Question

I want to make a multi-threaded downloader (in Python) and I need to tell each thread where to start and how many bytes to download. For that I get the remote file size and divide it, for example, to 2. Now, let’s say that the remote file size is 5: when I divide the number to 2, I get 2 as result. Now I can start the download but I will lose a byte (because 2*2=4, not 5). I can’t use float numbers because I can’t download half of a byte. How I could divide that number and to get a list with [2, 3], for example?

Answer 1

Use divmod:

>>> divmod(5, 2)
(2, 1)
>>>

This tells you, that 5 divided by 2 is 2, remainder 1, so the last piece will be 2 + 1 = 3.

>>> divmod(12345, 6)
(2057, 3)

Here, you’ll have 5 chunks at 2057 and a last slice at 2057+3.

This algorithm will also work for cases, where division is without remainder:

>>> divmod(12345, 5)
(2469, 0)

Here, you’ll have 4 chunks at 2469 plus a last slice at 2469+0.

So, your chunk sizes could be computed as:

def chunk_sizes(filesize, num_chunks):
    d, r = divmod(filesize, num_chunks)
    result = [d] * num_chunks
    result[-1] += r
    return result