为什么我的程序跳过了递归？

因此，我目前正在使用python和Tkinter制作排序算法可视化工具。我已经实现了选择，插入，冒泡和外壳排序。但是，由于递归性质，我在合并排序方面遇到了麻烦。在合并排序中，它会不断跳过合并排序的调用。

def merge_sort(self, array, lines):
    '''Sort the array/lines via merge sort. O(n log n).'''
    self.window.title("Merge Sort")
    # Basecase.
    if len(array) <= 1:
        return

    middle = len(array) // 2
    left = array[:middle]
    left_lines = lines[:middle]
    right = array[middle:]
    right_lines = lines[middle:]

    # Recursion.
    self.merge_sort(left, left_lines)
    self.merge_sort(right, right_lines)

    i = j = k = 0

    # Merge.
    while i < len(left) and j < len(right):
        if left[i] < right[j]:
            array[k], left[i] = left[i], array[k]
            lines[k], left_lines[i] = left_lines[i], lines[k]
            self.swap(lines[k], left_lines[i])
            yield
            i += 1
        else:
            array[k], right[j] = right[j], array[k]
            lines[k], right_lines[j] = right_lines[j], lines[k]
            self.swap(lines[k], right_lines[j])
            yield
            j += 1
        k += 1

    while i < len(left):
        array[k], left[i] = left[i], array[k]
        lines[k], left_lines[i] = left_lines[i], lines[k]
        self.swap(lines[k], left_lines[i])
        yield
        i += 1
        k += 1

    while j < len(right):
        array[k], right[j] = right[j], array[k]
        lines[k], right_lines[j] = right_lines[j], lines[k]
        self.swap(lines[k], right_lines[j])
        yield
        j += 1
        k += 1

This is my method, however it keeps skipping over self.merge_sort(left, left_lines) and self.merge_sort(right, right_lines).

我试过调试它，并使用print语句检查它调用self.merge_sort（）的次数，但是它始终只调用一次并退出。