Improves error message for parsing slice in annotations

[sobolevn]

Closes #10266

[Zac-HD]

Just want to check explicitly: is it possible to put a # type: ignore on this, or better yet to ignore all such errors from configuration? The latter would be especially useful for projects using array-related typing extensions.

[sobolevn]

is it possible to put a # type: ignore on this, or better yet to ignore all such errors from configuration?

Nope, does not look like it:

I think the reason is that it is raised in fastparse.py, which is quite a low level for type: ignore.
I will try to make it work (if no big changes are be required).

[sobolevn]

The same with: mypy ex.py --show-error-codes --disable-error-code syntax
It still shows this message.

[sobolevn]

Ok, looks like --disable-error-code can be used with this corner case quite easily:

From 006fbd50f6c8a2f99658367afaa15d29ecd5f851 Mon Sep 17 00:00:00 2001
From: sobolevn <[email protected]>
Date: Sat, 2 Oct 2021 10:31:47 +0300
Subject: [PATCH] Adds new error code for slice syntax

---
 mypy/errorcodes.py |  1 +
 mypy/errors.py     | 11 +++++++++++
 mypy/fastparse.py  | 13 +++++++++----
 3 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/mypy/errorcodes.py b/mypy/errorcodes.py
index d9e11a044..3ae079f08 100644
--- a/mypy/errorcodes.py
+++ b/mypy/errorcodes.py
@@ -135,6 +135,7 @@ NAME_MATCH: Final = ErrorCode(
 
 # Syntax errors are often blocking.
 SYNTAX: Final = ErrorCode("syntax", "Report syntax errors", "General")
+SLICE_SYNTAX: Final = ErrorCode("slice-syntax", "Report slice syntax errors", "General")
 
 # This is a catch-all for remaining uncategorized errors.
 MISC: Final = ErrorCode("misc", "Miscellaneous other checks", "General")
diff --git a/mypy/errors.py b/mypy/errors.py
index 3a0e0e14d..a68cfe2ac 100644
--- a/mypy/errors.py
+++ b/mypy/errors.py
@@ -374,6 +374,17 @@ class Errors:
                         return
             if file in self.ignored_files:
                 return
+            if (
+                not self.ignored_lines
+                and info.code
+                and self.is_error_code_enabled(info.code) is False
+            ):
+                # We also might want to ignore some errors during `fastparse`.
+                # At that moment `ignored_lines` might not be ready,
+                # so, we only check for specific error codes.
+                # For example, `Dict[{str: int}]` can be ignored this way.
+                return
+
         if info.only_once:
             if info.message in self.only_once_messages:
                 return
diff --git a/mypy/fastparse.py b/mypy/fastparse.py
index 386bfb948..870278284 100644
--- a/mypy/fastparse.py
+++ b/mypy/fastparse.py
@@ -1359,9 +1359,12 @@ class TypeConverter:
             return None
         return self.node_stack[-2]
 
-    def fail(self, msg: str, line: int, column: int) -> None:
+    def fail(self, msg: str, line: int, column: int,
+             blocker: bool = True, code: Optional[codes.ErrorCode] = None) -> None:
+        if code is None:
+            code = codes.SYNTAX
         if self.errors:
-            self.errors.report(line, column, msg, blocker=True, code=codes.SYNTAX)
+            self.errors.report(line, column, msg, blocker=blocker, code=code)
 
     def note(self, msg: str, line: int, column: int) -> None:
         if self.errors:
@@ -1562,12 +1565,14 @@ class TypeConverter:
             if (isinstance(sliceval, ast3.Slice) or
                 (isinstance(sliceval, ast3.Tuple) and
                  any(isinstance(x, ast3.Slice) for x in sliceval.elts))):
-                self.fail(INVALID_SLICE_ERROR, self.line, getattr(n, 'col_offset', -1))
+                self.fail(INVALID_SLICE_ERROR, self.line, getattr(n, 'col_offset', -1),
+                          blocker=False, code=codes.SLICE_SYNTAX)
                 return AnyType(TypeOfAny.from_error)
         else:
             # Python 3.8 or earlier use a different AST structure for subscripts
             if not isinstance(n.slice, Index):
-                self.fail(INVALID_SLICE_ERROR, self.line, getattr(n, 'col_offset', -1))
+                self.fail(INVALID_SLICE_ERROR, self.line, getattr(n, 'col_offset', -1),
+                          blocker=False, code=codes.SLICE_SYNTAX)
                 return AnyType(TypeOfAny.from_error)
             sliceval = n.slice.value
 
-- 
2.23.0

Let's merge this as is: just improving the error message. Later, we can review and merge / decline the patch above.
It might have issues in errors.py part 🤔

[Zac-HD]

Sounds good - thanks for your work on this @sobolevn (and maintainers)!

I'm definitely keen to get the second patch in too - though maybe not classified as a syntax error - so that I can use mypy on projects using the torchtyping package, but this patch is already a clear improvement IMO.