This is my solution to the Chapter 9 exercise in Automate the Boring Stuff:

Selective Copy

Write a program that walks through a folder tree and searches for files with a certain file extension (such as .pdf or .jpg). Copy these files from whatever location they are in to a new folder.

#selectiveCopy.py - Walks through a folder tree with the extension .PDF and copies them to a new folder.

import re
import os
import shutil

filePdf = re.compile(r"""^(.*?)          #text before extension
          [.][p][d][f]                   #extension .pdf
          """, re.VERBOSE)

def selectiveCopy(fileExtension):
    for pdfFiles in os.listdir("/Users//Desktop/newPDFs"):
        locatedFiles = filePdf.search(pdfFiles)
        if locatedFiles != None:
            files_to_copy = os.path.join("/Users/my_user_name/Desktop/newPDFs", pdfFiles)   `
            shutil.copy(files_to_copy, "/Users/my_user_name/Desktop/oldPDFs")   
            print("The following file has been copied " + files_to_copy)

I appreciate for probably all of you on here that it's not a very difficult exercise, but I've been working through this book slowly and have just decided it's probably a good idea to start putting them on a Github page. I wanted to know if I could improve the code to make it more efficient or if there are some things I've included that aren't necessarily considered "standard practice".

  • 1
    \$\begingroup\$ Hey, welcome to Code Review! The problem description says to traverse a directory tree. This sounds to me like you actually need to descend down into sub-directories. \$\endgroup\$
    – Graipher
    Commented Aug 17, 2018 at 12:50

2 Answers 2


If you want to recurse also into sub-folders, you should use os.walk:

import os
import shutil

def get_files_recursively(start_directory, filter_extension=None):
    for root, _, files in os.walk(start_directory):
        for file in files:
            if filter_extension is None or file.lower().endswith(filter_extension):
                yield os.path.join(root, file)

def selective_copy(source, target, file_extension=None):
    for file in get_files_recursively(source, file_extension):
        shutil.copy(file, target)
        print("The following file has been copied", file)

if __name__ == "__main__":

As stated in the documentation, this is actually faster than os.listdir since Python 3.5:

This function now calls os.scandir() instead of os.listdir(), making it faster by reducing the number of calls to os.stat().

I also

  • added a if __name__ == "__main__": guard to allow importing this module from another script without running the function
  • pulled the constants out into the calling code to make it re-usable
  • used is instead of != to compare to None
  • used str.endswith instead of a regular expression to avoid some overhead
  • changed the names to adhere to Python's official style-guide, PEP8.
  • \$\begingroup\$ I don't understand everything you did to be honest, but I'll look up some of the code, which will hopefully help me write a bit better. Thanks. \$\endgroup\$
    – Elarbe
    Commented Aug 17, 2018 at 13:25
  • \$\begingroup\$ @LRBrady The one thing I did not link to in my answer:get_files_recursively is a generator \$\endgroup\$
    – Graipher
    Commented Aug 17, 2018 at 13:53
  • 2
    \$\begingroup\$ A typo here: if filter is None \$\endgroup\$
    – hjpotter92
    Commented Aug 17, 2018 at 18:58


  • Your regex can be simplified:

    There is no need for the multiple [] brackets - this would suffice: r"^(.*?)\.pdf$".

    Note that I escape the . char (which will match anything) with a backslash \. to only match the specific . char, and the $ to be certain that .pdf is at the end of the string.

  • There is no need for Regex at all!

    1. You can either use the glob module to directly find all files with an extension Python3.5+

      def get_files(source, extension):
          for filename in glob.iglob(f'{source}/**/*{extension}', recursive=True):
              yield filename
    2. use something like .split(".") or filename.endswith('.extension') to find if the file uses that extension. As @Graipher showed in his answer.

  • Currently your solution works with hardcoded directories.

    To change the to directory to read or to write in, you'd need to change a lot in your code. Your solution would be much better if you could send these path locations as parameters.

  • \$\begingroup\$ I tried using the regex r"^(.*?)\.pdf" initially and it was matching a lot of other files that weren't PDFs, but I could have made an error or might have changed some code after that. Your last point what do you mean? sorry I'm still new so I don't understand. \$\endgroup\$
    – Elarbe
    Commented Aug 17, 2018 at 13:23
  • 2
    \$\begingroup\$ I added a $ to the regex so it wont match files like "afile.pdfsonething.txt" \$\endgroup\$
    – Ludisposed
    Commented Aug 17, 2018 at 13:25

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