How to read a print out statement from another program with python?

Question

I have an algorithm that is written in C++ that outputs a cout debug statement to the terminal window and I would like to figure out how to read that printout with python without it being piped/written to a file or to return a value.

Python organizes how each of the individual C++ algorithms are called while the data is kept on the heap and not onto disk. Below is an example of the a situation that is of similar output,

+-------------- terminal window-----------------+

(c++)runNewAlgo: Debug printouts on

(c++)runNewAlgo: closing pipes and exiting 

(c++)runNewAlgo: There are 5 objects of interest found

( PYTHON LINE READS THE PRINT OUT STATEMENT)

(python)main.py: Starting the next processing node, calling algorithm

(c++)newProcessNode: Node does work

+---------------------------------------------------+

Say the line of interest is "there are 5 objects of interest" and the code will be inserted before the python call. I've tried to use sys.stdout and subprocess.Popen() but I'm struggling here.

I don't understand. Your question is about Python, but you tag as C++. Are you mixing the languages? If not, only tag the language you will be using. — Thomas Matthews
– Thomas Matthews, Commented Dec 28, 2020 at 19:27
Will you start the C++ algorithm from within the python script or externally? Would piping the output to python work for you? — Marc
– Marc, Commented Dec 28, 2020 at 19:27
@ThomasMatthews I was the one that edited in the C++ tag. I did so because the OP wants to read the printout of a C++ program with Python, so therefore he is kind of mixing the languages. Did I do so incorrectly? — M-Chen-3
– M-Chen-3, Commented Dec 28, 2020 at 19:29
the languages are mixed in the API that is being used. The C++ algorithm is started from within the python script. Piping may be of use but this is run remotely which may cause a memory backup if not managed correctly. — kylernstlol
– kylernstlol, Commented Dec 28, 2020 at 19:34

Meldroc · Accepted Answer · 2020-12-28 19:27:56Z

0

Your easiest path would probably be to invoke your C++ program from inside your Python script.

More details here: How to call an external program in python and retrieve the output and return code?

answered Dec 28, 2020 at 19:27

Meldroc

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Brandon · Accepted Answer · 2020-12-28 19:30:33Z

0

You can use stdout from the returned process and read it line-by-line. The key is to pass stdout=subprocess.PIPE so that the output is sent to a pipe instead of being printed to your terminal (via sys.stdout).

Since you're printing human-readable text from your C++ program, you can also pass encoding='utf-8' as well to automatically decode each line using utf-8 encoding; otherwise, raw bytes will be returned.

import subprocess

proc = subprocess.Popen(['/path/to/your/c++/program'],
                        stdout=subprocess.PIPE, encoding='utf-8')
for line in proc.stdout:
  do_something_with(line)
  print(line, end='') # if you also want to see each line printed

answered Dec 28, 2020 at 19:30

Brandon

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