Tkinter可展开框架展开时X轴无故拉伸，如何阻止该变化？

我在Tkinter中实现了一个可展开框架，点击后会展示内部更多组件。但展开该框架时，它在X轴方向被拉长，尽管内部所有元素的尺寸都未超过框架原有尺寸。相关实现代码如下：

import tkinter as tk
from tkinter import ttk

class ToggledFrame(tk.Frame):

    def __init__(self, parent, text="", *args, **options):
        tk.Frame.__init__(self, parent, *args, **options)

        self.show = tk.IntVar()
        self.show.set(0)

        self.title_frame = ttk.Frame(self)
        self.title_frame.pack(fill="x", expand=1)

        ttk.Label(self.title_frame, text=text).pack(side="left", fill="x", expand=1)

        self.toggle_button = ttk.Checkbutton(self.title_frame, width=2, text='+', command=self.toggle,
                                            variable=self.show, style='Toolbutton')
        self.toggle_button.pack(side="left")

        self.sub_frame = tk.Frame(self, relief="sunken", borderwidth=1)

    def toggle(self):
        if bool(self.show.get()):
            self.sub_frame.pack(fill="x", expand=0)
            self.toggle_button.configure(text='-')
        else:
            self.sub_frame.forget()
            self.toggle_button.configure(text='+')

class EntryWithPlaceholder(tk.Entry):
    def __init__(self, master=None, placeholder="PLACEHOLDER", color='grey'):
        super().__init__(master)

        self.placeholder = placeholder
        self.placeholder_color = color
        self.default_fg_color = self['fg']

        self.bind("<FocusIn>", self.foc_in)
        self.bind("<FocusOut>", self.foc_out)

        self.put_placeholder()

    def put_placeholder(self):
        self.insert(0, self.placeholder)
        self['fg'] = self.placeholder_color

    def foc_in(self, *args):
        if self['fg'] == self.placeholder_color:
            self.delete('0', 'end')
            self['fg'] = self.default_fg_color

    def foc_out(self, *args):
        if not self.get():
            self.put_placeholder()

root = tk.Tk()
    
root.geometry("600x200")

root.columnconfigure(0, weight=1)
root.columnconfigure(1, weight=1)
root.columnconfigure(2, weight=1)

frame_1 = tk.Frame(root, pady=2, padx=2, width=200)
frame_1.grid(row=0, column=0, sticky='ew')

t1 = ToggledFrame(frame_1, text='Button 1', relief="raised", borderwidth=1)
t1.pack(fill="x", expand=1, pady=2, padx=2, anchor="n")

text1 = EntryWithPlaceholder(t1.sub_frame, "text")
text1.pack( expand=1, pady=2, padx=2, anchor="n")
    
root.mainloop()

解决方法 #

问题根源在于布局参数的约束不足，调整以下3处即可阻止横向拉伸：

1. 固定父框架尺寸：给frame_1添加grid_propagate(False)禁止它随子组件自动调整大小，同时修改sticky参数避免被父容器横向拉伸：

   frame_1 = tk.Frame(root, pady=2, padx=2, width=200)
   frame_1.grid(row=0, column=0, sticky='n')  # 仅顶部对齐，不横向填充
   frame_1.grid_propagate(False)  # 锁定框架宽度
   

2. 限制可展开框架的扩展权限：把t1.pack的expand参数从1改为0，禁止它横向扩展：

   t1.pack(fill="x", expand=0, pady=2, padx=2, anchor="n")
   

3. 规范子框架对齐方式：在toggle方法中给sub_frame.pack加上anchor="n"，确保子框架仅填充现有宽度、顶部对齐：

   self.sub_frame.pack(fill="x", expand=0, anchor="n")
   

修改后，展开框架时会严格保持原有宽度，不会出现X轴方向的不必要拉伸。

内容的提问来源于stack exchange，提问作者Liam