Python: calling inner() from outer()

Question

I have looked around on SO and surprisingly not found an answer to this question. I assume this is because normally inner/nested functions are used for something in particular (eg. maintaining an environment variable, factories) as opposed to something trivial like I'm trying to use them for. In any case, I can't seem to find any information on how to properly call an inner function from an outer function without having to declare inner() above outer() in the file. The problem is from this problem on HackerRank (https://www.hackerrank.com/challenges/circular-array-rotation/problem).

def circularArrayRotation(a, k, queries):

    def rotateArrayRightCircular(arr: list, iterations: int) -> list:
        """
        Perform a 'right circular rotation' on an array for number of iterations.
        Note: function actually moves last 'iterations' elements of array to front of array.

        >>>rotateArrayRightCircular([0,1,2], 1)
        [2,0,1]

        >>>rotateArrayRightCircular([0,1,2,3,4,5], 3)
        [3,4,5,0,1,2]

        >>>rotateArrayRightCircular([0,1,2,3,4,5], 6)
        [0,1,2,3,4,5]
        """
        return arr[-1 * iterations:] + arr[0:-1 * iterations]

    k = k % len(a)
    a = rotateArrayRightCircular(a, k)
    res = []
    for n in queries:
        res.append(a[n])
    return res

The code above does what I want it to, but it's somehow inelegant to me that I have to put the inner function call after the inner function definition. Various errors with different attempts:

# trying 'self.inner()'
Traceback (most recent call last):
  File "solution.py", line 52, in <module>
    result = circularArrayRotation(a, k, queries)
  File "solution.py", line 13, in circularArrayRotation
    a = self.rotateArrayRightCircular(a, k)
NameError: name 'self' is not defined

# Removing 'self' and leaving the definition of inner() after the call to inner()
Traceback (most recent call last):
  File "solution.py", line 52, in <module>
    result = circularArrayRotation(a, k, queries)
  File "solution.py", line 13, in circularArrayRotation
    a = rotateArrayRightCircular(a, k)
UnboundLocalError: local variable 'rotateArrayRightCircular' referenced before assignment

Any idea how I could include def inner() after the call to inner() without throwing an error?

You *always* have to define functions (and other variables) *before* you use them... — Aran-Fey, Oct 26 '18 at 07:53
Why _is_ your `rotateArrayRightCircular` function being defined inside `circularArrayRotation` ? Why not just define it outside `circularArrayRotation`, then you can have it afterwards if that's what your objective is. — khelwood, Oct 26 '18 at 08:16

score 2 · Answer 1 · answered Oct 26 '18 at 07:53

As a function is executed from top to bottom, and a function is put into existence as the function is processed, what you want is just not possible.

You could put the function before the outer one, making it outer itself, possibly adding some parameters (not necessary here). (BTW, it looks so generic that other parts of the code might want to use it as well, so why not outer?)

But otherwise, you are stuck. It is essetially the same situation as in

def f():
    print(a) # a doesn't exist yet, so this is an error
    a = 4

Well, you could do it this way:

def circularArrayRotation(a, k, queries):

    def inner_code():
        k = k % len(a)
        a = rotateArrayRightCircular(a, k)

        # BTW, instead of the following, you could just do
        # return [a[n] for n in queries]
        res = []
        for n in queries:
            res.append(a[n])
        return res


    def rotateArrayRightCircular(arr: list, iterations: int) -> list:
        """
        Perform a 'right circular rotation' on an array for number of iterations.
        Note: function actually moves last 'iterations' elements of array to front of array.

        >>>rotateArrayRightCircular([0,1,2], 1)
        [2,0,1]

        >>>rotateArrayRightCircular([0,1,2,3,4,5], 3)
        [3,4,5,0,1,2]

        >>>rotateArrayRightCircular([0,1,2,3,4,5], 6)
        [0,1,2,3,4,5]
        """
        return arr[-1 * iterations:] + arr[0:-1 * iterations]

    return inner_code()

but I don't see that you gain anything from it.

score 1 · Accepted Answer · answered Oct 26 '18 at 08:04

1

This is not possible in Python, but is possible in other languages like Javascript and PHP. It is called function hoisting.

answered Oct 26 '18 at 08:04

Hielke Walinga

2,677
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Thanks, that's what I wanted to know! :) – NotAnAmbiTurner Oct 29 '18 at 04:44

Python: calling inner() from outer()

2 Answers2