There are different ways to create a nested directory in different versions of python. Using the codes below, directories are created as shown in the image below.

Example 1: Using pathlib.Path.mkdir
For python 3.5 and above, you can use pathlib.Path.mkdir.
from pathlib import Path
Path("/root/dirA/dirB").mkdir(parents=True, exist_ok=True)
You should provide the full path (absolute path) of the directory (not relative path). If the directory already exists, the above code does not raise an exception.
Example 2: Using os.makedirs
For python 3.2 and above, you can use os.makedirs.
import os
os.makedirs("/root/dirA/dirB")
You should provide the full path (absolute path) of the directory (not relative path). If the directory already exists, the above code does not raise an exception.
Example 3: Using distutils.dir_util
import distutils.dir_util
distutils.dir_util.mkpath("/root/dirA/dirB")
You should provide the full path (absolute path) of the directory (not relative path). If the directory already exists, the above code does not raise an exception.
Example 4: Raising an exception if directory already exists
import os
try:
os.makedirs("/dirA/dirB")
except FileExistsError:
print("File already exists")
If the directory already exists, then FileExistsError is raised.