why i don't like os.walk()

[Skaperen]

i find no means to modify the list of subdirectories that it uses to descend down to the next level, such as sorting them or removing selected subdirectories

In principle, you can do this by changing the contents of the subdirectories list, for example

import os
for root, subdirs, files in os.walk('foo'):
    subdirs[:] = sorted(subdirs) # sorts the subdirs
    subdirs[:] = [d for d in subdirs if not d.startswith('tmp')] # remove some subdirs
    print(root, subdirs)

so this affects the generator state? how? what does the [:] on the LHS do? can i delete a directory from the directory list to have it not descend that one?