Add support for exception groups and except* (#14020)

Ref #12840

It looks like from the point of view of type checking support is quite
easy. Mypyc support however requires some actual work, so I don't
include it in this PR.

mypy/checker.py

@@ -4307,7 +4307,7 @@ def visit_try_without_finally(self, s: TryStmt, try_frame: bool) -> None:
                     with self.binder.frame_context(can_skip=True, fall_through=4):
                         typ = s.types[i]
                         if typ:
-                            t = self.check_except_handler_test(typ)
+                            t = self.check_except_handler_test(typ, s.is_star)
                             var = s.vars[i]
                             if var:
                                 # To support local variables, we make this a definition line,
@@ -4327,7 +4327,7 @@ def visit_try_without_finally(self, s: TryStmt, try_frame: bool) -> None:
             if s.else_body:
                 self.accept(s.else_body)
 
-    def check_except_handler_test(self, n: Expression) -> Type:
+    def check_except_handler_test(self, n: Expression, is_star: bool) -> Type:
         """Type check an exception handler test clause."""
         typ = self.expr_checker.accept(n)
 
@@ -4343,22 +4343,47 @@ def check_except_handler_test(self, n: Expression) -> Type:
                 item = ttype.items[0]
                 if not item.is_type_obj():
                     self.fail(message_registry.INVALID_EXCEPTION_TYPE, n)
-                    return AnyType(TypeOfAny.from_error)
-                exc_type = item.ret_type
+                    return self.default_exception_type(is_star)
+                exc_type = erase_typevars(item.ret_type)
             elif isinstance(ttype, TypeType):
                 exc_type = ttype.item
             else:
                 self.fail(message_registry.INVALID_EXCEPTION_TYPE, n)
-                return AnyType(TypeOfAny.from_error)
+                return self.default_exception_type(is_star)
 
             if not is_subtype(exc_type, self.named_type("builtins.BaseException")):
                 self.fail(message_registry.INVALID_EXCEPTION_TYPE, n)
-                return AnyType(TypeOfAny.from_error)
+                return self.default_exception_type(is_star)
 
             all_types.append(exc_type)
 
+        if is_star:
+            new_all_types: list[Type] = []
+            for typ in all_types:
+                if is_proper_subtype(typ, self.named_type("builtins.BaseExceptionGroup")):
+                    self.fail(message_registry.INVALID_EXCEPTION_GROUP, n)
+                    new_all_types.append(AnyType(TypeOfAny.from_error))
+                else:
+                    new_all_types.append(typ)
+            return self.wrap_exception_group(new_all_types)
         return make_simplified_union(all_types)
 
+    def default_exception_type(self, is_star: bool) -> Type:
+        """Exception type to return in case of a previous type error."""
+        any_type = AnyType(TypeOfAny.from_error)
+        if is_star:
+            return self.named_generic_type("builtins.ExceptionGroup", [any_type])
+        return any_type
+
+    def wrap_exception_group(self, types: Sequence[Type]) -> Type:
+        """Transform except* variable type into an appropriate exception group."""
+        arg = make_simplified_union(types)
+        if is_subtype(arg, self.named_type("builtins.Exception")):
+            base = "builtins.ExceptionGroup"
+        else:
+            base = "builtins.BaseExceptionGroup"
+        return self.named_generic_type(base, [arg])
+
     def get_types_from_except_handler(self, typ: Type, n: Expression) -> list[Type]:
         """Helper for check_except_handler_test to retrieve handler types."""
         typ = get_proper_type(typ)

mypy/fastparse.py

@@ -1254,7 +1254,6 @@ def visit_Try(self, n: ast3.Try) -> TryStmt:
         return self.set_line(node, n)
 
     def visit_TryStar(self, n: TryStar) -> TryStmt:
-        # TODO: we treat TryStar exactly like Try, which makes mypy not crash. See #12840
         vs = [
             self.set_line(NameExpr(h.name), h) if h.name is not None else None for h in n.handlers
         ]
@@ -1269,6 +1268,7 @@ def visit_TryStar(self, n: TryStar) -> TryStmt:
             self.as_block(n.orelse, n.lineno),
             self.as_block(n.finalbody, n.lineno),
         )
+        node.is_star = True
         return self.set_line(node, n)
 
     # Assert(expr test, expr? msg)

mypy/message_registry.py

@@ -44,6 +44,9 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 NO_RETURN_EXPECTED: Final = ErrorMessage("Return statement in function which does not return")
 INVALID_EXCEPTION: Final = ErrorMessage("Exception must be derived from BaseException")
 INVALID_EXCEPTION_TYPE: Final = ErrorMessage("Exception type must be derived from BaseException")
+INVALID_EXCEPTION_GROUP: Final = ErrorMessage(
+    "Exception type in except* cannot derive from BaseExceptionGroup"
+)
 RETURN_IN_ASYNC_GENERATOR: Final = ErrorMessage(
     '"return" with value in async generator is not allowed'
 )

mypy/nodes.py

@@ -1485,7 +1485,7 @@ def accept(self, visitor: StatementVisitor[T]) -> T:
 
 
 class TryStmt(Statement):
-    __slots__ = ("body", "types", "vars", "handlers", "else_body", "finally_body")
+    __slots__ = ("body", "types", "vars", "handlers", "else_body", "finally_body", "is_star")
 
     body: Block  # Try body
     # Plain 'except:' also possible
@@ -1494,6 +1494,8 @@ class TryStmt(Statement):
     handlers: list[Block]  # Except bodies
     else_body: Block | None
     finally_body: Block | None
+    # Whether this is try ... except* (added in Python 3.11)
+    is_star: bool
 
