I wrote a program to rename files with consecutive numbers, basically removing the gaps in the numbering. This could be a very annoying task to do it manually in windows. I only did this for practicing Python purposes.

Lets show what the program does in an example:

Say you have a folder E:\Spam which contains the following files:


Lets say for what reason you want to continues numbe all the files from type spam.txt

After running my program it gives out this:

eggs010.txt (unchanged wrong filename)
spam5.dat (unchanged wrong filetype)

Also an offset can be added to start the numbering for example at 5 or 0 instead of.


Removes gaps in numbering of multiple files in a specified directory.

Takes a directory to search for files which match prefix and type and
otherwise only contain numbers in the middle.
Files are renamed continuesyly and filled up with leading zeros if
It is possible to define with which number the numeration


would become (if start_number = 1):


To prevent overriting in the renaming process a temporary directory is
created during the renaming.

import os
import sys
import shutil
import stat
from typing import List

def get_filenames(directory: str) -> List[str]:
    get filenames in supplied absolute directory name
    return os.listdir('.')

def get_digits_minimum_length(filenames: List[str], file_prefix: str,
                              file_type: str, start_number: int) -> int:
    loops over all provided filenames to find the filename with the
    longest digit string e.g:
    spam00001.txt   -> len = 5
    spam02.txt      -> len = 2
    spam0000003.txt -> len = 7
    digits_length == 7
    digits_length: int = 0
    for filename in filenames:
        if filename.startswith(file_prefix) and filename.endswith(file_type):
            number_string: str = filename.lstrip(file_prefix)
            number_string = number_string.rstrip(file_type)
            if number_string.isdigit():
                current_length = len(number_string)
                if current_length > digits_length:
                    digits_length = current_length

    if len(str(start_number)) > digits_length:
        digits_length = start_number

    return digits_length

def make_filename(file_prefix: str, file_type: str, number: int,
                  number_length: int) -> str:
    Creates a new filename out of the parts.
    return file_prefix + str(number).zfill(number_length) + file_type

def make_folder(directory: str, folder_name: str):
    Creates a folder inside a directory
    new_directory: str = os.path.join(directory, folder_name)
    os.chmod(new_directory, stat.S_IWRITE)

def rename_and_move_file(directory: str, new_directory: str,
                         old_filename: str, new_filename: str):
    Renames a file from a specific directory into a new directory to
    prevent that renamoving overrites other existing files
    shutil.move(os.path.join(directory, old_filename),
                os.path.join(new_directory, new_filename))

def erase_directory(delete_directory: str):
    checks if directory exists and deletes it.
    if os.path.exists(delete_directory):

def extract_digits_and_move_to_temp(
        directory: str, temp_directory: str, file_prefix: str, file_type: str,
        digit_lenght: int):
    Remove pre und postfix from files, add leading zeros to the
    integer part to reach the same length for all digitsand copy them
    to temp directory
    for filename in get_filenames(directory):
        if filename.startswith(file_prefix) and filename.endswith(file_type):
            number_string: str = filename.lstrip(file_prefix)
            number_string = number_string.rstrip(file_type)
            if number_string.isdigit():
                rename_and_move_file(directory, temp_directory, filename,

def renumber_and_move_to_directory(
        start_number: int, directory: str, temp_directory: str,
        file_prefix: str, file_type: str, digit_lenght: int):
    Renumbers the files according to start file number and moves them
    back to directory
    next_number: int = start_number
    filenames = get_filenames(temp_directory)
    for filename in filenames:
        new_filename: str = make_filename(
            file_prefix, file_type, next_number, digit_lenght)
        rename_and_move_file(temp_directory, directory, filename, new_filename)
        next_number = next_number + 1

def filling_gaps(directory: str, file_prefix: str, file_type: str,
                 start_number: int = 1):
    Main logic of the filling gaps script.
    First a temporary directory is made for the file operations.
    Then the max len of digits (int + leading zeros) is calculated out
    of the files.
    Then all matched files are stripped everything but the int part and
    they are moved into the temp directory.
    In the temp directory they are numerated correctly again and moved
    back to the source directory.
    In the end the temporary directory is deleted
    if start_number < 0:
        raise Exception("Start file number must be >=0. Value: " +

    temp_folder_name = "temp"
    temp_directory = os.path.join(directory, temp_folder_name)

    if os.path.exists(temp_directory):
        print("Error: temp directory used for internal copy already exists."
              " Make sure it does not. directory name:" + temp_folder_name)

    make_folder(directory, temp_folder_name)

    digits_length: int = get_digits_minimum_length(
        get_filenames(directory), file_prefix, file_type, start_number)

        directory, temp_directory, file_prefix, file_type, digits_length)

        start_number, directory, temp_directory, file_prefix, file_type,


filling_gaps("E:\\Spam", "spam", ".txt", 1)

