If you’ve ever tried to build a code editor or a text processor in Tkinter, you’ve likely used the Text widget tags. Tags are incredibly powerful—they allow you to change the color, font, and behavior of specific ranges of text.
However, a common frustration arises when you apply multiple tags dynamically: they overlap and interfere with each other. In this guide, we’ll look at why this happens and how to “refresh” your tags correctly for features like syntax highlighting.
The Goal: Dynamic Highlighting
Imagine we want to build a simple highlighter that turns the word “Hello” red and the word “world” blue as the user types.
The basic logic is to bind a function to every keypress, scan the text, find the words, and apply the corresponding tag.
The Initial Implementation
import tkinter as tk
class HighlighterApp:
def __init__(self, root):
self.text = tk.Text(root)
self.text.pack(fill="both", expand=True)
# Trigger highlighting on every key release
self.text.bind("<KeyRelease>", self.highlight)
# Define our tag styles
self.text.tag_config("red_tag", foreground="red")
self.text.tag_config("blue_tag", foreground="blue")
def highlight(self, event=None):
# 1. Get all text from the widget
content = self.text.get("1.0", "end-1c")
# 2. Logic to find words and add tags...
# (This is where the overlapping issue usually starts)
The Problem: Overlapping Tags
As you type, Tkinter adds tags to specific ranges (e.g., from line 1.0 to 1.5). If you delete a word or change it, the old tag often “sticks” to those coordinates.
When you type a new word in the same spot, you might apply a new tag. Now you have two tags competing for the same text. Tkinter uses a priority system, so the newer or “higher” tag will win, often leading to words being colored incorrectly or staying colored when they shouldn’t be.
The Solution: The “Clean Slate” Approach
The most reliable way to handle dynamic syntax highlighting is to remove all relevant tags before re-applying them. This ensures that your highlighting logic is always working with a clean canvas.
The Optimized Logic
def highlight(self, event=None):
# 1. Remove existing tags from the entire widget
for tag in ["red_tag", "blue_tag"]:
self.text.tag_remove(tag, "1.0", "end")
content = self.text.get("1.0", "end-1c")
words = content.split()
# 2. Search and Re-apply
for word in ["Hello", "world"]:
start_pos = "1.0"
while True:
# Search for the word
start_pos = self.text.search(word, start_pos, stopindex="end")
if not start_pos:
break
# Calculate end position
end_pos = f"{start_pos}+{len(word)}c"
# Apply the appropriate tag
tag_name = "red_tag" if word == "Hello" else "blue_tag"
self.text.tag_add(tag_name, start_pos, end_pos)
# Move start_pos forward to find the next occurrence
start_pos = end_pos
tag_remove vs tag_delete
tag_remove(name, start, end): This keeps the tag’s configuration (color, font) in memory but removes it from the specific text range. This is what you should use for most highlighting tasks.tag_delete(name): This completely destroys the tag. You would have to calltag_configagain to use it. This is useful only when you want to clear out a large number of dynamic tags.
Conclusion
Syntax highlighting is a complex task, but mastering Tkinter tags is the first step. By remembering to clear your tags before each update, you prevent the “ghosting” effects that plague many beginner projects.
Pro Tip:
For large files, running a full re-highlight on every keypress can be slow. Consider only re-highlighting the current line or using a small delay (debouncing) to keep your editor snappy!
Suggested Posts
Troubleshooting Tkinter: Why is My GUI Output Not Showing?
Tkinter is the most common way for beginners to start building GUIs in Python. It’s powerful, but it...
How to Import Tkinter in Python 2 and 3: A Cross-Compatibility Guide
Tkinter is the standard GUI (Graphical User Interface) package that comes bundled with most Python i...
Display Most Frequent Words Using Pygame
""" Author: Abdur-Rahmaan Janhangeer Github: https://github.com/Abdur-rahmaanJ Instructions: pip...
