I assume that you are familiar with ’The Tale of the Three Brothers’?
Assuming I survive our hunt for the Horcruxes, I’ll find Mum and Dad and lift the enchantment.
Some, indeed, were disposed to praise his father’s action and assumed that Albus too was a Muggle-hater.
Harry could only assume that Sirius’s parents had been unable to remove the Permanent Sticking Charm that kept them on the wall because he was sure they would not have appreciated their eldest son’s taste in decoration.
Even assuming that she was a witch, however, it was odd behavior to come out on a night this cold, simply to look at an old ruin.
Dumbledore said in front of me that he never assumed he knew all of Hogwart’s secrets.
Above what Harry assumed was the title of the story (being unable to read runes, he could not be sure), there was a picture of what looked like a triangular eye, its pupil crossed with a vertical line.
He had never questioned his own assumption that Dumbledore wanted him alive.
Hermione had not packed any food in her magical bag, as she had assumed that they would be returning to Grimmauld Place that night, so they had had nothing to eat except some wild mushrooms that Hermione had collected from amongst the nearest trees and stewed in a Billycan.
Tom Riddle, who confided in no one and operated alone, might have been arrogant enough to assume that he, and only he, had penetrated the deepest mysteries of Hogwarts Castle.
There are no more uses of "assume" identified with this meaning in the book.
Show samples from other sources
I assume she was at school today, but I don’t remember actually seeing her.
Let’s assume that 80% of the people who said they will come to the party will actually show up.