Oh cool, there actually is a difference. It's a little bit inscrutable to me, and hard to determine what it should actually do, but at least it's something!

1. b'\u' does actually become b'\\u' at runtime, so it's good that the parser was representing that.
2. b'\u' raises a SyntaxWarning in Python, not a DeprecationWarning, so the fact that we ignore it with a DeprecationWarning is puzzling.
3. Presumably we're escalating that warning to an error somewhere. Maybe in whatever parser actually gets tested...? So maybe that can just be changed.
4. Both mypy on master and mypy after this change successfully pass a file with b'\u' with no warnings nor errors.

Thanks for taking the initiative of looking into this! A few notes to hopefully clarify what's going on:

• Invalid escape sequences emit a SynaxWarning in Python 3.12+, but emit a DeprecationWarning in earlier versions:

  $ python3 -Wall
  >>> import ast
  
  >>> ast.parse(r"b'\u'", feature_version=(3, 12))
  <unknown>:1: SyntaxWarning: invalid escape sequence '\u'
  <ast.Module object at 0x7cf24a69c790>
  
  >>> ast.parse(r"b'\u'", feature_version=(3, 11))
  <unknown>:1: DeprecationWarning: invalid escape sequence '\u'
  <ast.Module object at 0x7cf24a69c7d0>

  This code was designed to catch those warnings so that invalid escape sequences in the code being type checked don't clutter mypy's output.

  Note that you won't see DeprecationWarnings unless you run Python with warnings enabled (they're silenced by default).

• When you remove the warning filter, the uncaught DeprecationWarnings get promoted to errors since the test suite runs with -Werror.

• The reason you see test failures in all Python versions (not just <3.12) is because parse.test runs with python_version = (3, 9). (I'm not sure why that is. Possibly it would make more sense to use the current Python version to get move coverage?)

• There is actually one problem with this code, which is that it doesn't catch SyntaxWarnings on Python >=3.12:

  $ echo foo.py
  x = b"\u"
  
  $ mypy --cache-dir=/dev/null --python-version=3.11  foo.py
  Success: no issues found in 1 source file
  
  $ mypy --cache-dir=/dev/null --python-version=3.12  foo.py
  foo.py:1: SyntaxWarning: "\u" is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\u"? A raw string is also an option.
    x = b"\u"
  Success: no issues found in 1 source file

  To fix that, we could update the warning filter to something like this:

  category = SyntaxWarning if feature_version >= 12 else DeprecationWarning
  warnings.filterwarnings("ignore", category=category)

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

It's still passing on ≥ 3.12; curious.

Just in case my earlier comment wasn't clear, this code is working as intended, so I wouldn't spend more time trying to get the tests to pass with it removed

Oh, I understood that — although I appreciate the potential clarification. My last push was trying to investigate the necessity of changing what we catch to SyntaxWarning on Python >=3.12. So I'm surprised by the still-passing part, not the still-failing part 🙂.

But I need to ponder it further before drawing hard conclusions.

Ok, great! Just didn't want you to lose time working on something that couldn't be merged :-)

The reason the tests pass on 3.12+ is because SyntaxWarnings aren't promoted to exceptions in the test suite. See