Implement TypeIs (PEP 742)

docs/source/error_code_list2.rst

@@ -555,3 +555,19 @@ Correct usage:
 
 When this code is enabled, using ``reveal_locals`` is always an error,
 because there's no way one can import it.
+
+.. _code-type-is-not-subtype:
+
+Check that TypeIs narrows types [type-is-not-subtype]
+-----------------------------------------------------------------
+
+:pep:`742` requires that when a ``TypeIs`` is used, the narrowed
+type must be a subtype of the original type::
+
+    from typing_extensions import TypeIs
+
+    def f(x: int) -> TypeIs[str]:  # Error, str is not a subtype of int
+        ...
+
+    def g(x: object) -> TypeIs[str]:  # OK
+        ...

mypy/applytype.py

@@ -137,11 +137,15 @@ def apply_generic_arguments(
             arg_types=[expand_type(at, id_to_type) for at in callable.arg_types]
         )
 
-    # Apply arguments to TypeGuard if any.
+    # Apply arguments to TypeGuard and TypeIs if any.
     if callable.type_guard is not None:
         type_guard = expand_type(callable.type_guard, id_to_type)
     else:
         type_guard = None
+    if callable.type_is is not None:
+        type_is = expand_type(callable.type_is, id_to_type)
+    else:
+        type_is = None
 
     # The callable may retain some type vars if only some were applied.
     # TODO: move apply_poly() logic from checkexpr.py here when new inference
@@ -153,4 +157,5 @@ def apply_generic_arguments(
         ret_type=expand_type(callable.ret_type, id_to_type),
         variables=remaining_tvars,
         type_guard=type_guard,
+        type_is=type_is,
     )

mypy/checker.py

@@ -1209,6 +1209,22 @@ def check_func_def(
                     # visible from *inside* of this function/method.
                     ref_type: Type | None = self.scope.active_self_type()
 
+                if typ.type_is:
+                    arg_index = 0
+                    # For methods and classmethods, we want the second parameter
+                    if ref_type is not None and (not defn.is_static or defn.name == "__new__"):
+                        arg_index = 1
+                    if arg_index < len(typ.arg_types) and not is_subtype(
+                        typ.type_is, typ.arg_types[arg_index]
+                    ):
+                        self.fail(
+                            message_registry.TYPE_NARROWER_NOT_SUBTYPE.format(
+                                format_type(typ.type_is, self.options),
+                                format_type(typ.arg_types[arg_index], self.options),
+                            ),
+                            item,
+                        )
+
                 # Store argument types.
                 for i in range(len(typ.arg_types)):
                     arg_type = typ.arg_types[i]
@@ -2177,6 +2193,8 @@ def check_override(
             elif isinstance(original, CallableType) and isinstance(override, CallableType):
                 if original.type_guard is not None and override.type_guard is None:
                     fail = True
+                if original.type_is is not None and override.type_is is None:
+                    fail = True
 
         if is_private(name):
             fail = False
@@ -5629,7 +5647,7 @@ def combine_maps(list_maps: list[TypeMap]) -> TypeMap:
     def find_isinstance_check(self, node: Expression) -> tuple[TypeMap, TypeMap]:
         """Find any isinstance checks (within a chain of ands).  Includes
         implicit and explicit checks for None and calls to callable.
-        Also includes TypeGuard functions.
+        Also includes TypeGuard and TypeIs functions.
 
