traversing_directories

import os

files_by_type = {}

# for item in os.listdir('.'):  # goes in current level only
#     if os.path.isfile(os.path.join('.', item)):

level_count = 0
for _, _, file_names in os.walk("."):  # goes down to the bottom
    for item in file_names:
        extension = '.' + item.split('.')[-1]
        if extension not in files_by_type:
            files_by_type[extension] = []
        files_by_type[extension].append(item)
    level_count += 1
    if level_count == 2:  # goes one level deeper and stops, we can control how deep the search goes.
        break

desktop_dir = os.path.normpath(os.path.expanduser("~/Desktop"))
path_file = os.path.join(desktop_dir, 'report.txt')

with open(path_file, 'w') as output_file:
    for ext, f_names in sorted(files_by_type.items()):
        output_file.write(f"{ext}\n")
        for name in sorted(f_names):
            output_file.write(f"- - - {name}\n")