4.20 Swapping One File Extension for Another Throughout a Directory Tree

Credit: Julius Welby

4.20.1 Problem

You need to rename files throughout a subtree of directories, specifically changing the names of all files with a given extension so that they end in another extension.

4.20.2 Solution

Operating throughout a subtree of directories is easy enough, with the os.path.walk function from Python's standard library:

import os, string

def swapextensions(dir, before, after):
    if before[:1]!='.': before = '.'+before
    if after[:1]!='.': after = '.'+after
    os.path.walk(dir, callback, (before, -len(before), after))

def callback((before, thelen, after), dir, files):
    for oldname in files:
        if oldname[thelen:]==before:
            oldfile = os.path.join(dir, oldname)
            newfile = oldfile[:thelen] + after
            os.rename(oldfile, newfile)

if _ _name_ _=='_ _main_ _':
    import sys
    if len(sys.argv) != 4:
        print "Usage: swapext rootdir before after"
        sys.exit(100)
    swapextensions(sys.argv[1], sys.argv[2], sys.argv[3])

4.20.3 Discussion

This recipe shows how to change the file extensions of (i.e., rename) all files in a specified directory, all of its subdirectories, all of their subdirectories, and so on. This technique is useful for changing the extensions of a whole batch of files in a folder structure, such as a web site. You can also use it to correct errors made when saving a batch of files programmatically.

The recipe is usable either as a module, to be imported from any other, or as a script to run from the command line, and it is carefully coded to be platform-independent and compatible with old versions of Python as well as newer ones. You can pass in the extensions either with or without the leading dot (.), since the code in this recipe will insert that dot if necessary.

4.20.4 See Also

The author's web page at http://www.outwardlynormal.com/python/swapextensions.htm.