loop through sub directories, to sample files

The following code selects a random sample of files (in this case 50) from dir 1 and copies them to a new folder with the same name.

However, I have hundreds of folders which I need to sample from (and copy to a new folder with the same name).

How can I adjust the first part of the code so that I can loop through all sub directories, and move the samples to a new folder with the same name. (so the sample of sub dir 1 goes to dir 1, the sample of sub dir 2 goes to dir 2 etc.)

import os 
import shutil 
import random 
from shutil import copyfile

sourcedir = '/home/mrman/dataset-python/train/1/'
newdir  = '/home/mrman/dataset-python/sub-train/1'


filenames = random.sample(os.listdir(sourcedir), 50)
for i in filenames:
    shutil.copy2(sourcedir + i, newdir)