How do I write a file in python without loading it into memory?

My intuition was that writing a file line by line as possible in python, in the same way, that reading it is. I was wrong. Consider the following code.

with open("source_file.csv","r") as source, open("output_file.csv","w") as dest:
    while 1:
        buf = source.read(16*1024)
        if not buf:
                break
        dest.write(buf)

I observed how the output file is written via a $ tail -f output_file.csv. Data is displayed as it is being written. I interrupted the python script and noticed that none of that data was written on disk.

Here's my understanding of this so far :

• The read operation is indeed done line-by-line here without loading it into memory. The write operation, on the other hand, is loaded line-by-line into memory, before it is serialized and written on disk. I thought data could be written on disk chunk-by-chunk, to be memory efficient. Otherwise, any big file could not be copied (memory will eventually be insufficient), which is (Probably) what happens in my previous post. My questions :
• Is my analysis correct? If not, I would like additional information.
• How do we write chunk-by-chunk on disk?