Unbuffered stdout for Windows and displaying the output of subprocess calls

[YannickJadoul]

Cfr. side issue in #15:

• Forcing Python on Windows to not buffer its stdout, such that the output of cibuildwheel and that of its subprocess calls is nicely interleaved.
• Changing subprocess.check_output to check_call such that the stdout of these calls is not swallowed into a never-printed string by cibuildwheel.

[YannickJadoul]

@joerick There are several other workarounds, if you don't like the sys.stdout = os.fdopen(...) trick.

• Current solution, reopening stdout: https://stackoverflow.com/questions/881696/unbuffered-stdout-in-python-as-in-python-u-from-within-the-program
• Writing a simple wrapper around a stream: https://stackoverflow.com/questions/107705/disable-output-buffering
• Making a custom print function: https://stackoverflow.com/questions/230751/how-to-flush-output-of-python-print (this answer: https://stackoverflow.com/a/35467658)

[YannickJadoul]

Seems to be working, if you ask me. Two more little things:

• The lines of the run_tests.py (https://ci.appveyor.com/project/joerick/cibuildwheel/build/1.0.91#L1643) are still buffered and only output at the very end, because this solution is only applied to cibuildwheel itself. What to do with that? In principle, that is not part of the cibuildwheel library, so we can just as well change appveyor.yml to call python -u or set PYTHONUNBUFFERED=1. But is there then still a point to this PR?

• I've applied the solution only for the windows.py file (since OS X and Linux don't seem to have any problems), and applied a single flush() at the end of the preamble. The other option is either applying the fix when Windows is detected, or just applying it for all OSes.

[joerick]

Excellent PR @YannickJadoul! Thank you for the detailed write-up 👍

The lines of the run_tests.py (https://ci.appveyor.com/project/joerick/cibuildwheel/build/1.0.91#L1643) are still buffered and only output at the very end, because this solution is only applied to cibuildwheel itself. What to do with that? In principle, that is not part of the cibuildwheel library, so we can just as well change appveyor.yml to call python -u or set PYTHONUNBUFFERED=1.

Lets use python -u in appveyor.yml for that.

But is there then still a point to this PR?

Yes! for all the users don't have to worry about that themselves :)

[joerick]

Cool, I've made those changes. I'm good to merge and do a release once the lights go green.

[YannickJadoul]

Cool, seems to be working! And as far as I can see, the output is 1) there and 2) in the right order :-)

Thanks for the quick reaction and solution!

[joerick]

Released as 0.4.1 🎉