13 Dec 09 Python Subprocess Popen and Windows

I was writing a Python script to automate some Subversion-related tasks at work last week on Windows. The Python code was calling the Subversion command-line client applications using the Python subprocess module. I don’t know why exactly, but I had passed in the parameter shell=True to the subprocess.Popen function. I probably did this because I saw a lot of examples on the web, and you know, monkey-see, monkey-do. In hindsight, these examples were probably for Unix. It turned out that this caused a lot of problems. The Subversion clients started complaining when handed filenames that contained “special” characters like ampersands. And when they got to the files that had both ampersands and spaces, all kinds of crazy errors were produced. I was pulling my hair out, trying to escape the special characters and/or use quotes around the filename, but I could not come up with a combination that would work for all the different filenames the script encountered. Finally, as I stared at the code, I somehow got the idea to try switching that shell=True to shell=False.  Suddenly, everything worked perfectly. I undid all the escaping and quoting and everything worked as expected.

So I’m not sure what shell=True does on Windows, and it certainly isn’t documented. I do not recommend using it unless you know what is happening  in the implementation. Interestingly, shell=False is the default. I should have left it that way.

Tags: python, subprocess, svn, windows