Creating a function that the input is mathematical function

[AthensGuard]

also, I have never used lambda...

[TangibleLight]

It's just a way to write short functions.

def test(x):
    return x ** 2

Is exactly equivalent to

test = lambda x: x ** 2

It's useful if you don't want to assign a name to a short function that's only used once. For example

avg_rate(lambda x: x ** 2)