feat: add support for PEP758 (#1401)

PEP758 removes the requirement for parentheses to surround exceptions
in except and except* expressions when 'as' is not present.

This pr implements support for parsing these types of statements

Author: drinkmorewaterr

libcst/_nodes/tests/test_try.py

@@ -344,6 +344,34 @@ class TryTest(CSTNodeTest):
                 ),
                 "code": "try: pass\nexcept foo()as bar: pass\n",
             },
+            # PEP758 - Multiple exceptions with no parentheses
+            {
+                "node": cst.Try(
+                    cst.SimpleStatementSuite((cst.Pass(),)),
+                    handlers=[
+                        cst.ExceptHandler(
+                            cst.SimpleStatementSuite((cst.Pass(),)),
+                            type=cst.Tuple(
+                                elements=[
+                                    cst.Element(
+                                        value=cst.Name(
+                                            value="ValueError",
+                                        ),
+                                    ),
+                                    cst.Element(
+                                        value=cst.Name(
+                                            value="RuntimeError",
+                                        ),
+                                    ),
+                                ],
+                                lpar=[],
+                                rpar=[],
+                            ),
+                        )
+                    ],
+                ),
+                "code": "try: pass\nexcept ValueError, RuntimeError: pass\n",
+            },
         )
     )
     def test_valid(self, **kwargs: Any) -> None:
@@ -576,6 +604,38 @@ class TryStarTest(CSTNodeTest):
                 "parser": native_parse_statement,
                 "expected_position": CodeRange((1, 0), (5, 13)),
             },
+            # PEP758 - Multiple exceptions with no parentheses
+            {
+                "node": cst.TryStar(
+                    cst.SimpleStatementSuite((cst.Pass(),)),
+                    handlers=[
+                        cst.ExceptStarHandler(
+                            cst.SimpleStatementSuite((cst.Pass(),)),
+                            type=cst.Tuple(
+                                elements=[
+                                    cst.Element(
+                                        value=cst.Name(
+                                            value="ValueError",
+                                        ),
+                                        comma=cst.Comma(
+                                            whitespace_after=cst.SimpleWhitespace(" ")
+                                        ),
+                                    ),
+                                    cst.Element(
+                                        value=cst.Name(
+                                            value="RuntimeError",
+                                        ),
+                                    ),
+                                ],
+                                lpar=[],
+                                rpar=[],
+                            ),
+                        )
+                    ],
+                ),
+                "code": "try: pass\nexcept* ValueError, RuntimeError: pass\n",
+                "parser": native_parse_statement,
+            },
         )
     )
     def test_valid(self, **kwargs: Any) -> None:

native/libcst/src/parser/grammar.rs

@@ -554,12 +554,21 @@ parser! {
             }
 
         // Except statement
-
         rule except_block() -> ExceptHandler<'input, 'a>
             = kw:lit("except") e:expression() a:(k:lit("as") n:name() {(k, n)})?
                 col:lit(":") b:block() {
                     make_except(kw, Some(e), a, col, b)
             }
+            / kw:lit("except") e:expression() other:(c:comma() ex:expression() {(c, ex)})+ tc:(c:comma())?
+                col:lit(":") b:block() {
+                    let tuple = Expression::Tuple(Box::new(Tuple {
+                        elements: comma_separate(expr_to_element(e), other.into_iter().map(|(comma, expr)| (comma, expr_to_element(expr))).collect(), tc),
+                        lpar: vec![],
+                        rpar: vec![],
+                    }));
+
+                    make_except(kw, Some(tuple), None, col, b)
+            }
             / kw:lit("except") col:lit(":") b:block() {
                 make_except(kw, None, None, col, b)
             }
@@ -569,6 +578,16 @@ parser! {
                 a:(k:lit("as") n:name() {(k, n)})? col:lit(":") b:block() {
                     make_except_star(kw, star, e, a, col, b)
             }
+            / kw:lit("except") star:lit("*") e:expression() other:(c:comma() ex:expression() {(c, ex)})+ tc:(c:comma())?
+                col:lit(":") b:block() {
+                    let tuple = Expression::Tuple(Box::new(Tuple {
+                        elements: comma_separate(expr_to_element(e), other.into_iter().map(|(comma, expr)| (comma, expr_to_element(expr))).collect(), tc),
+                        lpar: vec![],
+                        rpar: vec![],
+                    }));
+
+                    make_except_star(kw, star, tuple, None, col, b)
+            }
 
         rule finally_block() -> Finally<'input, 'a>
             = kw:lit("finally") col:lit(":") b:block() {
@@ -1550,22 +1569,22 @@ parser! {
         rule separated<El, Sep>(el: rule<El>, sep: rule<Sep>) -> (El, Vec<(Sep, El)>)
             = e:el() rest:(s:sep() e:el() {(s, e)})* {(e, rest)}
 
-        rule traced<T>(e: rule<T>) -> T =
-            &(_* {
+                rule traced<T>(e: rule<T>) -> T =
+                    &(_* {
                 #[cfg(feature = "trace")]
                 {
                     println!("[PEG_INPUT_START]");
                     println!("{}", input);
                     println!("[PEG_TRACE_START]");
                 }
-            })
-            e:e()? {?
+                    })
+                    e:e()? {?
                 #[cfg(feature = "trace")]
-                println!("[PEG_TRACE_STOP]");
-                e.ok_or("")
-            }
+                    println!("[PEG_TRACE_STOP]");
+                    e.ok_or("")
+                    }
 
-    }
+                }
 }
 
 #[allow(clippy::too_many_arguments)]

native/libcst/tests/fixtures/terrible_tries.py

@@ -69,3 +69,25 @@
     pass
 
     #9
+
+try:
+    pass
+except (foo, bar):
+    pass
+
+try:
+    pass
+except foo, bar:
+    pass
+
+try:
+    pass
+except (foo, bar), baz:
+    pass
+else:
+    pass
+
+try:
+    pass
+except* something, somethingelse:
+    pass