You need to filter out directories; os.listdir() lists all names in a given path. You can use os.path.isdir() for this:
basepath = '/path/to/directory'
for fname in os.listdir(basepath):
path = os.path.join(basepath, fname)
# skip directories
os.walk() does the same work under the hood; unless you need to recurse down subdirectories, you don't need to use os.walk() here.