python os.rename(...) won't work !

Question

I am writing a Python function to change the extension of a list of files into another extension, like txt into rar, that's just an idle example. But I'm getting an error. The code is:

import os
def dTask():
    #Get a file name list
    file_list = os.listdir('C:UsersBDesktopsilsil2')
    #Change the extensions
    for file_name in file_list:
        entry_pos = 0;
        #Filter the file name first for '.'
        for position in range(0, len(file_name)):
            if file_name[position] == '.':
                break
        new_file_name = file_name[0:position]
        #Filtering done !
        #Using the name filtered, add extension to that name
        new_file_name = new_file_name + '.rar'
        #rename the entry in the file list, using new file name
        print 'Expected change from: ', file_list[entry_pos]
        print 'into File name: ', new_file_name
        os.rename(file_list[entry_pos], new_file_name)
        ++entry_pos
Error:
>>> dTask()
Expected change from:  New Text Document (2).txt
into File name:  New Text Document (2).rar
Traceback (most recent call last):
  File "<pyshell#10>", line 1, in <module>
    dTask()
  File "C:UsersBDesktopdTask.py", line 19, in dTask
    os.rename(file_list[entry_pos], new_file_name)
WindowsError: [Error 2] The system cannot find the file specified

I can succeed in getting the file name with another extension in variable level as you can see in the print-out, but not in reality because I can not end this process in OS level. The error is coming from os.rename(...). Any idea how to fix this ?

Answer 1

1. As the others have already stated, you either need to provide the path to those files or switch the current working directory so the os can find the files.

2. ++entry_pos doesn't do anything. There is no increment operator in Python. Prefix + is just there fore symmetry with prefix -. Prefixing something with two + is just two no-ops. So you're not actually doing anything (and after you change it to entry_pos += 1, you're still resetting it to zero in each iteration.

3. Also, your code is very inelegant - for example, you are using a separate index to file_list and fail to keep that in synch with the iteration variable file_name, even though you could just use that one! To show how this can be done better.

-

def rename_by_ext(to_ext, path):
    if to_ext[0] != '.':
        to_ext = '.'+to_ext
    print "Renaming files in", path
    for file_name in os.listdir(path):
        root, ext = os.path.splitext(file_name)
        print "Renaming", file_name, "to", root+ext
        os.rename(os.path.join(path, file_name), os.path.join(path, root+to_ext))
rename_by_ext('.rar', '...')