         Return value is a map of variables to their types if the condition
         is true and a map of variables to their types if the condition is false.
@@ -5681,7 +5699,7 @@ def find_isinstance_check_helper(self, node: Expression) -> tuple[TypeMap, TypeM
                 if literal(expr) == LITERAL_TYPE and attr and len(attr) == 1:
                     return self.hasattr_type_maps(expr, self.lookup_type(expr), attr[0])
             elif isinstance(node.callee, RefExpr):
-                if node.callee.type_guard is not None:
+                if node.callee.type_guard is not None or node.callee.type_is is not None:
                     # TODO: Follow *args, **kwargs
                     if node.arg_kinds[0] != nodes.ARG_POS:
                         # the first argument might be used as a kwarg
@@ -5707,15 +5725,31 @@ def find_isinstance_check_helper(self, node: Expression) -> tuple[TypeMap, TypeM
                                 # we want the idx-th variable to be narrowed
                                 expr = collapse_walrus(node.args[idx])
                             else:
-                                self.fail(message_registry.TYPE_GUARD_POS_ARG_REQUIRED, node)
+                                kind = (
+                                    "guard" if node.callee.type_guard is not None else "narrower"
+                                )
+                                self.fail(
+                                    message_registry.TYPE_GUARD_POS_ARG_REQUIRED.format(kind), node
+                                )
                                 return {}, {}
                     if literal(expr) == LITERAL_TYPE:
                         # Note: we wrap the target type, so that we can special case later.
                         # Namely, for isinstance() we use a normal meet, while TypeGuard is
                         # considered "always right" (i.e. even if the types are not overlapping).
                         # Also note that a care must be taken to unwrap this back at read places
                         # where we use this to narrow down declared type.
-                        return {expr: TypeGuardedType(node.callee.type_guard)}, {}
+                        if node.callee.type_guard is not None:
+                            return {expr: TypeGuardedType(node.callee.type_guard)}, {}
+                        else:
+                            assert node.callee.type_is is not None
+                            return conditional_types_to_typemaps(
+                                expr,
+                                *self.conditional_types_with_intersection(
+                                    self.lookup_type(expr),
+                                    [TypeRange(node.callee.type_is, is_upper_bound=False)],
+                                    expr,
+                                ),
+                            )
         elif isinstance(node, ComparisonExpr):
             # Step 1: Obtain the types of each operand and whether or not we can
             # narrow their types. (For example, we shouldn't try narrowing the

mypy/checkexpr.py

@@ -1451,13 +1451,12 @@ def check_call_expr_with_callee_type(
             object_type=object_type,
         )
         proper_callee = get_proper_type(callee_type)
-        if (
-            isinstance(e.callee, RefExpr)
-            and isinstance(proper_callee, CallableType)
-            and proper_callee.type_guard is not None
-        ):
+        if isinstance(e.callee, RefExpr) and isinstance(proper_callee, CallableType):
             # Cache it for find_isinstance_check()
-            e.callee.type_guard = proper_callee.type_guard
+            if proper_callee.type_guard is not None:
+                e.callee.type_guard = proper_callee.type_guard
+            if proper_callee.type_is is not None:
+                e.callee.type_is = proper_callee.type_is
         return ret_type
 
     def check_union_call_expr(self, e: CallExpr, object_type: UnionType, member: str) -> Type:
@@ -5277,7 +5276,7 @@ def infer_lambda_type_using_context(
         # is a constructor -- but this fallback doesn't make sense for lambdas.
         callable_ctx = callable_ctx.copy_modified(fallback=self.named_type("builtins.function"))
 
-        if callable_ctx.type_guard is not None:
+        if callable_ctx.type_guard is not None or callable_ctx.type_is is not None:
             # Lambda's return type cannot be treated as a `TypeGuard`,
             # because it is implicit. And `TypeGuard`s must be explicit.
             # See https://github.com/python/mypy/issues/9927

mypy/constraints.py

@@ -1020,8 +1020,12 @@ def visit_callable_type(self, template: CallableType) -> list[Constraint]:
             template_ret_type, cactual_ret_type = template.ret_type, cactual.ret_type
             if template.type_guard is not None:
                 template_ret_type = template.type_guard
+            elif template.type_is is not None:
+                template_ret_type = template.type_is
             if cactual.type_guard is not None:
                 cactual_ret_type = cactual.type_guard
+            elif cactual.type_is is not None:
+                cactual_ret_type = cactual.type_is
             res.extend(infer_constraints(template_ret_type, cactual_ret_type, self.direction))
 
             if param_spec is None:

mypy/errorcodes.py

@@ -281,5 +281,11 @@ def __hash__(self) -> int:
     sub_code_of=MISC,
 )
 
