(Beginner) Rename file using Python - Refactor for a better answer

Question

(First post... very new to programming)

I needed to rename a bunch of files from 'This is a filename-123456.ext' to '123456-This is a filename.ext'

I managed to solve the problem using Python with the code below. I had to make 2 scripts because sometimes there are 5 numbers, but for the most part 6.

import os
for filename in os.listdir('.'): #not sure how to rename recursive sub-directories
    if filename != 'ren6.py': #included to not rename the script file
        start = filename[:-11]
        number = filename[-10:-4]
        ext = filename[-4:]
        newname = str(number) + '-' + str(start)+str(ext) #Unnecessary variable creation?
        os.rename(filename,newname)

I'm still learning and very curious of more efficient and elegant examples of to accomplish the same thing.

Answer 1

It may be safer and more powerful to use regular expressions. This will only rename files that match the given pattern, which is [ANY SEQUENCE OF CHARACTERS][A DASH][NUMBERS][EXTENSION]

An added benefit to using this method is that you can run it multiple times on the same directory and it won't affect already renamed files.

You might also want to do a check to make sure the file you're renaming it to doesn't already exist (so that you don't overwrite an existing file).

import re


for filename in os.listdir('.')
    m = re.match(r'^(?P<name>.+)-(?P<num>\d+)(?P<ext>\.\w+)$', filename)
    if m:
        newname = '{num}-{name}{ext}'.format(**m.groupdict())
        if not os.path.exists(newname):
            os.rename(filename, newname)

I'll break down the regular expression

^(?P<name>.+)

The ^ indicates we will start matching at the beginning of the filename (as opposed to matching a middle part of the filename). The () make this a regex group, so that we can access just that one part of the string match. The ?P<name> is just a way to apply a label to a particular group, so that we can refer it to by name later on. In this case, we've given this group a label of name.

. will match any character, and + tells it to match 1 or more characters.

-

This will only match the - character

(?P<num>\d+)

Again, we've made this a group and given it a label of num. \d will only match numbers and the + means it will match 1 or more numbers.

(?P<ext>\.\w+)$

Another group, another label. The \. will only match a . and the \w will match word characters (i.e. letters, numbers, underscores). Again, the + means it will match 1 or more characters. The $ ensures it matches all the way to the end of the string.