Fix find_python_executable error

[Edward-Knight]

Fixes #330

I also took the liberty of adding type hints.

Behaviour before:

$ flit install --python pyhton
Python executable None not found
$ flit install --python ls
ls: cannot access 'import sys; print(sys.executable)': No such file or directory
Traceback (most recent call last):
  File "/PATH/bin/flit", line 8, in <module>
    sys.exit(main())
  File "/PATH/lib/python3.7/site-packages/flit/__init__.py", line 168, in main
    python = find_python_executable(args.python)
  File "/PATH/lib/python3.7/site-packages/flit/__init__.py", line 33, in find_python_executable
    universal_newlines=True,
  File "/PATH/lib/python3.7/subprocess.py", line 411, in check_output
    **kwargs).stdout
  File "/PATH/lib/python3.7/subprocess.py", line 512, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['/PATH/ls', '-c', 'import sys; print(sys.executable)']' returned non-zero exit status 2.
$ flit install --python .
Python executable None not found

Behaviour after:

$ flit install --python pyhton
Python executable 'pyhton' not found
$ flit install --python ls
ls: cannot access 'import sys; print(sys.executable)': No such file or directory
CalledProcessError occurred trying to find the absolute filepath of Python executable 'ls'
$ flit install --python .
PermissionError occurred trying to find the absolute filepath of Python executable '.'

[Edward-Knight]

I can add tests for those exceptions and error messages if needed :)

[takluyver]

Thanks! Comments inline.

Yes, one or two tests of handling bad cases would be good. It doesn't need to be exhaustive, and I try to avoid testing the precise wording of human-readable error messages.

[takluyver]

Thanks - are you still interested in restoring the which() call and adding a test? Or should I pick it up?

[Edward-Knight]

Hey, yeah I'll add it in tonight if that's okay? Been a bit busy the past couple of days

[takluyver]

That's fine, just checking you were still aware of it :-)

…

On Wed, 1 Apr 2020, 18:33 Edward Knight, ***@***.***> wrote: Hey, yeah I'll add it in tonight if that's okay? Been a bit busy the past couple of days — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#331 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AACQB5MEHGTYQYCYZGXCOA3RKN3HBANCNFSM4LVW63DQ> .

[Edward-Knight]

Will have to postpone till tomorrow, apologies!

[Edward-Knight]

I'll have some time on the weekend to add in the tests hopefully :)

[takluyver]

Thanks, that looks good.