Read output from python script in C++ Qt app

Question

I have a python script that outputs to the console every 10 seconds or so. I want to run that script from my c++ code, and keep it running to continue getting updates until I close my app. If I use popen, I can get the output line by line, without having to wait for the script to "finish", but I cannot kill it when I close my app now.

Is there a way to read the output of a script as it is spit out, keeping the script running until I close my qt app?

Thanks!

Have you looked at QProcess? The doc states how to close the process from code, and it has standard read/write channels to communicate with it. — MatthiasB
– MatthiasB, Commented Feb 14, 2014 at 13:47
@MatthiasBonora Thank you!! I had seen that class, but when I tried it I had a completely separate issue going on, in that python wasn't flushing the stdout buffer. Once I fixed that, QProcess worked like a charm and is awesome! If you want, say that as an answer so I can give you credit. — phil.harlow
– phil.harlow, Commented Feb 14, 2014 at 16:13

MatthiasB · Accepted Answer · 2014-02-14 16:26:52Z

1

QProcess allows you to call a external process from code. The documentation includes information on how to close it within the program. Also you can use the read/write channels for communication.

There are some examples in the documentation and method descriptions, for getting started.

answered Feb 14, 2014 at 16:26

MatthiasB

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3 Comments

phil.harlow Over a year ago

This was the solution, I just passed over it earlier because I had two separate issues. First was that my python and C scripts weren't flushing the stdout buffer, so once I added sys.stdout.flush() in python, and setbuf(stdout, NULL) in C, my scripts were properly writing to the std out. Once that was working, using QProcess's readyReadStandardOutput signal and the readAllStandardOutput method I could properly see all my output! Thanks @MatthiasBonora

Kuba hasn't forgotten Monica Over a year ago

@phil.harlow Console buffers in Python are guaranteed to flush as the application exits - so if your script ran to completion and didn't abort early/crash, it should have flushed its buffers. Same applies to the C runtime - any half-decent implementation will flush the stdout/stderr buffers once main returns. I think your problem wasn't quite where you thought it was.

phil.harlow Over a year ago

@KubaOber you are right, in that I would see ALL of my python script's output at once when closing my app. The problem is I wanted to see those messages as they were pushed out by python, line by line, so adding sys.stdout.flush() after my printf statements in python now pushed the buffer and let it show up in c++. That was one problem that caused me to think the QProcess readyReadStandardOutput signal wasn't working, because it was only firing once at the end. Once I fixed the python buffer flush I was able to figure out the QProcess signal and all is well!

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