Add optional error code for legacy type comments

CHANGELOG.md

@@ -32,6 +32,26 @@ necessary.
 Contributed by Marc Mueller (PR [20410](https://github.com/python/mypy/pull/20410))
 and (PR [20405](https://github.com/python/mypy/pull/20405)).
 
+### Added optional error code `type-comment`
+
+A new disabled by default error code `type-comment` was added. If enabled with
+`--enable-error-code type-comment`, mypy will generate errors if legacy type comments instead of
+type annotations are used.
+
+```py
+a = 2  # type: int
+a: int = 2
+
+def func(a, b):
+    # type: (int, str) -> bool
+    ...
+
+def func(a: int, b: str) -> bool:
+    ...
+```
+
+Contributed by Marc Mueller (PR [20616](https://github.com/python/mypy/pull/20616)).
+
 ## Mypy 1.19
 
 We’ve just uploaded mypy 1.19.0 to the Python Package Index ([PyPI](https://pypi.org/project/mypy/)).

docs/source/error_code_list2.rst

@@ -699,3 +699,29 @@ Example:
         @printing_decorator  # E: Untyped decorator makes function "add_forty_two" untyped  [untyped-decorator]
         def add_forty_two(value: int) -> int:
             return value + 42
+
+.. _code-type-comment:
+
+Check that no legacy type comments are used [type-comment]
+----------------------------------------------------------
+
+If enabled with :option:`--enable-error-code type-comment <mypy --enable-error-code>`,
+mypy generates an error if legacy type comments are used. Tools like
+[com2ann](https://github.com/ilevkivskyi/com2ann) can help with translating type comments to
+type annotations.
+
+More information about type comments are available in the
+[typing specification](https://typing.python.org/en/latest/spec/historical.html#type-comments).
+
+Example:
+
+.. code-block:: python
+
+    o = 2  # type: int
+
+    for x, y in points:  # type: float, float
+        ...
+
+    def func(a, b):
+        # type: (str, int) -> bool
+        ...

mypy/errorcodes.py

@@ -280,6 +280,12 @@ def __hash__(self) -> int:
     "Error when a string is used where a TypeForm is expected but a string annotation cannot be recognized",
     "General",
 )
+TYPE_COMMENT: Final = ErrorCode(
+    "type-comment",
+    "Error when legacy type comments are used instead of type annotations",
+    "General",
+    default_enabled=False,
+)
 
 # Syntax errors are often blocking.
 SYNTAX: Final = ErrorCode("syntax", "Report syntax errors", "General")

mypy/fastparse.py

@@ -927,6 +927,9 @@ def do_func_def(
             arg_types = [None] * len(args)
             return_type = None
         elif n.type_comment is not None:
+            self.fail(
+                message_registry.TYPE_COMMENT_SOFT_DEPRECATED, lineno, n.col_offset, blocker=False
+            )
             try:
                 func_type_ast = ast3_parse(n.type_comment, "<func_type>", "func_type")
                 assert isinstance(func_type_ast, FunctionType)
@@ -1138,7 +1141,13 @@ def make_argument(
             arg_type = None
             if annotation is not None:
                 arg_type = TypeConverter(self.errors, line=arg.lineno).visit(annotation)
-            else:
+            elif type_comment is not None:
+                self.fail(
+                    message_registry.TYPE_COMMENT_SOFT_DEPRECATED,
+                    arg.lineno,
+                    arg.col_offset,
+                    blocker=False,
+                )
                 arg_type = self.translate_type_comment(arg, type_comment)
         if argument_elide_name(arg.arg):
             pos_only = True
@@ -1266,6 +1275,13 @@ def visit_Delete(self, n: ast3.Delete) -> DelStmt:
     def visit_Assign(self, n: ast3.Assign) -> AssignmentStmt:
         lvalues = self.translate_expr_list(n.targets)
         rvalue = self.visit(n.value)
+        if n.type_comment is not None:
+            self.fail(
+                message_registry.TYPE_COMMENT_SOFT_DEPRECATED,
+                n.lineno,
+                n.col_offset,
+                blocker=False,
+            )
         typ = self.translate_type_comment(n, n.type_comment)
         s = AssignmentStmt(lvalues, rvalue, type=typ, new_syntax=False)
         return self.set_line(s, n)
@@ -1293,6 +1309,13 @@ def visit_AugAssign(self, n: ast3.AugAssign) -> OperatorAssignmentStmt:
 
