Functions as a argument

Question

Just started learning python and came across this

def f(g):
    return g(2)

def square(x):
    return x ** 2

print(f(square)) # which gives 4

How does inputting square into the f function equate to 4?

To understand replace `g` with `square` so, `f(square)` returns `square(2)` — Ch3steR, Feb 06 '22 at 17:01
Functions are objects too. Related https://stackoverflow.com/questions/24881604/when-should-i-use-function-currying-in-python — OneCricketeer, Feb 06 '22 at 17:02
functions are also objects, and when you use return values, their values are stored in the function obj. — Crystallean, Feb 06 '22 at 17:06
Because `f` does `g(2)` with that argument, and `square(2)` gives you `2 ** 2` which is 4 — juanpa.arrivillaga, Feb 06 '22 at 18:24

score 3 · Answer 1 · answered Feb 06 '22 at 17:03

3

When you call a function, the value of the argument is assigned to the named parameter. The call

print(f(square))

can be thought of as "expanding" to

g = square
print(g(2))

answered Feb 06 '22 at 17:03

chepner

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score 0 · Answer 2 · answered Feb 06 '22 at 17:01

0

Calling f(square) is square(2) which is 2**2 so 4

answered Feb 06 '22 at 17:01

rapaterno

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score 0 · Answer 3 · answered Feb 06 '22 at 17:07

In python functions are first-class values, considered objects as anything else, hence you can pass them as parameters to another function.

In your case, you are "composing" f with g, which in turn has an invocation with fixed 2 inside f. In this sense, f is a function whose purpose is to feed 2 into another function, given as argument.

Knowing that g squares a number, it's easy to see how g(2) is 4, then f(g) will return 4.

Functions as a argument

3 Answers3