Lambdas, sorted key=, and higher-order functions

Functions are values

Here is the idea this whole lesson builds on: in Python, a function is a value, just like a number or a string. You can store it in a variable, put it in a list, or pass it to another function. A function that takes or returns another function is called a higher-order function.

def shout(text):
    return text.upper() + "!"

say = shout          # no parentheses: we pass the function itself
print(say("hi"))     # HI!

Writing shout hands over the function. Writing shout(...) calls it. That distinction matters everywhere below.

The payoff is immediate and practical: custom sort orders, leaderboards by score, files by size, orders by date, are all one key= away once you can hand one function to another.

sorted(key=...)

You used sorted(nums) in Python for Beginners. Its superpower is the key parameter: pass a function, and Python calls it on each item to decide the sort order, without changing the items:

words = ["pear", "fig", "banana"]
sorted(words, key=len)   # ['fig', 'pear', 'banana']

When the key is tiny and used once, defining a whole def is heavy. A lambda is a one-expression, inline function:

lambda w: w[-1]      # takes w, returns its last character

lambda arguments: expression and nothing more. No statements, no return keyword, the expression is the return value.

Where you will meet this again

• max(items, key=...) and min(items, key=...) accept the same key idea.
• sorted(d.items(), key=lambda kv: kv[1]) sorts a dict by value, a pattern you will use in the Counter lesson (6-1) and the capstone (unit 10).
• Comprehensions from unit 1 usually beat map and filter, but you should recognize map(f, xs) and filter(f, xs) when reading other people's code: they apply or test f one item at a time as you loop over the result, instead of all at once up front (unit 4 covers this on-demand style in depth).

One caution from the style police: if a lambda gets hard to read, promote it to a named def. Lambdas are for tiny keys, not logic.