“The system cannot find the file specified” when invoking subprocess.Popen in python

Question

I'm trying to use svnmerge.py to merge some files. Under the hood it uses python, and when I use it I get an error - "The system cannot find the file specified". Colleagues at work are running the same version of svnmerge.py, and of python (2.5.2, specifically r252:60911) without an issue.

I found this link, which describes my problem. Trying what was outlined there, I confirmed Python could find svn (it's in my path):

P:\>python 
Python 2.5.2 (r252:60911, Feb 21 2008, 13:11:45) [MSC v.1310 32 bit (Intel)] on win32 
Type "help", "copyright", "credits" or "license" for more information. 
>>> import os 
>>> i,k = os.popen4("svn --version") 
>>> i.close() 
>>> k.readline() 
'svn, version 1.4.2 (r22196)\n' 

Looking at the svnmerge.py code, though, I noticed for python versions 2.4 and higher it was following a different execution path. Rather than invoking os.popen4() it uses subprocess.Popen(). Trying that reproduces the error:

C:\>python
Python 2.5.2 (r252:60911, Feb 21 2008, 13:11:45) [MSC v.1310 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> p = subprocess.Popen("svn --version", stdout=subprocess.PIPE, 
>>> close_fds=False, stderr=subprocess.PIPE)
Traceback (most recent call last):
  File "", line 1, in 
  File "C:\Python25\lib\subprocess.py", line 594, in __init__
    errread, errwrite)
  File "C:\Python25\lib\subprocess.py", line 816, in _execute_child
    startupinfo)
WindowsError: [Error 2] The system cannot find the file specified
>>>

For now, I've commented out the 2.4-and-higher specific code, but I'd like to find a proper solution.

If it's not obvious, I'm a complete python newbie, but google hasn't helped. Any pointers?

Answer 1

It's a bug, see the documentation of subprocess.Popen. There either needs to be a "shell=True" option, or the first argument needs to be a sequence ['svn', '--version']. As it is now, Popen is looking for an executable named, literally, "svn --version" which it doesn't find.

I don't know why it would work for your colleagues though, if they are running the same OS and version of Python... FWIW it gives me the same error message on a mac, and either of the two ways I gave fixes it.