Renaming Files and/or Directory

Question

I'm quite new to Python and would love to gather feedback on my code. I've written a piece a script that allows me to search all file or directory names within a folder matching a particular pattern. The caller can specify if the function should be recursive or not.

It's only a basic script I would love to know how it could be improved!

import sys
import os
import string

def rename(source, pattern, replacement, dirs = False, recurse = False):
    """
    Rename a file or directory

    source = source directory
    pattern = old string
    replacement = new string
    dirs = apply to directorys
    recurse = move recursivley
    """
    dir = os.path.abspath(source)

    for item in os.listdir(dir):
            # Get the full path
            item = os.path.join(source, item)
            if os.path.isfile(item) and pattern in item:
                 os.rename(item, string.replace(item, pattern, replacement))
            elif os.path.isdir(item):
                if recurse:
                    # Move to the next level, before renaming
                    rename_items(item, pattern, replacement, dirs, recurse)
                if pattern in item and dirs:
                    os.rename(item, string.replace(item, pattern, replacement))

if __name__ == '__main__':

    source = sys.argv[1]
    pattern = sys.argv[2]
    replacement = sys.argv[3]

    rename(source, pattern, replacement, True, True)

Answer 1

What if the script renames file "a.zip" to "b.zip", and both files "a.zip" and "b.zip" exist in the same directory?