import tkinter as tk
from tkinter import ttk

root = tk.Tk()

tree = ttk.Treeview(root)
tree.pack(fill="both", expand=True)

items = []
for i in range(10):
    item = tree.insert("", "end", text="Item {}".format(i+1))
    items.append(item)

items_to_select = items[:]


def select_all_items(event):
    tree.selection_set(items_to_select)

tree.bind('<Button-1>', select_all_items)


root.mainloop()