def read_blocks(source, count = None, blocksize = 512):
    '''Read and yield block-sized strings from open file object,
    up to a total of count bytes.
    '''
    while count is None or count > 0:
        if count is not None:
            blocksize = min(count, blocksize)

        data = source.read(blocksize)
        
        if len(data) == 0:
            return # End of file
        
        if count is not None:
            count -= len(data)
        
        yield data

def write_blocks(dest, blocks):
    '''Write a series of blocks to open file object.'''
    for block in blocks:
        dest.write(block)