1. Print all the filenames in the dir directory and subdirectories:

>>> import os
>>> for root, dirs, files in os.walk('.'):
...     for file in files:
...         print(file)
...
file1.txt
file2.txt
file6.pdf
file3.txt
file4.txt
file5.pdf

2. Print the full path of the files, joining with the root:

>>> for root, dirs, files in os.walk('.'):
...     for file in files:
...         full_file_path = os.path.join(root, file)
...         print(full_file_path)
...
./dir/file1.txt
./dir/file2.txt
./dir/file6.pdf
./dir/subdir/file3.txt
./dir/subdir/file4.txt
./dir/subdir/file5.pdf

3. Print only the .pdf files:

>>> for root, dirs, files in os.walk('.'):
...     for file in files:
...         if file.endswith('.pdf'):
...             full_file_path = os.path.join(root, file)
...             print(full_file_path)
...
./dir/file6.pdf
./dir/subdir/file5.pdf

4. Print only files that contain an even number:

>>> import re
>>> for root, dirs, files in os.walk('.'):
...     for file in files:
...         if re.search(r'[13579]', file):
...             full_file_path = os.path.join(root, file)
...             print(full_file_path)
...
./dir/file1.txt
./dir/subdir/file3.txt
./dir/subdir/file5.pdf