Why does it run the mult function twice to get a result of 16?

Question

What is the output of this code?
def test(func, arg):
return func(func(arg))

def mult(x):
return x * x

print(test(mult, 2))

Answer 1

To understand this you need to imagine replacing parameter 'func' with 'mult' within the `test` function body.

return func(func(arg)) ↓
return mult(mult(arg))

First we check the inner call for `mult`

mult(arg)

Is equivalent to

mult(2) returns 2 * 2 => 4

Then we pass that result from inner call (4) to the outer call for `mult`, before we return it.

mult(mult(arg))

Is equivalent to

mult(4) # 4 is result from inner call

mult(4) returns 4 * 4 => 16

And finally we return that result (16) to the `test` function caller.

Hth, cmiiw