+TYPE_NARROWER_NOT_SUBTYPE: Final[ErrorCode] = ErrorCode(
+    "type-is-not-subtype",
+    "Warn if a TypeIs function's narrowed type is not a subtype of the original type",
+    "General",
+)
+
 # This copy will not include any error codes defined later in the plugins.
 mypy_error_codes = error_codes.copy()

mypy/expandtype.py

@@ -342,6 +342,7 @@ def visit_callable_type(self, t: CallableType) -> CallableType:
                     arg_names=t.arg_names[:-2] + repl.arg_names,
                     ret_type=t.ret_type.accept(self),
                     type_guard=(t.type_guard.accept(self) if t.type_guard is not None else None),
+                    type_is=(t.type_is.accept(self) if t.type_is is not None else None),
                     imprecise_arg_kinds=(t.imprecise_arg_kinds or repl.imprecise_arg_kinds),
                     variables=[*repl.variables, *t.variables],
                 )
@@ -375,6 +376,7 @@ def visit_callable_type(self, t: CallableType) -> CallableType:
             arg_types=arg_types,
             ret_type=t.ret_type.accept(self),
             type_guard=(t.type_guard.accept(self) if t.type_guard is not None else None),
+            type_is=(t.type_is.accept(self) if t.type_is is not None else None),
         )
         if needs_normalization:
             return expanded.with_normalized_var_args()

mypy/fixup.py

@@ -270,6 +270,8 @@ def visit_callable_type(self, ct: CallableType) -> None:
                 arg.accept(self)
         if ct.type_guard is not None:
             ct.type_guard.accept(self)
+        if ct.type_is is not None:
+            ct.type_is.accept(self)
 
     def visit_overloaded(self, t: Overloaded) -> None:
         for ct in t.items:

mypy/message_registry.py

@@ -262,7 +262,7 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 
 CONTIGUOUS_ITERABLE_EXPECTED: Final = ErrorMessage("Contiguous iterable with same type expected")
 ITERABLE_TYPE_EXPECTED: Final = ErrorMessage("Invalid type '{}' for *expr (iterable expected)")
-TYPE_GUARD_POS_ARG_REQUIRED: Final = ErrorMessage("Type guard requires positional argument")
+TYPE_GUARD_POS_ARG_REQUIRED: Final = ErrorMessage("Type {} requires positional argument")
 
 # Match Statement
 MISSING_MATCH_ARGS: Final = 'Class "{}" doesn\'t define "__match_args__"'
@@ -324,3 +324,6 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 ARG_NAME_EXPECTED_STRING_LITERAL: Final = ErrorMessage(
     "Expected string literal for argument name, got {}", codes.SYNTAX
 )
+TYPE_NARROWER_NOT_SUBTYPE: Final = ErrorMessage(
+    "Narrowed type {} is not a subtype of input type {}", codes.SYNTAX
+)

mypy/messages.py

@@ -2634,6 +2634,8 @@ def format_literal_value(typ: LiteralType) -> str:
         elif isinstance(func, CallableType):
             if func.type_guard is not None:
                 return_type = f"TypeGuard[{format(func.type_guard)}]"
+            elif func.type_is is not None:
+                return_type = f"TypeIs[{format(func.type_is)}]"
             else:
                 return_type = format(func.ret_type)
             if func.is_ellipsis_args:
@@ -2850,6 +2852,8 @@ def [T <: int] f(self, x: int, y: T) -> None
     s += " -> "
     if tp.type_guard is not None:
         s += f"TypeGuard[{format_type_bare(tp.type_guard, options)}]"
+    elif tp.type_is is not None:
+        s += f"TypeIs[{format_type_bare(tp.type_is, options)}]"
     else:
         s += format_type_bare(tp.ret_type, options)
 

mypy/nodes.py

@@ -1755,6 +1755,7 @@ class RefExpr(Expression):
         "is_inferred_def",
         "is_alias_rvalue",
         "type_guard",
+        "type_is",
     )
 
     def __init__(self) -> None:
@@ -1776,6 +1777,8 @@ def __init__(self) -> None:
         self.is_alias_rvalue = False
         # Cache type guard from callable_type.type_guard
         self.type_guard: mypy.types.Type | None = None
+        # And same for TypeIs
+        self.type_is: mypy.types.Type | None = None
 
