Python functions with multiple parameter brackets

Question

I've been having trouble understanding what h(a)(b) means. I'd never seen one of those before yesterday, and I couldn't declare a function this way:

def f (a)(b):     return a(b) 

When I tried to do def f (a, b):, it didn't work either. What do these functions do? How can I declare them? And, finally, what's the difference between f(a, b)and f(a)(b)?

Answer 1

Functions with multiple parameter brackets don't exist, as you saw when you tried to define one. There are, however, functions which return (other) functions:

def func(a):     def func2(b):         return a + b     return func2 

Now when you call func() it returns the inner func2 function:

>>> func2 = func(1)  # You don't have to call it func2 here >>> func2(2) 3 

But if you don't need the inner function later on, then there's no need to save it into a variable and you can just call them one after the other:

>>> func(1)(2)   # func(1) returns func2 which is then called with (2) 3 

This is a very common idiom when defining decorators that take arguments.

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Notice that calling func() always creates a new inner function, even though they're all named func2 inside of the definition of our func:

>>> f1 = func(1) >>> f2 = func(1) >>> f1(1), f2(1) (2, 2) >>> f1 is f2 False 

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And, finally, what's the difference between f(a, b)and f(a)(b)?

It should be clear now that you know what f(a)(b) does, but to summarize:

• f(a, b) calls f with two parameters a and b
• f(a)(b) calls f with one parameter a, which then returns another function, which is then called with one parameter b