Generating one MD5/SHA1 checksum of multiple files

Question

I have looked through several topics about calculating checksums of files in Python but none of them answered the question about one sum from multiple files. I have several files in sub directories and would like to determine if there was any change in one or more of them. Is there a way to generate one sum from multiple files?

EDIT: This is the way I do it to get a list of sums:

checksums = [(fname, hashlib.md5(open(fname, 'rb').read()).digest()) for fname in flist]

Answer 1

Slightly cleaner than Artur's answer. There's no need to treat the first element specially.

def calculate_checksum(filenames):
    hash = hashlib.md5()
    for fn in filenames:
        if os.path.isfile(fn):
            hash.update(open(fn, "rb").read())
    return hash.digest()

(You can remove the os.path.isfile() if you like.)

Answer 2

So I made it :) This way one hash sum is generated for a file list.

hash_obj = hashlib.md5(open(flist[0], 'rb').read())
for fname in flist[1:]:
    hash_obj.update(open(fname, 'rb').read())
checksum = hash_obj.digest()

Thank you PM 2Ring for your input!

Note that md5 has been cracked so use it only for non security critical purposes.