ttk optionmenu组件最初在下拉列表中包含所有值。选中任何一个值后，列表中的第一个值将消失，不再出现...
有人知道为什么吗？这是该组件设计的特点吗？请尝试使用以下内容：
import tkinter.ttk as ttk
import tkinter as tk
a = tk.Tk()
options = ['1', '2', '3']
value = tk.StringVar()
masterframe = ttk.Frame()
masterframe.pack()
dropdown = ttk.OptionMenu(masterframe, value, *options)
dropdown.pack()
a.mainloop()
注意 - 另一个用户在此处提出了同样的问题：OptionMenu不显示第一个选项(使用Tkinter)
他们似乎已经找到了一种解决方法，但是并不明白为什么会发生这种情况。
更新：实际上，只有在使用ttk小部件时才会出现此行为。Tk小部件可以正常工作（尽管看起来非常丑陋）。
ttk.OptionMenu命令的签名如下：
def __init__(self, master, variable, default=None, *values, **kwargs):
这是文档字符串：
"""使用master作为父级构建一个带有主题的OptionMenu小部件，资源textvariable设置为变量，初始选定的值由default参数指定，菜单值由*values和其他关键字给出。
请注意，在值列表之前出现的default选项。不要在值列表中添加空白项，而是将您想要的任何值添加为默认值：
options = ['1', '2', '3']
dropdown = ttk.OptionMenu(masterframe, value, options[1], *options)
# test-optionmenu.py
import tkinter as tk
from tkinter import ttk
root = tk.Tk()
# tk OptionMenu lists
optionList1 = ('a', 'b', 'c')
optionList2 = ('d', 'e', 'f')
optionList3 = ('g', 'h', 'j')
# ttk OptionMenu lists
optionList4 = ('1', '2', '3')
optionList5 = ('4', '5', '6')
optionList6 = ('7', '8', '9')
optionList7 = ('z', 'x', 'y')
# Set up the StringVars for each OptionMenu
v1 = tk.StringVar()
v2 = tk.StringVar()
v3 = tk.StringVar()
v4 = tk.StringVar()
v5 = tk.StringVar()
v6 = tk.StringVar()
v7 = tk.StringVar()
# tk.OptionMenu requires the default option
# to be declared via the set() method.
v2.set(optionList2[1]) # Default tk.OptionMenu value for om2
v3.set(optionList3[2]) # Default tk.OptionMenu item value for om3
v5.set(optionList5[1]) # Default ttk.OptionMenu item value for om5
# -------------------------------------------
# tk OptionMenu om1 does not automatically
# assign a default based on your list, so
# this does not display a list item initially
om1 = tk.OptionMenu(root, v1, *optionList1)
# -------------------------------------------
# om2 demonstrates the 'default' parameter
# from a former ttk.OptionMenu is ignored by
# tk.OptionMenu because it needs to have the
# default list item set via v2.set() call.
# Notice the 'e' is displayed initially as
# set, but now 'd' is duplicated in the list
# as a remnant of the former ttk.OptionMenu
# 'default' parameter.
om2 = tk.OptionMenu(root, v2, optionList2[0], *optionList2)
# -------------------------------------------
# om3 is a tk.OptionMenu and has no 'default'
# paramter declared like om2 above does. Its
# default is set to option 2, or 'j', and is
# initially displayed on the dropdown. None
# of the OptionMenu list items are duplicated.
om3 = tk.OptionMenu(root, v3, *optionList3)
# -------------------------------------------
# om4 shows how the 'default' parameter
# from a former ttk.OptionMenu is ignored by
# tk.OptionMenu because it needs to have the
# default list item set via v4.set() call.
# Since v4 is not set, nothing at all is
# diplayed initially and when the OptionMenu
# is clicked, it will show that '1' is listed
# twice.
om4 = tk.OptionMenu(root, v4, optionList4[0], *optionList4)
# -------------------------------------------
# by changing a tk.OptionMenu to a
# ttk.OptionMenu, without properly declaring
# the default OptionMenu item in the ttk way
# will result in item '5' not being displayed
# initially, and '4' will not be displayed in
# the OptionMenu choices after a choice has
# been made.
om5 = ttk.OptionMenu(root, v5, *optionList5)
# -------------------------------------------
# om6 is the same as om5, except it does not
# have a tk.OptionMenu default declared. It
# is only here for consiceness.
om6 = ttk.OptionMenu(root, v6, *optionList6)
# -------------------------------------------
# om7 is the proper way to declare a
# ttk.OptionMenu, with a default option
# initially set.
om7 = ttk.OptionMenu(root, v7, optionList7[2], *optionList7)
# Pack'em all up
om1.pack()
om2.pack()
om3.pack()
om4.pack()
om5.pack()
om6.pack()
om7.pack()
# Execute the mainloop
root.mainloop()
我知道代码可能有点丑陋，如果你确实想要更改它，那就去尝试吧。:)
补充其他人的答案，因为它们对我没有用。我发现如果你不在StringVar/IntVar中设置widget选项，它就不会显示已设置的标准值。这可能看起来很傻，但我花了很多时间才弄清楚这一点。希望能有所帮助，再见。
master = tk.Tk()
var = tk.StringVar(master)
master.mainloop()
虽然这篇文章有点旧，但我觉得我应该为这个话题做出贡献。在使用 tk.OptionMenu 时，让第一个（或者你列表中的任何一个项目）默认显示在 Mac 上非常简单，只需利用 tk.StringVar 即可。
例如：
options = ['option1', 'option2', 'option3', 'option4']
root = Tk()
var = StringVar()
var.set(options[0])
OptionMenu(root, var, *options)
mainloop()
selected = var.get()
var.set来确定“set”值。希望这可以帮助您！看起来使用 ttk.OptionMenu 时，选项列表中的第一项必须留空：
import tkinter.ttk as ttk
import tkinter as tk
a = tk.Tk()
options = ['', '1', '2', '3']
value = tk.StringVar()
value.set(options[1])
masterframe = ttk.Frame()
masterframe.pack()
dropdown = ttk.OptionMenu(masterframe, value, *options)
dropdown.pack()
a.mainloop()
来源：https://pyinmyeye.blogspot.com/2012/08/tkinter-menubutton-demo.html
ttk.OptionMenu 期望在变量参数后和值列表前提供默认值。虽然你的解决方案有效，但它掩盖了实际发生的情况。 - Bryan Oakley