I would like to know if this is good code? Is the approach a good one? Do you understand the code from the comments and the docstrings? Are there any bad practices you can spot Are there any improvements / better approaches to try?

Also I tested this script by manually adding the example script again and again how could this be better tested?

Please let me know what you think.


2 Answers 2



You have a two bugs in get_digits_minimum_length():

if len(str(start_number)) > digits_length:
    digits_length = start_number

First of all, you probably meant digits_length = len(start_number). But even that is not enough, because the end number might have more digits than the starting number. Therefore, it should be:

digits_length = max(digits_length, len(str(start_number + len(filenames) - 1)))

File manipulation

A better way to make a temporary directory within directory is tempfile.mkdtemp(dir=directory). It guarantees that the name of the new directory will not collide with any existing file or directory. (It automatically generates a different name as necessary to make that happen.)

The temporary directory needs to have permissions stat.S_IRWXU. On Unix, stat.S_IWRITE (or stat.IWUSR) is insufficient, since you will not be able to call listdir() on the temporary directory.

The program calls get_filenames() three times. Not only is that slow and wasteful, it also presents a possibility for inconsistencies due to race conditions, if a file gets created or removed while the program is running.

shutil.move() is overkill, when os.rename() should work just as well. Also, it would be better to just do os.rmdir() rather than shutil.rmtree() to clean up the temporary directory, because os.rmdir() asserts that the directory is empty. (You wouldn't want to accidentally delete any of your files, would you? And if you did want to force a cleanup, with tempfile.TemporaryDirectory() as temporary_directory: … would be a more elegant way to write it.)

Global state

os.chdir() affects the global state of a process. I'd try to avoid doing it at all, if possible. And if you do do it, then I'd call it from a prominent place in your code, so that some innocuous-looking utility function (get_filenames()) does not have unexpected insidious side-effects on the rest of your code. After calling os.chdir(), you don't have to ever mention directory again in your code anymore, because every operation is relative to the current directory.

Design and efficiency

I am slightly annoyed by some of your very short helper functions: make_filename(), make_folder(), and erase_directory(). In my opinion, they make the code harder to read, because they add very little value over the standard library call, but I have to devote mental effort into keeping track of what they do. Each of those functions is called from just one place, which makes their existence even less worthwhile.

I imagine that there might be use cases where this program is repeatedly executed for some directory. In that case, you would be moving files into and out of the temporary directory, most of them for naught. A better strategy would be to put more work into mapping the source filenames into their desired destinations (as in my filename_map() below), so that no more filesystem manipulation is done than necessary.

Suggested solution

import os
import re
import stat
import tempfile

def filename_map(prefix, suffix, start_number):
    Make a dictionary that maps source filenames to their renumbered
    destination filename.
    pattern = re.compile(re.escape(prefix) + '([0-9]+)' + re.escape(suffix), re.I)
    num_to_fn = {
        pattern.fullmatch(fn).group(1): fn
        for fn in os.listdir()
        if pattern.fullmatch(fn)
    digits_length = max(
        max((len(num) for num in num_to_fn), default=0),
        len(str(start_number + len(num_to_fn) - 1))
    return {
        num_to_fn[num]: prefix + str(i).zfill(digits_length) + suffix
        for i, num in enumerate(sorted(num_to_fn, key=int), start_number)

def filling_gaps(prefix, suffix, start_number=1):
    Rename files in the current directory whose names consist of the given
    prefix, followed by some digits, and the given suffix, such that they
    are consecutively numbered from the specified start_number.
    if start_number < 0:
        raise ArgumentError("Start number {0} is less than 0", start_number)

