Python stream hasher

stream_hash.py

def local_file_hash(filepath):
    '''
    Return a hash of the file's contents, if file exists
    Otherwise return None
    '''
    hasher = hashlib.sha256()
    blocksize=(1 << 16)
    try:
        with open(filepath, "rb") as afile:
            buf = afile.read(blocksize)
            while len(buf) > 0:
                hasher.update(buf)
                buf = afile.read(blocksize)
            return hasher.digest().encode('hex')
    except IOError as exception:
        if exception.errno == errno.ENOENT: # file doesn't exist
            return None
        else:
            raise