Trouble following the call stack in a Binary Search Tree Inorder Tree Traversal call

I was hoping someone could clarify how the call stack works in this recursive call for me please. I think I am starting to wrap my head around recursion and backtracking which has been an issue for me and I think I have dwindled the problem down to one specific area.

So, the confusion came up when I was practicing my BST Inorder Traversals in Python.

Here is the implementation of a BST Tree I used to represent the following array [10, 14, 19, 27, 31, 35, 42].

I provide 3 Inorder Traversal solutions because I was trying to understand how the recursive call stack worked. For options 1 and 2 when I keep the result outside of the recursive call I get how the solution works. But for Option 3 when we do res = and res = res + I am confused how the call stack keeps track of this to return an answer. I think once I get how the res is saved in the call stack I will be able to understand how recursion works. Could someone walk me through how this works please.

FYI: I don't run the solution with all three functions named the same it was for the sake of the example.

class Node:

    def __init__(self, data):
        self.left = None
        self.right = None
        self.data = data

    def insert(self, data):
        node = Node(data)
        if self.data:
            if data < self.data:
                if self.left is None:
                    self.left = node
                else:
                    self.left.insert(data)
            elif data > self.data:
                if self.right is None:
                    self.right = node
                else:
                    self.right.insert(data)
        else:
            self.data = data

    def printTree(self):
        if self.left:
            self.left.printTree()
        print(self.data)
        if self.right:
            self.right.printTree()

# Option 1) I can follow the call stack if I implement it like this keeping the result out of the helper function
    def inorderTraversalRecursive(self, root):
        res = []

        def inorder(root):
            if not root:
                return
            inorder(root.left)
            res.append(root.data)
            inorder(root.right)

        inorder(root)
        return res

# Option 2) I can also follow the stack when I implement it like this and print the root with end=''
    def inorderTraversalRecursive(self, root):
        if root is None:
            return
        self.inorderTraversalRecursive(root.left)
        print(root.data, end=' ')
        self.inorderTraversalRecursive(root.right)

# Option 3) I have an issue understanding once I put the result into the if root statement
    def inorderTraversalRecursive(self, root):
        res = []
        if root:
            res = self.inorderTraversalRecursive(root.left)
            res.append(root.data)
            res = res + self.inorderTraversalRecursive(root.right)
        return res


#Creating the Tree
root = Node(27)
root.insert(14)
root.insert(35)
root.insert(10)
root.insert(19)
root.insert(31)
root.insert(42)
print(root.inorderTraversalRecursive(root))

Solution

Here's how I rewrote the function in the hope that it will help to understand it better:

def inorderTraversalRecursive(self, root):
    if root:
        return self.inorderTraversalRecursive(root.left)\
              +[root.data]\
              +self.inorderTraversalRecursive(root.right)
    return []

P.S.1. In your code the name of the function (variant 3) is inorderTraversal - probably a spelling mistake.

P.S.2. I wonder why there is if self.data: block in the insert function. If self.data is set to 0 it won't be recorded?

P.S.3. I hope i did everything right. I dare say, Recursion is a slippery subject.