     # For(expr target, expr iter, stmt* body, stmt* orelse, string? type_comment)
     def visit_For(self, n: ast3.For) -> ForStmt:
+        if n.type_comment is not None:
+            self.fail(
+                message_registry.TYPE_COMMENT_SOFT_DEPRECATED,
+                n.lineno,
+                n.col_offset,
+                blocker=False,
+            )
         target_type = self.translate_type_comment(n, n.type_comment)
         node = ForStmt(
             self.visit(n.target),
@@ -1305,6 +1328,13 @@ def visit_For(self, n: ast3.For) -> ForStmt:
 
     # AsyncFor(expr target, expr iter, stmt* body, stmt* orelse, string? type_comment)
     def visit_AsyncFor(self, n: ast3.AsyncFor) -> ForStmt:
+        if n.type_comment is not None:
+            self.fail(
+                message_registry.TYPE_COMMENT_SOFT_DEPRECATED,
+                n.lineno,
+                n.col_offset,
+                blocker=False,
+            )
         target_type = self.translate_type_comment(n, n.type_comment)
         node = ForStmt(
             self.visit(n.target),
@@ -1332,6 +1362,13 @@ def visit_If(self, n: ast3.If) -> IfStmt:
 
     # With(withitem* items, stmt* body, string? type_comment)
     def visit_With(self, n: ast3.With) -> WithStmt:
+        if n.type_comment is not None:
+            self.fail(
+                message_registry.TYPE_COMMENT_SOFT_DEPRECATED,
+                n.lineno,
+                n.col_offset,
+                blocker=False,
+            )
         target_type = self.translate_type_comment(n, n.type_comment)
         node = WithStmt(
             [self.visit(i.context_expr) for i in n.items],
@@ -1343,6 +1380,13 @@ def visit_With(self, n: ast3.With) -> WithStmt:
 
     # AsyncWith(withitem* items, stmt* body, string? type_comment)
     def visit_AsyncWith(self, n: ast3.AsyncWith) -> WithStmt:
+        if n.type_comment is not None:
+            self.fail(
+                message_registry.TYPE_COMMENT_SOFT_DEPRECATED,
+                n.lineno,
+                n.col_offset,
+                blocker=False,
+            )
         target_type = self.translate_type_comment(n, n.type_comment)
         s = WithStmt(
             [self.visit(i.context_expr) for i in n.items],

mypy/message_registry.py

@@ -311,6 +311,10 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 TYPE_COMMENT_SYNTAX_ERROR_VALUE: Final = ErrorMessage(
     'Syntax error in type comment "{}"', codes.SYNTAX
 )
+TYPE_COMMENT_SOFT_DEPRECATED: Final = ErrorMessage(
+    "Using type comments with mypy is (soft-)deprecated, use inline annotations instead",
+    codes.TYPE_COMMENT,
+)
 ELLIPSIS_WITH_OTHER_TYPEPARAMS: Final = ErrorMessage(
     "Ellipses cannot accompany other parameter types in function type signature", codes.SYNTAX
 )

test-data/unit/check-errorcodes.test

@@ -1335,3 +1335,28 @@ def process(response1: int,response2: int) -> int: # E: Overloaded function sign
 
 def process(response1,response2)-> Union[float,int]:
    return response1 + response2
+
+[case testLegacyTypeComments]
+# flags: --enable-error-code type-comment
+def f(): ...
+async def g(): ...
+
+a = 2  # type: int  # E: Using type comments with mypy is (soft-)deprecated, use inline annotations instead  [type-comment]
+
+for b in ():  # type: int  # E: Using type comments with mypy is (soft-)deprecated, use inline annotations instead  [type-comment]
+    ...
+
+with f() as foo:  # type: int  # E: Using type comments with mypy is (soft-)deprecated, use inline annotations instead  [type-comment]
+    ...
+
+def func(d, e):  # E: Using type comments with mypy is (soft-)deprecated, use inline annotations instead  [type-comment]
+    # type: (str, int) -> bool
+    ...
+
+async def func2():  # E: Using type comments with mypy is (soft-)deprecated, use inline annotations instead  [type-comment]
+    # type: () -> None
+
+    async for c in g():  # type: int  # E: Using type comments with mypy is (soft-)deprecated, use inline annotations instead  [type-comment]
+        ...
+
+[builtins fixtures/tuple.pyi]