This problem happens in launching Python from Windows. 
A quoted pathname is used as parameter. If and only if 
the pathname is ended with a trailing \, it is treated 
as an escape character.

p.py
-------------------------------------------------------
import sys
from pprint import pprint
pprint(sys.argv)
-------------------------------------------------------

Note slash inside the quote is treated as path 
separators

g:\usr\bin>c:\python24\python.exe  g:\usr\bin\p.py "
tung\wai\yip" wai
['g:\\usr\\bin\\p.py', 'tung\\wai\\yip', 'wai']


However the trailing \ is treated as an escape 
character for the quote. Only command line parameter 
is presented in sys.argv.

g:\usr\bin>c:\python24\python.exe  g:\usr\bin\p.py "
tung\wai\yip\" wai
['g:\\usr\\bin\\p.py', 'tung\\wai\\yip" wai']

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Is that Python-specific? I thought sys.argv is the cmdline
args as Python gets them from Windows.

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After having a quick look it appears as though python is in
fact stripping the trailing '\'. the argv array that the
python interpreter is passed contains the backslash so it
must be doing away with it.

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Whoops, my bad. Its a windows issue, not a python one.

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OK, closing.

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I don't completely follow your logic. But I have reproduce 
this with Java so it looks very likely that it is a Windows 
bug. In addition, a workaround is to add as extra \ at the 
end of the first argument if you know it is going to end in 
\. This is illustrated in the last example.

P.java
------------------------------------------------------------
public class P {
  public static void main(String argv[]) throws Exception {
    for (int i=0; i < argv.length; i++) {
      System.out.println("argv " + i + " " + argv[i]);
    }
  }
}
-----------------------------------------------------------
BAD
c:\>java P "tung\wai\yip\" wai
argv 0 tung\wai\yip" wai

GOOD
c:\>java P "tung\wai\yip\\" wai
argv 0 tung\wai\yip\
argv 1 wai
-----------------------------------------------------------