gh-81548: Deprecate octal sequences with value larger than 0o377

[serhiy-storchaka]

Closes #81548.

[gvanrossum]

I'm glad you're doing this!

[serhiy-storchaka]

Interesting, you posted your comment just between two commits which I have pushed with one git push.

[slateny]

I think this line should be :gh: instead of :issue: - got a failed CI here

[disconnect3d]

If possible, I would suggest extending the information about this deprecation warning as it is not obvious for people reading this why is this made and when does the deprecation warning fires off.

1. The former could be something like:

This is done because high octal escape values may give unexpected results. For example the b'\407' literal is b'\x07' as the 0o407 (263) value is treated as a single byte and 263 % 256 = 7.

2. The latter issue, when the deprecation warning fires off, can be seen here:

$ cat a.py
print("\407", b"\407")

$ python3.11 a.py
ć b'\x07'

$ python3.11 -W default a.py
//a.py:1: DeprecationWarning: invalid octal escape sequence '\407'
  print("\407", b"\407")
//a.py:1: DeprecationWarning: invalid octal escape sequence '\407'
  print("\407", b"\407")
ć b'\x07'

I believe this comes from the default warning filter. Idk if we really want it described there, but imho it never hurts to provide more information.

cc: @serhiy-storchaka

[pablogsal]

If possible, I would suggest extending the information about this deprecation warning as it is not obvious for people reading this why is this made and when does the deprecation warning fires off.

1. The former could be something like:

This is done because high octal escape values may give unexpected results. For example the b'\407' literal is b'\x07' as the 0o407 (263) value is treated as a single byte and 263 % 256 = 7.

2. The latter issue, when the deprecation warning fires off, can be seen here:

$ cat a.py
print("\407", b"\407")

$ python3.11 a.py
ć b'\x07'

$ python3.11 -W default a.py
//a.py:1: DeprecationWarning: invalid octal escape sequence '\407'
  print("\407", b"\407")
//a.py:1: DeprecationWarning: invalid octal escape sequence '\407'
  print("\407", b"\407")
ć b'\x07'

I believe this comes from the default warning filter. Idk if we really want it described there, but imho it never hurts to provide more information.

cc: @serhiy-storchaka

Will give this a go soon 👍

[arhadthedev]

Will give this a go soon 👍

@pablogsal