The tree will be initially populated by the populate_node() method, which lists the entries in the current directory. If an entry is a directory, it also adds an empty child to show it as an expandable node.

When a node that represents a directory is opened, it lazily loads the contents of the directory by calling populate_node() again. This time, instead of adding the items as top-level nodes, they are nested inside the opened node:

import os
import tkinter as tk
import tkinter.ttk as ttk

class App(tk.Tk):
    def __init__(self, path):
        super().__init__()
        self.title("Ttk Treeview")

        abspath = os.path.abspath(path)
        self.nodes = {}
        self.tree = ttk.Treeview(self)
        self.tree.heading("#0", text=abspath, anchor=tk.W)
        ysb = ttk.Scrollbar(self, orient=tk.VERTICAL,
                            command=self.tree.yview)
        xsb = ttk.Scrollbar(self, orient=tk.HORIZONTAL,
                            command=self.tree.xview)
        self.tree.configure(yscroll=ysb.set, xscroll=xsb.set)

        self.tree.grid(row=0, column=0, sticky=tk.N + tk.S + tk.E +     tk.W)
        ysb.grid(row=0, column=1, sticky=tk.N + tk.S)
        xsb.grid(row=1, column=0, sticky=tk.E + tk.W)
        self.rowconfigure(0, weight=1)
        self.columnconfigure(0, weight=1)

        self.tree.bind("<<TreeviewOpen>>", self.open_node)
        self.populate_node("", abspath)

    def populate_node(self, parent, abspath):
        for entry in os.listdir(abspath):
            entry_path = os.path.join(abspath, entry)
            node = self.tree.insert(parent, tk.END, text=entry, open=False)
            if os.path.isdir(entry_path):
                self.nodes[node] = entry_path
                self.tree.insert(node, tk.END)

    def open_node(self, event):
        item = self.tree.focus()
        abspath = self.nodes.pop(item, False)
        if abspath:
            children = self.tree.get_children(item)
            self.tree.delete(children)
            self.populate_node(item, abspath)

if __name__ == "__main__":
    app = App(path=".")
    app.mainloop()

When you run the preceding example, it will display the filesystem hierarchy from the directory where the script is located, but you can explicitly set the directory you want via the path argument of the App constructor.