`SyntaxWarning` problems are hidden after byte-code compilation

[andersk]

Bug report

Bug description:

$ echo '"\s"' > test.py
$ python --version
Python 3.13.1
$ python -m test
/tmp/test.py:1: SyntaxWarning: invalid escape sequence '\s'
  "\s"
$ python -m test
$ python -m test
$ rm -r __pycache__
$ python -m test
/tmp/test.py:1: SyntaxWarning: invalid escape sequence '\s'
  "\s"
$ python -m test

I know that’s because byte-code compilation bypasses the parser. However, the resulting behavior makes it difficult to discover SyntaxWarning problems within modules, especially ones in third-party modules for which pip hides the compilation output.

$ pip install glom==22.1.0
Collecting glom==22.1.0
  Using cached glom-22.1.0-py2.py3-none-any.whl.metadata (4.9 kB)
Collecting boltons>=19.3.0 (from glom==22.1.0)
  Using cached boltons-25.0.0-py3-none-any.whl.metadata (6.5 kB)
Collecting attrs (from glom==22.1.0)
  Using cached attrs-25.1.0-py3-none-any.whl.metadata (10 kB)
Collecting face>=20.1.0 (from glom==22.1.0)
  Using cached face-24.0.0-py3-none-any.whl.metadata (1.1 kB)
Using cached glom-22.1.0-py2.py3-none-any.whl (100 kB)
Using cached boltons-25.0.0-py3-none-any.whl (194 kB)
Using cached face-24.0.0-py3-none-any.whl (54 kB)
Using cached attrs-25.1.0-py3-none-any.whl (63 kB)
Installing collected packages: boltons, attrs, face, glom
Successfully installed attrs-25.1.0 boltons-25.0.0 face-24.0.0 glom-22.1.0
$ python -m glom
{}
$ rm -r /tmp/v/lib/python3.13/site-packages/glom/__pycache__
$ python -m glom
/tmp/v/lib/python3.13/site-packages/glom/matching.py:764: SyntaxWarning: invalid escape sequence '\ '
  """The :class:`Switch` specifier type routes data processing based on
{}
$ python -m glom
{}

It would be better if compilation could record the warning in the .pyc file such that it gets re-emitted on every load, even though the parser was bypassed.

(This was mentioned in #77093 (comment), but it seems nobody took up the suggestion to file this as a separate issue.)

CPython versions tested on:

3.13

Operating systems tested on:

Linux

[ncoghlan]

This is intentional, as the targets of syntax warnings are developers, not end users that are merely running the code.

If the code is being actively worked on, the warning will be emitted each time the file is changed, and it will also be picked up under static analysis by linters and type checkers.