python3.9 -d:

Python 3.9.7 (default, Oct 21 2021, 20:51:19)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
Loaded pyinteractive.py.

>>>

python3.10 -d:

Python 3.10.0 (default, Oct 21 2021, 23:13:32) [GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
line[1] = "\"\"\" pyinteractive.py\n"  tok->done = 10
line[2] = "\n"  tok->done = 10
line[3] = "    This file is executed on interactive startup by Python\n"  tok->done = 10
...

(not that in both cases a PYTHONINTERACTIVE script is loaded)

Is that intended ?

What's even worse is that those debug lines get written to stdout, not stderr.

Yeah, those are expected in 3.10, but this should not appear in release versions (only on debug ones).

Python -d is normally used to debug the parser:

-d     : turn on parser debugging output (for experts only, only works on
         debug builds); also PYTHONDEBUG=x

New changeset 86dfb55d2e091cf633dbd7aabcd49d96fb1f9d81 by Pablo Galindo Salgado in branch 'main':
bpo-45562: Only show debug output from the parser in debug builds (GH-29140)
https://github.com/python/cpython/commit/86dfb55d2e091cf633dbd7aabcd49d96fb1f9d81

Yes, I know that (at the moment) it's only documented to work in the parser, but since Py_DebugFlag is a general purpose flag, this use could easily be extended to other parts of the interpreter as well, e.g. for parsing the command line or instrumenting the interpreter to collect debug stats.

In any case, thanks for the quick fix, Pablo.

New changeset ae78ffdc9399802621eabcd1668e44a91ec5f45e by Miss Islington (bot) in branch '3.10':
bpo-45562: Only show debug output from the parser in debug builds (GH-29140) (#29149)
https://github.com/python/cpython/commit/ae78ffdc9399802621eabcd1668e44a91ec5f45e

Thanks you for the catch!

I had left a comment on Github about using stderr instead of stdout, to make the output more consistent (other parser error messages go to stderr).

Note sure whether that's something you still want to change before closing the issue.

Yeah, let me fix that in another PR

New changeset 10bbd41ba8c88bc102df108a4e0444abc7c5ea43 by Pablo Galindo Salgado in branch 'main':
bpo-45562: Print tokenizer debug messages to stderr (GH-29250)
https://github.com/python/cpython/commit/10bbd41ba8c88bc102df108a4e0444abc7c5ea43

New changeset 038f45230892235e806ef227dfd9484b95687823 by Miss Islington (bot) in branch '3.10':
bpo-45562: Print tokenizer debug messages to stderr (GH-29250) (GH-29252)
https://github.com/python/cpython/commit/038f45230892235e806ef227dfd9484b95687823

Hi Pablo,

I think you missed one instance:

   print_escape(stdout, tok->cur, tok->inp - tok->cur);

Cheers

New changeset cdc7a5827754bec83970bb052d410d55f85b3fff by Pablo Galindo Salgado in branch 'main':
bpo-45562: Ensure all tokenizer debug messages are printed to stderr (GH-29270)
https://github.com/python/cpython/commit/cdc7a5827754bec83970bb052d410d55f85b3fff

Thanks, Pablo :-)

New changeset d8ca47c943c4ad396a9c70aff35bbbf4b1868876 by Miss Islington (bot) in branch '3.10':
bpo-45562: Ensure all tokenizer debug messages are printed to stderr (GH-29270)
https://github.com/python/cpython/commit/d8ca47c943c4ad396a9c70aff35bbbf4b1868876