Do I need to pass the full path of a file in another directory to open()?

I have a folder with ten files in it which I want to loop through. When I print out the name of the file my code works fine:

import os
indir = '/home/des/test'
for root, dirs, filenames in os.walk(indir):
    for f in filenames:
        print(f)

Which prints:

1
2
3
4
5
6
7
8
9
10

But if I try to open the file in the loop I get an IO error:

import os
indir = '/home/des/test'
for root, dirs, filenames in os.walk(indir):
    for f in filenames:
        log = open(f, 'r')

Traceback (most recent call last):
File "/home/des/my_python_progs/loop_over_dir.py", line 6, in <module>
log = open(f, 'r')
IOError: [Errno 2] No such file or directory: '1'
>>> 

Do I need to pass the full path of the file even inside the loop to open() them?