Avoid python3 error with no stdin

[mattaezell]

Under python3, sys.stdin is None if there is no stdin attached to the process:

$ python3 -c 'import sys; assert not sys.stdin.isatty()' 0<&-
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'isatty'

Originally reported at https://bugs.schedmd.com/show_bug.cgi?id=11021

[thiell]

Great finding @mattaezell! I will review your patch ASAP. We should include the fix in 1.8.4.

[thiell]

It's interesting. I can't find any documentation to explain when sys.stdin can be None in the Python documentation. Looks like other developers are facing the same problem with Python 3: https://stackoverflow.com/questions/17458728/when-is-sys-stdin-none-in-python/17459258
I guess we have no choice but checking whether it is defined, like you're doing in your patch indeed. We'll need to find a way to make a test too.

[thiell]

It's set to None here:
https://hg.python.org/cpython/file/dbceba88b96e/Python/pythonrun.c#l1201

    /* Under some conditions stdin, stdout and stderr may not be connected
     * and fileno() may point to an invalid file descriptor. For example
     * GUI apps don't have valid standard streams by default.
     */

[thiell]

Ah! It is actually documented under sys.__stdin__ in the Python 3 documentation. It was not documented in Python 2.

Note: Under some conditions stdin, stdout and stderr as well as the original values stdin, stdout and stderr can be None. It is usually the case for Windows GUI apps that aren’t connected to a console and Python apps started with pythonw.

[thiell]

looks better :)