    fn_map = {
        src_fn: dst_fn
        for src_fn, dst_fn in filename_map(prefix, suffix, start_number).items()
        if src_fn != dst_fn
    if not fn_map:
        return  # Nothing to rename

    temp_directory = tempfile.mkdtemp(dir='.', prefix='renumber')
    os.chmod(temp_directory, stat.S_IRWXU)
    for src_fn, dst_fn in fn_map.items():
        os.rename(src_fn, os.path.join(temp_directory, dst_fn))
    for dst_fn in fn_map.values():
        os.rename(os.path.join(temp_directory, dst_fn), dst_fn)

filling_gaps('spam', '.txt', 1)
  • \$\begingroup\$ prefix + str(i).zfill(digits_length) + suffix could be f"{prefix}{i:0{digits_length}}{suffix}" as an f-string. \$\endgroup\$
    – Graipher
    Dec 9, 2018 at 10:03


I don't like this style

def get_digits_minimum_length(filenames: List[str], file_prefix: str,
                              file_type: str, start_number: int) -> int:

I'm more a fan of

def get_digits_minimum_length(
    filenames: List[str], file_prefix: str, file_type: str, start_number: int
) -> int:



def get_digits_minimum_length(
    filenames: List[str], 
    file_prefix: str, 
    file_type: str, 
    start_number: int,
) -> int:

But since recently, I use a code formatter (black) to do this


Using pathlib.Path instead of os can make things a lot easier. It has builtin globbing, and you can easily generate the new name

separate the program

All in all this is a rather simple program:

  1. Find the files who are structured "{prefix}{number}.{suffix}"
  2. Extract number
  3. Generate a new number
  4. Generate the new name
  5. Move the files

Your program makes a few strange splits between the functions.

If you need to do this via a temporary directory, use tempfile.TemporaryDirectory as a context manager

All in all this can be done quite simple

def find_files(directory, prefix, suffix):
    """find the files in `directory` wich start with `prefix` and end with `.suffix`"""
    directory = Path(directory)

    filename_pattern = re.compile(
        re.escape(prefix) + "(?P<number>[0-9]+)\." + re.escape(suffix), re.I
    for file in directory.glob(f"{prefix}*.{suffix}"):
        match = filename_pattern.search(file.name)
        if not match:
        yield int(match.group("number")), file

This generator yield the number (as int) and the filename (as Path) of all the files that comply with the prefix and suffix

def rename_files(files, prefix, suffix, start_num=0):
    """Generates the new filename"""
    files = sorted(files)
    last_file_num = files[-1][0]
    max_num = max(last_file_num, start_num + len(files))
    num_length = len(str(max_num))
    for new_number, (_, file) in enumerate((files), start_num):
        new_name = f"{prefix}{str(new_number).zfill(num_length)}.{suffix}"
        yield file, new_name

This one yields the file and the new proposed name. To calculate how long the number needs to be, it takes into account the lingest existing number and the starting number + amount of files to rename.

It also keeps the existing order, ordered via the number, not lexigraphically, so spam1.txt comes before spam02.txt

the method to move the files takes this generator as input, and does the moving on the fly:

def move(renames, practice=True):
    for file, new_name in renames:
        new_file = file.with_name(new_name)
        print(f"renaming {file.name} to {new_name}")
        if not practice:
            os.rename(file, new_file)

This was tested with

if __name__ == "__main__":
    prefix = "spam"
    suffix = "txt"
    files = find_files(DATA_DIR, prefix, suffix)
    renames = rename_files(files, prefix, suffix)
    move(renames, practice=False)
renaming spam00.txt to spam0000.txt
renaming spam002.txt to spam0001.txt
renaming spam004.txt to spam0002.txt
renaming spam005.txt to spam0003.txt
renaming spam7.txt to spam0004.txt
renaming spam009.txt to spam0005.txt
renaming spam037.txt to spam0006.txt
renaming spam045.txt to spam0007.txt
renaming spam089.txt to spam0008.txt
renaming spam2000.txt to spam0009.txt

I tested this with:

DATA_DIR = Path("data/test")
testfiles = """eggs010.txt

for filename in testfiles.split("\n"):
    file = DATA_DIR / filename.strip()

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