This means that the sizes are rounded down whenever the size is not a multiply of the sector size. This sounds like a really bad idea as it can mean that the partition we end up creating can be smaller than the contents we want to fit (especially that the method name is sizeToSectors). I think the default should be to round the size up.
The code for sizeToSectors() in pyparted is:
bytes_ * __exponents[unit] // sector_size
This means that the sizes are rounded down whenever the size is not a multiply of the sector size. This sounds like a really bad idea as it can mean that the partition we end up creating can be smaller than the contents we want to fit (especially that the method name is sizeToSectors). I think the default should be to round the size up.