is_cgi() in CGIHTTPRequestHandler class separates given path into (dir, rest) then checks if dir is in cgi_directories. However, it divides based on the first seen '/', multi-level directories like /sub/dir/cgi-bin/hello.py is divided into head=/sub, rest=dir/cgi-bin/hello.py then check whether '/sub' exists in cgi_directories = [..., '/sub/dir/cgi-bin'].
If the function divides by last seen '/', it works correctly as head=/sub/dir/cgi-bin, rest=hello.py

CGI programs are stored in a directory which must be configured in the web server. The path is typically SERVER_ROOT/cgi-bin, so the URL looks like
http://www.domain/cgi-bin/script

So IMHO, is_cgi's assumption is correct.
IMHO, this is not the wrong code.

In addition, the code is not about the path on the file system,
but about the web request path.

https://bugs.python.org/msg216960 will help you to understand.

Thank you for your message and the info about 21323. I agree with the idea that cgi files are conventionally placed at the cgi-bin of the root but there is no explicit regulation so other servers, apache for example, handle this kind of sub directories successfully. In short, there is no violation in the form of /subdir/cgi-bin so this is nice to have if the http.server processes this case correctly. By the way, I didn't consider the case of continuous slashes so PR must be modified.

Yes, IMHO, but this code is related to the http.server.CGIHTTPRequestHandler.
This code looks like to be executed on the http.server.CGIHTTPRequestHandler not the apache server.

According to docs,
This defaults to ['/cgi-bin', '/htbin'] and describes directories to treat as containing CGI scripts.

reference: https://docs.python.org/3.9/library/http.server.html?highlight=cgihttprequesthandler#http.server.CGIHTTPRequestHandler.cgi_directories

Hi Donghee,
Since you said this is not a bug, I changed the title describing this is a matter of improvement.

For your comment, I would say sorry first that I have made you confused. My mention about apache is just to give you an example for the other module that does similar thing(HTTP request handler with cgi support). I would never say the Python server shall support the sub-directory parsing for cgi scripts because apache does but I am saying good is good.

>> Yes, IMHO, but this code is related to the http.server.CGIHTTPRequestHandler.
This code looks like to be executed on the http.server.CGIHTTPRequestHandler not the apache server.

So you may see the PR changes the CGIHTTPRequestHandler class. Please refer to the code.

>> According to docs,
This defaults to ['/cgi-bin', '/htbin'] and describes directories to treat as containing CGI scripts.

This is the one I was looking at. As described, I added '/sub/dir/cgi-bin' into the cgi_directories expecting it to be treated the directory is for CGI scripts but the CGIHTTPRequestHandler does not process it. That is the reason for this issue report. Again, there is no note that restrict the path in cgi_directories shall be a single depth, so I think this will definitely give a benefit to handle the multi-level directories just like https://bugs.python.org/issue21323 enables processing the sub directories of /cgi-bin.
Does this make sense to you now?

Thanks, Siwon Kang, I now understood what you want to say.
I left some comments on your PR 17312.

@martin.panter, @asvetlov

I add the core developers who looks like to be experts on this module.
Can you follow up on Siwon's PR?

Thanks for your understanding.

New changeset 91daa9d7224626dad4bb820924c01b3438ca6e3f by Miss Islington (bot) (Siwon Kang) in branch 'master':
bpo-38863: Improve is_cgi() in http.server (GH-17312)
https://github.com/python/cpython/commit/91daa9d7224626dad4bb820924c01b3438ca6e3f

The change introduced a reference leak, see bpo-38962. Pablo fixed it:

commit 24f5cac7254177a4c9956d680c0a9b6dadd85c6f (HEAD -> master, upstream/master)
Author: Pablo Galindo <Pablogsal@gmail.com>
Date:   Wed Dec 4 09:29:10 2019 +0000

    bpo-38962: Fix reference leak in test_httpservers (GH-17454)

Thanks, Pablo!

Obviously, I should have taken back what I added to cgi_directories. Thank you Pablo and Victor!