Output from C++ library not shown on IPython console

[gtarabat]

Issue Report Checklist

• Searched the issues page for similar reports
• Read the relevant sections of the Spyder Troubleshooting Guide and followed its advice
• Reproduced the issue after updating with conda update Spyder (or pip, if not using Anaconda)
• Could not reproduce inside jupyter qtconsole (if console-related)
• Tried basic troubleshooting (if a bug/error)
  • Restarted Spyder
  • Reset preferences with Spyder --reset
  • Reinstalled the latest version of Anaconda
  • Tried the other applicable steps from the Troubleshooting Guide
• Completed the Problem Description, Steps to Reproduce and Version sections below

Problem Description

I create a Python binding of a C++ code using the library pybind11. The C++ code has a single function that receives an integer N and prints all the integers less than N, one on each line. I run the python part inside Spyder.

When N is small, e.g. N=100, the expected output is obtained. When N is large, e.g. N=100000, no output is obtained and the script hangs.

The problem does not appear if I run from IPython or from terminal or from a jupyter notebook. The same problem appears with pip and conda.

What steps reproduce the problem?

1. conda install pybind11

2. Open a file with name printx.cpp and paste the following code:

#include <iostream>
#include <pybind11/pybind11.h>

void printloop(int N){
  for(int i=0; i<N; i++)
    printf("%d\n",i);
}

PYBIND11_MODULE(printx, m) {
    m.doc() = "Minimal working example to reproduce a bug in Spyder using pybind11.";
    m.def("printloop", &printloop, "Print integers in a loop.");
}

3. Compile the library:

c++ -O3 -Wall -shared -std=c++11 -undefined dynamic_lookup `python3 -m pybind11 --includes` printx.cpp -o printx`python3-config --extension-suffix`

4. Run the following from Spyder:

import printx as x
x.printloop(100000);

What is the expected output? What do you see instead?

The expected output is 100000 integers printed on the screen, one on each line. Instead, nothing is printed on screen and the python script hangs.

Versions

• Spyder version: 3.3.3
• Python version: 3.7.3
• Qt version: 5.9.7
• PyQt version: 5.9.2
• Operating System name/version: macOS Mojave (10.14.5)

Dependencies

IPython >=4.0     :  7.4.0 (OK)
cython >=0.21     :  0.29.6 (OK)
jedi >=0.9.0      :  0.13.3 (OK)
matplotlib >=2.0.0:  3.0.3 (OK)
nbconvert >=4.0   :  5.4.1 (OK)
numpy >=1.7       :  1.16.2 (OK)
pandas >=0.13.1   :  0.24.2 (OK)
psutil >=0.3      :  5.6.1 (OK)
pycodestyle >=2.3 :  2.5.0 (OK)
pyflakes >=0.6.0  :  2.1.1 (OK)
pygments >=2.0    :  2.3.1 (OK)
pylint >=0.25     :  2.3.1 (OK)
qtconsole >=4.2.0 :  4.4.3 (OK)
rope >=0.9.4      :  0.12.0 (OK)
sphinx >=0.6.6    :  1.8.5 (OK)
sympy >=0.7.3     :  1.3 (OK)

[ccordoba12]

This is fixed in our latest version. Please update by opening Terminal.app and running there

conda install spyder=3.3.5

[gtarabat]

I updated to spyder=3.3.5 but the problem remains.

[ccordoba12]

Could you use std::out instead of printfto see if that makes a difference? Thanks!

[gtarabat]

Thank you Carlos. Using std::cout has the same behavior as with printf.

[groussea]

I get a similar problem although the outputs are visible at the end of the C++ run in my case.
However, it would be nice to show the outputs in spyder-kernel when code is running as it works with iPython in an external system terminal.

[moorepants]

We are having a possibly related issue here: mechmotum/cyipopt#88 FYI

[ricewenzhou]

I make a .pyd with pybind11. The std::cout works fine in Ipython when it is called from the pwsh, but it does not work in spyder. ipython 7.4. spyder 4.2.3.

[ccordoba12]

There's already a pull request to solve this, we'll try to merge it in the next weeks.