Lambdas, map, filter and sorted

Small anonymous functions

A lambda is a function written inline, with no name:

double = lambda n: n * 2      # works, but just use def

Assigning a lambda to a name is worse than def in every way. Where lambdas earn their place is as an argument to something else — most often sorted:

people.sort(key=lambda p: p["age"])
sorted(words, key=len)
sorted(scores, reverse=True)

The key function is called once per item, and sorting happens on what it returns. Note that key=len needs no lambda at all — pass the function itself when one already exists.

map(fn, items) and filter(fn, items) apply a function across a collection, though a comprehension usually reads better.

Your turn: write by_length(words) returning the words sorted shortest to longest, and by_last_letter(words) sorting by the final character.

Sorting structured data

key comes into its own with dictionaries and objects, where there is no obvious natural order.

Returning a tuple from the key sorts by several fields at once — the first element first, then the second as a tie-breaker:

sorted(people, key=lambda p: (p["city"], p["name"]))

Your turn: write ranked(players) sorting by score from high to low, with ties broken by name alphabetically.