     def __init__(
         self,
@@ -1511,6 +1513,7 @@ def __init__(
         self.handlers = handlers
         self.else_body = else_body
         self.finally_body = finally_body
+        self.is_star = False
 
     def accept(self, visitor: StatementVisitor[T]) -> T:
         return visitor.visit_try_stmt(self)

mypy/strconv.py

@@ -276,6 +276,8 @@ def visit_del_stmt(self, o: mypy.nodes.DelStmt) -> str:
 
     def visit_try_stmt(self, o: mypy.nodes.TryStmt) -> str:
         a: list[Any] = [o.body]
+        if o.is_star:
+            a.append("*")
 
         for i in range(len(o.vars)):
             a.append(o.types[i])

mypy/treetransform.py

@@ -373,14 +373,16 @@ def visit_raise_stmt(self, node: RaiseStmt) -> RaiseStmt:
         return RaiseStmt(self.optional_expr(node.expr), self.optional_expr(node.from_expr))
 
     def visit_try_stmt(self, node: TryStmt) -> TryStmt:
-        return TryStmt(
+        new = TryStmt(
             self.block(node.body),
             self.optional_names(node.vars),
             self.optional_expressions(node.types),
             self.blocks(node.handlers),
             self.optional_block(node.else_body),
             self.optional_block(node.finally_body),
         )
+        new.is_star = node.is_star
+        return new
 
     def visit_with_stmt(self, node: WithStmt) -> WithStmt:
         new = WithStmt(

mypyc/irbuild/statement.py

@@ -616,6 +616,8 @@ def transform_try_stmt(builder: IRBuilder, t: TryStmt) -> None:
     # constructs that we compile separately. When we have a
     # try/except/else/finally, we treat the try/except/else as the
     # body of a try/finally block.
+    if t.is_star:
+        builder.error("Exception groups and except* cannot be compiled yet", t.line)
     if t.finally_body:
 
         def transform_try_body() -> None:

test-data/unit/check-python311.test

@@ -1,6 +1,53 @@
-[case testTryStarDoesNotCrash]
+[case testTryStarSimple]
 try:
     pass
 except* Exception as e:
-    reveal_type(e)  # N: Revealed type is "builtins.Exception"
+    reveal_type(e)  # N: Revealed type is "builtins.ExceptionGroup[builtins.Exception]"
+[builtins fixtures/exception.pyi]
+
+[case testTryStarMultiple]
+try:
+    pass
+except* Exception as e:
+    reveal_type(e)  # N: Revealed type is "builtins.ExceptionGroup[builtins.Exception]"
+except* RuntimeError as e:
+    reveal_type(e)  # N: Revealed type is "builtins.ExceptionGroup[builtins.RuntimeError]"
+[builtins fixtures/exception.pyi]
+
+[case testTryStarBase]
+try:
+    pass
+except* BaseException as e:
+    reveal_type(e)  # N: Revealed type is "builtins.BaseExceptionGroup[builtins.BaseException]"
+[builtins fixtures/exception.pyi]
+
+[case testTryStarTuple]
+class Custom(Exception): ...
+
+try:
+    pass
+except* (RuntimeError, Custom) as e:
+    reveal_type(e)  # N: Revealed type is "builtins.ExceptionGroup[Union[builtins.RuntimeError, __main__.Custom]]"
+[builtins fixtures/exception.pyi]
+
+[case testTryStarInvalidType]
+class Bad: ...
+try:
+    pass
+except* (RuntimeError, Bad) as e:  # E: Exception type must be derived from BaseException
+    reveal_type(e)  # N: Revealed type is "builtins.ExceptionGroup[Any]"
+[builtins fixtures/exception.pyi]
+
+[case testTryStarGroupInvalid]
+try:
+    pass
+except* ExceptionGroup as e:  # E: Exception type in except* cannot derive from BaseExceptionGroup
+    reveal_type(e)  # N: Revealed type is "builtins.ExceptionGroup[Any]"
+[builtins fixtures/exception.pyi]
+
+[case testTryStarGroupInvalidTuple]
+try:
+    pass
+except* (RuntimeError, ExceptionGroup) as e:  # E: Exception type in except* cannot derive from BaseExceptionGroup
+    reveal_type(e)  # N: Revealed type is "builtins.ExceptionGroup[Union[builtins.RuntimeError, Any]]"
 [builtins fixtures/exception.pyi]

test-data/unit/fixtures/exception.pyi

@@ -1,23 +1,28 @@
+import sys
 from typing import Generic, TypeVar
 T = TypeVar('T')
 
 class object:
     def __init__(self): pass
 
 class type: pass
-class tuple(Generic[T]): pass
+class tuple(Generic[T]):
+    def __ge__(self, other: object) -> bool: ...
 class function: pass
 class int: pass
 class str: pass
 class unicode: pass
 class bool: pass
 class ellipsis: pass
 
-# Note: this is a slight simplification. In Python 2, the inheritance hierarchy
-# is actually Exception -> StandardError -> RuntimeError -> ...
 class BaseException:
     def __init__(self, *args: object) -> None: ...
 class Exception(BaseException): pass
 class RuntimeError(Exception): pass
 class NotImplementedError(RuntimeError): pass
 
+if sys.version_info >= (3, 11):
+    _BT_co = TypeVar("_BT_co", bound=BaseException, covariant=True)
+    _T_co = TypeVar("_T_co", bound=Exception, covariant=True)
+    class BaseExceptionGroup(BaseException, Generic[_BT_co]): ...
+    class ExceptionGroup(BaseExceptionGroup[_T_co], Exception): ...