Python giving FileNotFoundError for file name returned by os.listdir

It is because os.listdir does not return the full path to the file, only the filename part; that is 'foo.txt', when open would want 'E:/somedir/foo.txt' because the file does not exist in the current directory.

Use os.path.join to prepend the directory to your filename:

path = r'E:/somedir'

for filename in os.listdir(path):
    with open(os.path.join(path, filename)) as f:
        ... # process the file

(Also, you are not closing the file; the with block will take care of it automatically).