Add small validator utility for PEG grammars (GH-23519)

1 files changed, 51 insertions, 0 deletions

diff --git a/Lib/test/test_peg_generator/test_grammar_validator.py b/Lib/test/test_peg_generator/test_grammar_validator.py
new file mode 100644
index 0000000..2e72ff8
--- /dev/null
+++ b/Lib/test/test_peg_generator/test_grammar_validator.py
@@ -0,0 +1,51 @@
+import unittest
+from test import test_tools
+
+test_tools.skip_if_missing('peg_generator')
+with test_tools.imports_under_tool('peg_generator'):
+    from pegen.grammar_parser import GeneratedParser as GrammarParser
+    from pegen.validator import SubRuleValidator, ValidationError
+    from pegen.testutil import parse_string
+    from pegen.grammar import Grammar
+
+
+class TestPegen(unittest.TestCase):
+    def test_rule_with_no_collision(self) -> None:
+        grammar_source = """
+        start: bad_rule
+        sum:
+            | NAME '-' NAME
+            | NAME '+' NAME
+        """
+        grammar: Grammar = parse_string(grammar_source, GrammarParser)
+        validator = SubRuleValidator(grammar)
+        for rule_name, rule in grammar.rules.items():
+            validator.validate_rule(rule_name, rule)
+
+    def test_rule_with_simple_collision(self) -> None:
+        grammar_source = """
+        start: bad_rule
+        sum:
+            | NAME '+' NAME
+            | NAME '+' NAME ';'
+        """
+        grammar: Grammar = parse_string(grammar_source, GrammarParser)
+        validator = SubRuleValidator(grammar)
+        with self.assertRaises(ValidationError):
+            for rule_name, rule in grammar.rules.items():
+                validator.validate_rule(rule_name, rule)
+
+    def test_rule_with_collision_after_some_other_rules(self) -> None:
+        grammar_source = """
+        start: bad_rule
+        sum:
+            | NAME '+' NAME
+            | NAME '*' NAME ';'
+            | NAME '-' NAME
+            | NAME '+' NAME ';'
+        """
+        grammar: Grammar = parse_string(grammar_source, GrammarParser)
+        validator = SubRuleValidator(grammar)
+        with self.assertRaises(ValidationError):
+            for rule_name, rule in grammar.rules.items():
+                validator.validate_rule(rule_name, rule)