     @property
     def fullname(self) -> str:

mypy/semanal.py

@@ -879,6 +879,13 @@ def analyze_func_def(self, defn: FuncDef) -> None:
                         )
                         # in this case, we just kind of just ... remove the type guard.
                         result = result.copy_modified(type_guard=None)
+                    if result.type_is and ARG_POS not in result.arg_kinds[skip_self:]:
+                        self.fail(
+                            "TypeIs functions must have a positional argument",
+                            result,
+                            code=codes.VALID_TYPE,
+                        )
+                        result = result.copy_modified(type_is=None)
 
                     result = self.remove_unpack_kwargs(defn, result)
                     if has_self_type and self.type is not None:

mypy/subtypes.py

@@ -683,10 +683,23 @@ def visit_callable_type(self, left: CallableType) -> bool:
             if left.type_guard is not None and right.type_guard is not None:
                 if not self._is_subtype(left.type_guard, right.type_guard):
                     return False
+            elif left.type_is is not None and right.type_is is not None:
+                # For TypeIs we have to check both ways; it is unsafe to pass
+                # a TypeIs[Child] when a TypeIs[Parent] is expected, because
+                # if the narrower returns False, we assume that the narrowed value is
+                # *not* a Parent.
+                if not self._is_subtype(left.type_is, right.type_is) or not self._is_subtype(
+                    right.type_is, left.type_is
+                ):
+                    return False
             elif right.type_guard is not None and left.type_guard is None:
                 # This means that one function has `TypeGuard` and other does not.
                 # They are not compatible. See https://github.com/python/mypy/issues/11307
                 return False
+            elif right.type_is is not None and left.type_is is None:
+                # Similarly, if one function has typeNarrower and the other does not,
+                # they are not compatible.
+                return False
             return is_callable_compatible(
                 left,
                 right,

mypy/typeanal.py

@@ -663,7 +663,10 @@ def try_analyze_special_unbound_type(self, t: UnboundType, fullname: str) -> Typ
                 )
                 return AnyType(TypeOfAny.from_error)
             return RequiredType(self.anal_type(t.args[0]), required=False)
-        elif self.anal_type_guard_arg(t, fullname) is not None:
+        elif (
+            self.anal_type_guard_arg(t, fullname) is not None
+            or self.anal_type_is_arg(t, fullname) is not None
+        ):
             # In most contexts, TypeGuard[...] acts as an alias for bool (ignoring its args)
             return self.named_type("builtins.bool")
         elif fullname in ("typing.Unpack", "typing_extensions.Unpack"):
@@ -981,7 +984,8 @@ def visit_callable_type(self, t: CallableType, nested: bool = True) -> Type:
                 variables = t.variables
             else:
                 variables, _ = self.bind_function_type_variables(t, t)
-            special = self.anal_type_guard(t.ret_type)
+            type_guard = self.anal_type_guard(t.ret_type)
+            type_is = self.anal_type_is(t.ret_type)
             arg_kinds = t.arg_kinds
             if len(arg_kinds) >= 2 and arg_kinds[-2] == ARG_STAR and arg_kinds[-1] == ARG_STAR2:
                 arg_types = self.anal_array(t.arg_types[:-2], nested=nested) + [
@@ -1036,7 +1040,8 @@ def visit_callable_type(self, t: CallableType, nested: bool = True) -> Type:
                 # its type will be the falsey FakeInfo
                 fallback=(t.fallback if t.fallback.type else self.named_type("builtins.function")),
                 variables=self.anal_var_defs(variables),
-                type_guard=special,
+                type_guard=type_guard,
+                type_is=type_is,
                 unpack_kwargs=unpacked_kwargs,
             )
         return ret
@@ -1059,6 +1064,22 @@ def anal_type_guard_arg(self, t: UnboundType, fullname: str) -> Type | None:
             return self.anal_type(t.args[0])
         return None
 
+    def anal_type_is(self, t: Type) -> Type | None:
+        if isinstance(t, UnboundType):
+            sym = self.lookup_qualified(t.name, t)
+            if sym is not None and sym.node is not None:
+                return self.anal_type_is_arg(t, sym.node.fullname)
+        # TODO: What if it's an Instance? Then use t.type.fullname?
+        return None
+
+    def anal_type_is_arg(self, t: UnboundType, fullname: str) -> Type | None:
+        if fullname in ("typing_extensions.TypeIs", "typing.TypeIs"):
+            if len(t.args) != 1:
+                self.fail("TypeIs must have exactly one type argument", t, code=codes.VALID_TYPE)
+                return AnyType(TypeOfAny.from_error)
+            return self.anal_type(t.args[0])
+        return None
+
     def anal_star_arg_type(self, t: Type, kind: ArgKind, nested: bool) -> Type:
         """Analyze signature argument type for *args and **kwargs argument."""
         if isinstance(t, UnboundType) and t.name and "." in t.name and not t.args: