Implementing merge sort function in python class, errors

Question

So I have a function defined that works great at doing merge sort on a linear array if it is implemented by its lonesome, but if I put it into a class it bugs out. I think it's a great example of what I don't quite understand about how classes work; possibly in regards to namespace management(?).

See below:

def sort(array):
    print('Splitting', array)
    if len(array) > 1:
        m = len(array)//2
        left = array[:m]
        right = array[m:]

        sort(left)
        sort(right)

        i = 0
        j = 0
        k = 0

        while i < len(left) and j < len(right):
            if left[i] < right[j]:
                array[k] = left[i]
                i += 1
            else:
                array[k] = right[j]
                j += 1
            k += 1

        while i < len(left):
            array[k] = left[i]
            i += 1
            k += 1

        while j < len(right):
            array[k] = right[j]
            j += 1
            k += 1
    print('Merging', array)

arr = [1,6,5,2,10,8,7,4,3,9]
sort(arr)

Produces the expected correct output:

Splitting  [1, 6, 5, 2, 10, 8, 7, 4, 3, 9]
Splitting  [1, 6, 5, 2, 10]
Splitting  [1, 6]
Splitting  [1]
Merging  [1]
Splitting  [6]
Merging  [6]
Merging  [1, 6]
Splitting  [5, 2, 10]
Splitting  [5]
Merging  [5]
Splitting  [2, 10]
Splitting  [2]
Merging  [2]
Splitting  [10]
Merging  [10]
Merging  [2, 10]
Merging  [2, 5, 10]
Merging  [1, 2, 5, 6, 10]
Splitting  [8, 7, 4, 3, 9]
Splitting  [8, 7]
Splitting  [8]
Merging  [8]
Splitting  [7]
Merging  [7]
Merging  [7, 8]
Splitting  [4, 3, 9]
Splitting  [4]
Merging  [4]
Splitting  [3, 9]
Splitting  [3]
Merging  [3]
Splitting  [9]
Merging  [9]
Merging  [3, 9]
Merging  [3, 4, 9]
Merging  [3, 4, 7, 8, 9]
Merging  [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

However, I get an error when I attempt to use this function in a class; something to do with namespace managment, I think. See below:

class MergeSort(object):

    def __init__(self, array):
        self.array = array

    def sort(self):
        print('Splitting', self.array)
        if len(self.array) > 1:
            m = len(self.array)//2
            left = self.array[:m]
            right = self.array[m:]

            sort(left)
            sort(right)

            i = 0
            j = 0
            k = 0

            while i < len(left) and j < len(right):
                if left[i] < right[j]:
                    self.array[k] = left[i]
                    i += 1
                else:
                    self.array[k] = right[j]
                    j += 1
                k += 1

            while i < len(left):
                self.array[k] = left[i]
                i += 1
                k += 1

            while j < len(right):
                self.array[k] = right[j]
                j += 1
                k += 1
        print('Merging', self.array)

x = MergeSort([1,6,5,2,10,8,7,4,3,9])
x.sort()

Produces the error output:

Splitting [1, 6, 5, 2, 10, 8, 7, 4, 3, 9]
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-15-89509f86277e> in <module>()
      1 x = MergeSort([1,6,5,2,10,8,7,4,3,9])
----> 2 x.sort()

<ipython-input-14-2bba116f00ce> in sort(self)
     11             right = self.array[m:]
     12 
---> 13             sort(left)
     14             sort(right)
     15 

NameError: name 'sort' is not defined

My initial instinct, after google searching around was to change subroutines sort(left) and sort(right) by adding prefixive self., but that generates a positional argument error. Would love a comment or two on what it is that I'm not understanding here. And cheers for good votes if my question is not stupid, and down votes if it is.

Answer 1

The reason sort(left) doesn't work is that, as you surmised, you can't call a method on self without specifying self . Leaving that off means it looks for a local or global name sort , doesn't find one, and raises a NameError .

The reason self.sort(left) doesn't work is that the API you defined doesn't work that way. Your class takes the list as a constructor argument, and then takes a sort with no arguments, that operates on the list passed in at construction time. So, you have no way to call your own sort with a different array. If you try self.sort(left) , you're passing the wrong number of arguments, just like calling abs(1, 2) , and you get the same TypeError .

You have to use your API the way you designed it: Create a new MergeSort sorter object with the new list, then call sort on that new object:

leftsorter = MergeSort(left)
leftsorter.sort()
rightsorter = MergeSort(right)
rightsorter.sort()

Answer 2

Replacing the sort(left) and sort(right) components of the sort() function within my class with

leftsorter = MergeSort(left)
leftsorter.sort()
rightsorter = MergeSort(right)
rightsorter.sort()

(Thank you abarnert)

While at the same time removing debugging print statements from the code (thank you Evgany P), and by avoiding reusing a built in function name sort() to avoid confusion, I have a working MergeSort class.

class MergeSort(object):

    def __init__(self, array):
        self.array = array

    def merge_sort(self):
        if len(self.array) > 1:
            m = len(self.array)//2
            left = self.array[:m]
            right = self.array[m:]

            leftsorter = MergeSort(left)
            leftsorter.merge_sort()
            rightsorter = MergeSort(right)
            rightsorter.merge_sort()

            i = 0
            j = 0
            k = 0

            while i < len(left) and j < len(right):
                if left[i] < right[j]:
                    self.array[k] = left[i]
                    i += 1
                else:
                    self.array[k] = right[j]
                    j += 1
                k += 1

            while i < len(left):
                self.array[k] = left[i]
                i += 1
                k += 1

            while j < len(right):
                self.array[k] = right[j]
                j += 1
                k += 1

x = MergeSort([3,5,6,2,1,4,10,9,8,7])
x.merge_sort()
x.array

Out[ ]: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Well done everybody!

Answer 3

You need to call self.sort() within the class.

A larger problem is that none of your function or class method return anything, just prints, are you satisfied with that?