Hey there! Let's work through this Treeview column update issue together and fix that frustrating error you're seeing.

Why You're Getting That Error #

That TypeError usually happens when you accidentally treat a float value like a list or tuple (trying to use square brackets to index into it). For example, if you tried something like float(current_value)[2] (which doesn't make sense) or tried to directly edit a single element in the immutable tuple returned by tree.item(...)['values'], you'll hit this problem. Treeview doesn't let you modify individual columns directly—you have to update the full set of values for the row while preserving the columns you don't want to change.

How to Update a Single Column Correctly #

Here's the step-by-step approach to make this work:

1. Grab the currently selected Treeview row (since you're triggering this on Enter, you'll want the row the user is working on).
2. Fetch the existing values for that row using tree.item(item_id, 'values')—this returns a tuple of all column values.
3. Calculate your third column value using the first two columns (don't forget to convert them to numbers first!).
4. Create a new tuple of values, keeping the first two columns unchanged and replacing the third with your calculated result.
5. Update the Treeview item with this new tuple.

Working Code Example #

Let's put this into action with a complete sample setup:

import tkinter as tk
from tkinter import ttk

root = tk.Tk()
root.title("Treeview Auto-Calculation")

# Create Treeview with 3 columns
tree = ttk.Treeview(root, columns=('col1', 'col2', 'col3'), show='headings')
tree.heading('col1', text='First Value')
tree.heading('col2', text='Second Value')
tree.heading('col3', text='Calculated Total')
tree.pack(padx=10, pady=10)

# Add a test row to experiment with
tree.insert('', 'end', values=(15.75, 22.25, ''))

def on_enter_triggered(event):
    # Get the selected row (handle case where no row is selected)
    try:
        selected_item = tree.selection()[0]
    except IndexError:
        print("No row selected! Please select a row first.")
        return
    
    # Fetch current values for the selected row
    current_values = tree.item(selected_item, 'values')
    
    try:
        # Convert first two columns to floats for calculation
        val1 = float(current_values[0])
        val2 = float(current_values[1])
        
        # Calculate the total (adjust this formula to your needs)
        calculated_total = val1 + val2
        
        # Create new values tuple: keep first two, replace third
        new_values = (current_values[0], current_values[1], round(calculated_total, 2))
        
        # Update the Treeview row with the new values
        tree.item(selected_item, values=new_values)
    except ValueError:
        print("Oops! Please enter valid numbers in the first two columns.")
    except IndexError:
        print("Make sure the first two columns have values entered!")

# Bind Enter key to the calculation function
tree.bind('<Return>', on_enter_triggered)

root.mainloop()

Key Tips #

• Always add validation (the try/except blocks here handle cases where users enter non-numeric text or leave columns empty).
• Remember that tree.item() expects a full sequence (tuple or list) for the values parameter—you can't modify just one element directly, so you have to pass the complete set of values each time.
• If you're using inline editing (like ttk.Entry for Treeview cells), make sure to save the edited value to the Treeview before running the calculation.

内容的提问来源于stack exchange，提问作者Aleksandar Beat