`py -V:X.Y script.py` fails with "No suitable Python runtime found" when script arguments contain forward slashes

[xyzzy-foo]

Summary

When using the Python Launcher (py.exe) with a version flag (-V:X.Y), any script argument containing a forward slash (e.g. ./output, /tmp, C:/foo) causes exit code 103 ("No suitable Python runtime found"), even though the specified Python version is installed.

Reproduction

# test_simple.py
import sys
print('ran ok, argv:', sys.argv[1:])

py -V:3.14 test_simple.py files      # OK
py -V:3.14 test_simple.py ./files    # FAIL: "No suitable Python runtime found"
py -V:3.14 test_simple.py --help     # OK  (no slash)
py -V:3.14 test_simple.py .\files    # OK  (backslash is fine)
py -V:3.14 test_simple.py /files     # FAIL
py -V:3.14 test_simple.py C:/files   # FAIL

py test_simple.py ./files            # OK  (no -V: flag = unaffected)

Root cause

In PC/launcher2.c, parseCommandLine() parses -V:company/tag as follows:

const wchar_t *argStart = wcschr(arg, L':') + 1;   // points past ':' in "-V:3.14"
const wchar_t *tagStart = wcschr(argStart, L'/');   // BUG: scans entire remaining cmdline

arg points into the full command-line string (from GetCommandLineW), not just the current token. tail (the whitespace-bounded end of the -V: token) is computed earlier but is not used to bound the wcschr search.

For py -V:3.14 script.py ./files, argStart is "3.14 script.py ./files...". The unbounded wcschr finds the / in ./files, and misparses:

company = "3.14 script.py ."   (should be empty)
tag     = "files"               (should be "3.14")

selectEnvironment then finds no Python matching this bogus company/tag and returns RC_NO_PYTHON.

Fix

Limit the / search to within tail:

const wchar_t *argStart = wcschr(arg, L':') + 1;
// Search for '/' only within the current token boundary
const wchar_t *tagStart = NULL;
for (const wchar_t *p = argStart; p < tail; ++p) {
    if (*p == L'/') { tagStart = p; break; }
}

Environment

• Python 3.14.4 (Windows 11, version 25H2)
• py.exe version: 3.14.4 (C:\Windows\py.EXE)
• Also present in launcher2.c on main and 3.13 branches (same code)

[serhiy-storchaka]

You can perhaps simply use wmemchr().

@xyzzy-foo, do you want to provide a PR?

cc @zooba

[zooba]

This launcher is also deprecated and will be removed after 3.15, so no major urgency. The issue has been in there for a long time. Would accept a PR though.