bpo-46307: Add string.Template.get_identifiers() method (GH-30493)

Add `string.Template.get_identifiers()` method that returns the identifiers within the template. By default, raises an error if it encounters an invalid identifier (like `substitute()`). The keyword-only argument `raise_on_invalid` can be set to `False` to ignore invalid identifiers (like `safe_substitute()`).

Automerge-Triggered-By: GH:warsaw

Lib/string.py

@@ -141,6 +141,35 @@ class Template:
                              self.pattern)
         return self.pattern.sub(convert, self.template)
 
+    def is_valid(self):
+        for mo in self.pattern.finditer(self.template):
+            if mo.group('invalid') is not None:
+                return False
+            if (mo.group('named') is None
+                and mo.group('braced') is None
+                and mo.group('escaped') is None):
+                # If all the groups are None, there must be
+                # another group we're not expecting
+                raise ValueError('Unrecognized named group in pattern',
+                    self.pattern)
+        return True
+
+    def get_identifiers(self):
+        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:
+                # add a named group only the first time it appears
+                ids.append(named)
+            elif (named is None
+                and mo.group('invalid') is None
+                and mo.group('escaped') is None):
+                # If all the groups are None, there must be
+                # another group we're not expecting
+                raise ValueError('Unrecognized named group in pattern',
+                    self.pattern)
+        return ids
+
 # Initialize Template.pattern.  __init_subclass__() is automatically called
 # only for subclasses, not for the Template class itself.
 Template.__init_subclass__()