Python Tkinter按键绑定异常：左箭头触发prev_cmd失效问题

我在TreeView中按父节点拆分文本内容，每个父节点最多包含10个子文本节点。通过next_cmd和prev_cmd函数实现切换上一个/下一个文本元素并高亮TreeView中的当前项，该功能支持按钮点击或按下左/右箭头触发。按钮点击时功能均正常，但按下左箭头触发prev_cmd时，仅会高亮父节点，无法定位到对应的子文本节点；而按下右箭头触发next_cmd在两种触发方式下均正常工作。

复现代码 #

import math
import tkinter as tk
from tkinter import ttk

root = tk.Tk()

text = ["Hello", "World", "Hello", "World","Hello", "World","Hello", "World","Hello", "World","Hello", "World","Hello", "World",
        "Hello", "World", "Hello", "World","Hello", "World","Hello", "World","Hello", "World","Hello", "World","Hello", "World",
        "Hello", "World", "Hello", "World","Hello", "World","Hello", "World","Hello", "World","Hello", "World","Hello", "World",
        "Hello", "World", "Hello", "World","Hello", "World","Hello", "World","Hello", "World","Hello", "World","Hello", "World"]
text_index = 0

mainframe = ttk.Frame(root)

text_preview = ttk.Treeview(mainframe, columns=("text"), show="headings", style="RateFrame.Treeview",
                                selectmode="browse")


def next_cmd(event=None):
    # Works as intended with both using the right-arrow-key as well as pressing the next button.
    global text_index
    global text
    if text_index == len(text) - 1:
        # Do nothing, if end of text was reached.
        return 
    else:
        # Highlight the next child element in the treeview
        text_index = text_index + 1
        child_iid = "child_" + str(text_index)
        text_preview.focus(child_iid)
        text_preview.selection_set(child_iid)

def prev_cmd(event=None):
    # Only pressing the prev-button works as intended, but not using the left-arrow-key.
    global text_index
    global text
    if text_index == 0:
        # Do nothing, if beginning of text was reached.
        return 
    else:
        # Highlight the previous child element in the treeview
        text_index = text_index - 1
        child_iid = "child_" + str(text_index)
        text_preview.focus(child_iid)
        text_preview.selection_set(child_iid)


def populate_treeview():
    upper_limit = math.ceil(len(text) / 10)
    for i in range(upper_limit):
        text_preview.insert("", "end", iid=("parent_" + str(i)), open=False, 
                                values=("Questions " + str(i * 10 + 1) + " - " + str(len(text)),))

        # Populate the children:
        for j in range(10):
            k = j + 10 * i # Offset k; 0 <= k < len(text)
            if k >= len(text):
                break
            text_preview.insert(("parent_" + str(i)), "end", iid=("child_" + str(k)), open=False,
                                    values=(text[k],))

    # Focus the first child element and open it's parent, when the script starts.
    text_preview.item("parent_0", open=True)
    text_preview.focus("child_0")
    text_preview.selection_set("child_0")
    text_preview.focus_set()

    

text_preview.heading("text", text="Navigation")
text_preview.bind("<Right>", next_cmd)
text_preview.bind("<Left>", prev_cmd)

mainframe.pack(fill="both", expand=True)
text_preview.pack(fill="both", expand=True)

prev_btn = ttk.Button(mainframe, text="prev", command=prev_cmd)
next_btn = ttk.Button(mainframe, text="next", command=next_cmd)

prev_btn.pack(side="left")
next_btn.pack(side="right")

populate_treeview()

root.mainloop()

问题原因 #

TreeView控件对左箭头有默认行为：当焦点处于子节点时，按下左箭头会自动将焦点切换到其父节点。这个默认事件会先于我们自定义的prev_cmd执行，导致我们设置的子节点焦点被默认行为覆盖。而右箭头的默认行为是展开父节点（若当前焦点是父节点），不会干扰子节点的切换逻辑，因此next_cmd能正常工作。

解决方案 #

在自定义的prev_cmd函数末尾返回'break'，终止事件的传递，阻止TreeView的默认左箭头行为执行。同时为了一致性，也可以给next_cmd添加同样的返回值。

修改后的函数代码如下：

def next_cmd(event=None):
    global text_index
    global text
    if text_index == len(text) - 1:
        return 
    text_index += 1
    child_iid = "child_" + str(text_index)
    text_preview.focus(child_iid)
    text_preview.selection_set(child_iid)
    return 'break'  # 阻止默认行为

def prev_cmd(event=None):
    global text_index
    global text
    if text_index == 0:
        return 
    text_index -= 1
    child_iid = "child_" + str(text_index)
    text_preview.focus(child_iid)
    text_preview.selection_set(child_iid)
    return 'break'  # 阻止默认行为

这样修改后，按下左箭头就能正确定位到对应的子节点，和按钮点击的行为保持一致。

内容的提问来源于stack exchange，提问作者Timo