Task: Tree traversal
Go: package main

import "fmt"
    
type node struct {
    value       int
    left, right *node
}

func (n *node) iterPreorder(visit func(int)) {
    if n == nil {
        return
    }
    visit(n.value)
    n.left.iterPreorder(visit)
    n.right.iterPreorder(visit)
}

func (n *node) iterInorder(visit func(int)) {
    if n == nil {
        return
    }
    n.left.iterInorder(visit)
    visit(n.value)
    n.right.iterInorder(visit)
}

func (n *node) iterPostorder(visit func(int)) {
    if n == nil {
        return
    }
    n.left.iterPostorder(visit)
    n.right.iterPostorder(visit)
    visit(n.value)
}

func (n *node) iterLevelorder(visit func(int)) {
    if n == nil {
        return
    }
    for queue := []*node{n}; ; {
        n = queue[0]
        visit(n.value)
        copy(queue, queue[1:])
        queue = queue[:len(queue)-1]
        if n.left != nil {
            queue = append(queue, n.left)
        }
        if n.right != nil {
            queue = append(queue, n.right)
        }
        if len(queue) == 0 {
            return
        }
    }
}

func main() {
    tree := &node{1,
        &node{2,
            &node{4,
                &node{7, nil, nil},
                nil},
            &node{5, nil, nil}},
        &node{3,
            &node{6,
                &node{8, nil, nil},
                &node{9, nil, nil}},
            nil}}
    fmt.Print("preorder:    ")
    tree.iterPreorder(visitor)
    fmt.Println()
    fmt.Print("inorder:     ") 
    tree.iterInorder(visitor)
    fmt.Println()
    fmt.Print("postorder:   ")
    tree.iterPostorder(visitor)
    fmt.Println() 
    fmt.Print("level-order: ")
    tree.iterLevelorder(visitor)
    fmt.Println()
}

func visitor(value int) {
    fmt.Print(value, " ")
}

Python: from collections import namedtuple
 
Node = namedtuple('Node', 'data, left, right')
tree = Node(1,
            Node(2,
                 Node(4,
                      Node(7, None, None),
                      None),
                 Node(5, None, None)),
            Node(3,
                 Node(6,
                      Node(8, None, None),
                      Node(9, None, None)),
                 None))
 
def printwithspace(i):
    print(i, end=' ')

def dfs(order, node, visitor):
    if node is not None:
        for action in order:
            if action == 'N':
                visitor(node.data)
            elif action == 'L':
                dfs(order, node.left, visitor)
            elif action == 'R':
                dfs(order, node.right, visitor)
                
def preorder(node, visitor = printwithspace):
    dfs('NLR', node, visitor)
 
def inorder(node, visitor = printwithspace):
    dfs('LNR', node, visitor)
 
def postorder(node, visitor = printwithspace):
    dfs('LRN', node, visitor)
 
def ls(node, more, visitor, order='TB'):
    "Level-based Top-to-Bottom or Bottom-to-Top tree search"
    if node:
        if more is None:
            more = []
        more += [node.left, node.right]
    for action in order:
        if action == 'B' and more:
            ls(more[0], more[1:], visitor, order)
        elif action == 'T' and node:
            visitor(node.data)

def levelorder(node, more=None, visitor = printwithspace):
    ls(node, more, visitor, 'TB') 
 

def reverse_preorder(node, visitor = printwithspace):
    dfs('RLN', node, visitor)
    
def bottom_up_order(node, more=None, visitor = printwithspace, order='BT'):
    ls(node, more, visitor, 'BT')


if __name__ == '__main__':
    w = 10
    for traversal in [preorder, inorder, postorder, levelorder, 
                      reverse_preorder, bottom_up_order]:
        if traversal == reverse_preorder:
            w = 20
            print('\nThe generalisation of function dfs allows:')
        if traversal == bottom_up_order:
            print('The generalisation of function ls allows:')
        print(f"{traversal.__name__:>{w}}:", end=' ')
        traversal(tree)
        print()