os.walk() silent about non-directory

[serhiy-storchaka]

os.walk() silently produces an empty iterator if the argument is not a path to directory:

>>> import os
>>> list(os.walk('./non-existing'))
[]
>>> list(os.walk('./python'))
[]
>>> list(os.walk('/dev/null'))
[]

It can causes bugs like #99203, when a function produces some incorrect result instead of raising an exception.

[serhiy-storchaka]

Note that os.fwalk() raises an exception for non-existing entity, but not for file.

>>> list(os.fwalk('./non-existing'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/os.py", line 475, in fwalk
    orig_st = stat(top, follow_symlinks=False, dir_fd=dir_fd)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: './non-existing'
>>> list(os.fwalk('./python'))
[]

glob() also produces an empty result:

>>> glob.glob('./non-existing/*')
[]
>>> glob.glob('./python/*')
[]

[achhina]

I would like to work on this.

@serhiy-storchaka what do you think the expected behavior for this should be? Looking at the docs, it seems to imply os.fwalk should mimic os.walk, however I agree that silently producing an iterator is probably not the best way to deal with this. Also, would there be any backward compatibility concerns for folks who are relying on this feature in os.walk, if we decide to modify it?