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I wrote the following script called "bonkers.py":
--
 import sys
 print(sys.argv[0])
--
then ran
 % python3 -m unittest bonkers
It printed
--
 ['python -m unittest', 'bonkers']
--
Shouldn't it say "python3", not "python"? Maybe it should use sys.executable? (Though that gets tricky if there are spaces in the filename.)

It looks like the offending line is here:
http://hg.python.org/cpython/file/6ccb04c4cbae/Lib/unittest/__main__.py#l5
if sys.argv[0].endswith("__main__.py"):
 sys.argv[0] = "python -m unittest"

Certainly. But what is the right thing to do?
I talked to Michael about it in person this morning, and our consensus was: use basename of sys.executable.
Patch attached.

(I was just including the line for the convenience of anyone that might happen to come across the issue. It was not to inform you of course! :) )
FWIW, if you already know from talking to Michael or looking at the code, I think a comment saying why sys.argv[0] is being rewritten in the first place would be useful. For example, is it strictly to have a nicer-looking usage string for --help, or are there other reasons?
$ ./python.exe -m unittest --help
Usage: python -m unittest [options]
...

Yes it is for nicer help output, and a comment in the code is certainly warranted.

Michael, you're the one who came up with the feature. If you write the comment I'll fold it into the patch.

Added comment explaining why we alter the executable string.

New changeset c76f1d78ff78 by Michael Foord in branch 'default':
Closes issue 16064. No longer hard code executable name in unittest help output.
http://hg.python.org/cpython/rev/c76f1d78ff78