import hashlib


def sha1_checksum(path_name):
    """
    Determine and return SHA1 checsum for file

    Parameters
    ----------
    path_name : str
        file path

    Returns
    -------
    sha1 hash
    """ 
    BLOCKSIZE = 65536
    hasher = hashlib.sha1()
    with open(path_name, 'rb') as afile:
        buf = afile.read(BLOCKSIZE)
        while len(buf) > 0:
            hasher.update(buf)
            buf = afile.read(BLOCKSIZE)

    return hasher.hexdigest()