Python program to rename file names while overwriting if there already is that file

On Windows os.rename won't replace the destination file if it exists. You have to remove it first. You can catch the error and try again after removing the file:

import os

original = sys.argv[1]
output = sys.argv[2]

try:
    os.rename(original, output)
except WindowsError:
    os.remove(output)
    os.rename(original, output)

You can use shutil.move, it will overwrite on windows:

from shutil import move

move(src,dest)

Demo:

In [10]: ls    
Directory of C:\Users\padraic\Desktop

11/05/2015  20:20    <DIR>          .
11/05/2015  20:20    <DIR>          ..
11/05/2015  20:20                 0 bar.txt
11/05/2015  20:20                 0 foo.txt
               2 File(s)              0 bytes
               2 Dir(s)  47,405,617,152 bytes free

In [11]: shutil.move("bar.txt","foo.txt")    
In [12]: ls 
Directory of C:\Users\padraic\Desktop    
11/05/2015  20:20    <DIR>          .
11/05/2015  20:20    <DIR>          ..
11/05/2015  20:20                 0 foo.txt
               1 File(s)              0 bytes
               2 Dir(s)  47,405,613,056 bytes free
In [13]: shutil.move("foo.txt","bar.txt")
In [14]: ls
 Volume in drive C has no label.
 Volume Serial Number is 3C67-52B9

 Directory of C:\Users\padraic\Desktop

11/05/2015  20:24    <DIR>          .
11/05/2015  20:24    <DIR>          ..
11/05/2015  20:20                 0 bar.txt
               1 File(s)              0 bytes
               2 Dir(s)  47,405,568,000 bytes free