Using cyqwin 3.3.4-2, and python3:

Python 3.9.10 (main, Jan 20 2022, 21:37:52)
[GCC 11.2.0] on cygwin

Trying this bash command line:

> python3 C:/path/to/script.py

results in this error:

"python3: can't open file '/cygdrive/c/path/to/curdir/C:/path/to/script.py': [Errno 2] No such file or directory"

OTOH, calling it like

> python3 /cygdrive/c/path/to/script.py

gives the expected output:

"usage: script.py [-h] ..."

It seems that python3 doesn't recognize "C:/path/to/script.py" to be a proper full path under cygwin, while most other cygwin apps handle those fine. E.g.,

> nano C:/path/to/script.py

opens the script for editing without problems.

The mentioned path syntax is useful and supported under cygwin, so it would be nice if python3 could support it, too. Especially useful it is in mixed development environment, mixing Windows native tools and cygwin ones; using such path style allows to use same paths for both kinds of tools, simplifying scripts.