# Python functions with multiple parameter brackets

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### Question :

Python functions with multiple parameter brackets

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)`?

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.

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
``````

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`

`f(a)(b)` just means that the expression `f(a)` returns a value that is itself callable. It’s a short form of

``````g = f(a)
g(b)
``````

You might be more comfortable adding a pair of redundant parentheses to emphasize that this is not a single syntactic construct.

``````(f(a))(b)  # f(a) is evaluated first, then the result is applied to b
``````

It is exactly analogous to the same doubling of square brackets for indexing nested dictionaries.

``````d1[x][y]
``````

is equivalent to

``````d2 = d1[x]
d2[y]
``````

Lets say we have an expression like

``````f(a)(b)
``````

then, `f(a)` returns a function itself which gets invoked with argument `b`. Consider the following example

``````def f(a):
def g(b):
return a * b
return g
``````

Then `f(5)(4)` evaluates to `5 * 4`, since `f(5)` returns a function which is basically

``````def g(b):
return 5 * b
``````

One could now do stuff like this

``````mult_by_5 = f(5)
[mult_by_5(x) for x in range(10)]
``````

Let’s be fancy, what about more nested functions?:

``````def f(a):
def g(b):
def h(c):
return a * b *c
return h
return g
f(2)(3)(4) # 24
``````