Stefan - 1 year ago 134
Python Question

# Function chaining in python

On codewars.com I encountered the following task:

Create a function
`add`
that adds numbers together when called in succession. So
`add(1)`
should return
`1`
,
`add(1)(2)`
should return
`1+2`
, ...

While I'm familiar with the basics of Python, I've never encountered a function that is able to be called in such succession, i.e. a function
`f(x)`
that can be called as
`f(x)(y)(z)...`
. And thus far, I'm not even sure how to interpret this notation. As a mathematican, I'd suspect that
`f(x)(y)`
is a function that assigns to every
`x`
a function
`g_{x}`
and then returns
`g_{x}(y)`
and likewise for
`f(x)(y)(z)`
. Should this interpretation be correct, Python would allow me to dynamically create functions which seems very interesting to me. I've searched the web for the past hour, but wasn't able to find a lead in the right direction. Since I don't know how this programming concept is called, however, this may not be too surprising.

Therefore I'd like to ask you: How do you call this concept and where can I read more about it?

Answer Source

One suggestion (the way I'd do this) would be with a custom `int` subclass that defines a `__call__` which returns a new instance of itself with the updated value:

``````class CustomInt(int):
def __call__(self, v):
return CustomInt(self + v)
``````

Function `add` can now be defined to return that `CustomInt` instance, which, as a callable, can be called in succession:

``````# as @Caridorc noted in a comment
# add could be simply written as: add = CustomInt
>>> def add(v):
...    return CustomInt(v)
>>> add(1)
1
>>> add(1)(2)
3
>>> add(1)(2)(3)(44)  # and so on..
50
``````

In addition, as an `int` subclass, the returned value retains the `__repr__` and `__str__` behavior of `int`s.

Maybe this implementation suits you, alternatives likely exist and would be interesting to examine. I don't know whether this is function chaining as much as it's callable chaining, but, since functions are callables I guess there's no harm done.

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