named)
+            elif mo.group('invalid') is not None and raise_on_invalid:
+                self._invalid(mo)
+        return ids
+
 # Initialize Template.pattern.  __init_subclass__() is automatically called
 # only for subclasses, not for the Template class itself.
 Template.__init_subclass__()

Lib/test/test_string.py

@@ -475,6 +475,27 @@ class PieDelims(Template):
         self.assertEqual(s.substitute(dict(who='tim', what='ham')),
                          'tim likes to eat a bag of ham worth $100')
 
+    def test_get_identifiers(self):
+        eq = self.assertEqual
+        raises = self.assertRaises
+        s = Template('$who likes to eat a bag of ${what} worth $$100')
+        ids = s.get_identifiers()
+        eq(ids, ['who', 'what'])
+
+        # repeated identifiers only included once
+        s = Template('$who likes to eat a bag of ${what} worth $$100; ${who} likes to eat a bag of $what worth $$100')
+        ids = s.get_identifiers()
+        eq(ids, ['who', 'what'])
+
+        # invalid identifiers are raised
+        s = Template('$who likes to eat a bag of ${what} worth $100')
+        raises(ValueError, s.get_identifiers)
+
+        # invalid identifiers are ignored with raise_on_invalid=False
+        s = Template('$who likes to eat a bag of ${what} worth $100')
+        ids = s.get_identifiers(raise_on_invalid=False)
+        eq(ids, ['who', 'what'])
+
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Library/2022-01-10-07-51-43.bpo-46307.SKvOIY.rst

@@ -0,0 +1 @@
+Add :meth:`string.Template.get_identifiers` method.