The most common place you will pass a function is the key argument of sorted, min and max. The key function is called on each item, and the items are compared by what it returns:

words = ["banana", "Apple", "fig", "cherry"]
print(sorted(words))
print(sorted(words, key=len))
print(sorted(words, key=str.lower))
print(max(words, key=len))

scores = {"Ada": 92, "Grace": 87, "Linus": 75}
print(max(scores, key=scores.get))

Notice key=len, not key=len(): you pass the function, and sorted calls it. max(scores, key=scores.get) finds the key with the biggest value: a neat replacement for the "biggest so far" loop.