I am trying to write a python script that checks out a revision from

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Asked: May 27, 20262026-05-27T13:45:19+00:00 2026-05-27T13:45:19+00:00

I am trying to write a python script that checks out a revision from

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I am trying to write a python script that checks out a revision from a mercurial repo for a visual studio 2010 native c++ project, builds it, and then runs a program under various scenarios. Then I will compare results from other builds, etc. So I am just getting started with a prototype, and I have:

from subprocess import call
import os
import tempfile
import sys


def main():

    temp_repo_name = 'temprepo'
    d = tempfile.mkdtemp()
    os.chdir(os.path.normpath(d))

    command1 = ['hg', 'clone', r'C:\temp\1\jxg_hcr', temp_repo_name]

    devnull = open(os.devnull,'w')

    rc1 = call(command1,stdout=devnull, stderr=devnull)

    if rc1 != 0:
        print('could not clone repo into temporary directory.  Terminating Program')
        sys.exit(1)

    devnull.close()

    devnull = open(os.devnull,'w')

    os.chdir(temp_repo_name)

    command2 = [r'msbuild', r'hcr_dll.sln', r'/t:Rebuild',r'/p:Configuration=Release']
    rc2 = call(command2,stdout=devnull, stderr=devnull)

    print rc2
    if rc2 != 0:
        print('could not build repo.  Terminating Program')
        sys.exit(1)

    devnull.close()

if __name__ == '__main__':
    main()

When I run this, I get the following output on my console:

C:\programming\eclipse_workspace\hcr_cli_build>python hcr_cli_build.py
0
1
could not build repo.  Terminating Program

But when I change the line for command2 to

rc2 = call(command2)

I get

0
<bunch of build output>
0

and it builds successfully. I do not know why it would fail when I redirect.

Any Ideas? Maybe just a silly error I cannot see?

NOTE: I don’t think I really need to close and re-open devnull each time, but it was just something I was trying out when I was trying to figure out the issue. I get the same results when I just keep it open the whole time and close it at the very end.

EDIT1: This also fails from the command line when I tried it per the suggestion of David Hess.

EDIT2: I have also verified this same issue occurs when I create an empty C# console application with the boiler plate Main function. It will build via the gui and on the command line when msbuild is not redirected to NUL, but when I do redirect, it returns 1 and does not build.

Bounty Edit: I am most interested in why this is happening. Obviously, I would also like to be able to succeed silently and just output a message saying “Build went okay” if the return code was 0, so if nobody can tell me why it is happening, the bounty will go to the best solution.

I also tried a bash script on mintty.exe:

#!/bin/bash
for i in 1 2 3 4
 do
  echo "doing $i"
  msbuild.exe /c/temp/$i/jxg_hcr/hcr_dll.sln //t:Rebuild //p:Configuration=Release
  echo $?
 done

works (it prints all the output and builds successfully and then prints 0 to the console), but

#!/bin/bash
for i in 1 2 3 4
 do
  echo "doing $i"
  msbuild.exe /c/temp/$i/jxg_hcr/hcr_dll.sln //t:Rebuild //p:Configuration=Release > /dev/null
  echo $?
 done

does not perform the build and just returns 1 to the console.