Test setup:
- Windows 2000 (french)
- Python 2.5 (reproducible with 2.4.2 and 2.4.4)

One python source file, test.py:
---
  import os
  print os.system("echo test")
---

Console output:
---
C:\test>test.py
test
0

C:\test>test.py > test.log
Descripteur non valide

C:\test>type test.log
0

C:\test>c:\python25\python test.py
test
0

C:\test>c:\python25\python test.py > test.log

C:\test>type test.log
test
0

C:\test>
---

The "Descripteur non valide" error would translate to "invalid file descriptor".

I would expect the 2nd call to perform the same way the 4th does.

Do you have an explanation for the cause of the problem?

This is a windows bug.  See the Microsoft Knowledge Base:

http://support.microsoft.com/kb/321788

This entry talks about Windows 2000.  I do not use Windows 2000 anymore, so I cannot test on this version.  I have created the registry entry that is mentioned on WinXP SP2 (German version, fwiw), and it does fix the problem.  I had to open a new command window after changing the registry.

So we can do nothing about the bug, but we should definitely document that somewhere and link to the Knowledge Base.

What would be the correct location?

Issue 1590068 seems to be the same problem.

Documented in README in r70902.