Can spellcasters prepare spells not all at once?

@JeremyECrawford Can spellcasters prepare spells not all at once, but leave some "slots" for preparing spells later during the day?

— Lebedev Dmitry (@LebMit) October 12, 2015

You prepare your list of spells only at the end of a long rest. #DnD https://t.co/gLoqulfGqb

— Jeremy Crawford (@JeremyECrawford) October 20, 2015