Tkinter OptionMenu: setting the width by the longest menu item in the list

Question

Tkinter's OptionMenu sets its width based on the currently active menu item. I'm looking for a way to make it set its width based on the widest menu item, rather than the currently selected one to prevent the widget from resizing itself when the user selects a different menu item.

The suggested answer in the question Tkinter Option Menu Widget Changing Widths hides the problem, but doesn't solve it. (Hint: Not a duplicate!) The accepted answer is to set the widget to be sticky to both W and E. This hides the problem as long as the suggested size for the column is wider than all of the items in the box, but doesn't actually solve the problem.

I have (and prefer to have) a grid layout column with various widgets, which is dynamically sized based on the widths of the widgets. However, I'd prefer that the width reported by the OptionMenu widget is that of it widest item, not the currently selected ones, as the width of the whole column changes, if the user chooses an item which is wider than any of the other items.

All suggested methods I have found so far have either been to sticky the widget W and E which doesn't (in itself) solve the problem in my case, and setting an arbitrary constant width unrelated to the item lengths, which I don't want to do.

nitro2k01 · Accepted Answer · 2015-03-07T17:06:00.900

Thank you two who left answers, but neither using a monotype font or just setting the width to the length of the string are desirable solutions. The latter is too imprecise because the width parameter takes a number in a special "text unit" corresponding to the width of the the character "0" in the given font. So my final solution does the following:

Figure out the font that the element is using.
Use measure to find the width of each string. (Given here in pixels.)
Divide the longest width by the width of the 0 character. You now have the width in "text units".

This gives the following code. Could probably be more Pythonic and be a class method of a subclass of OptionMenuetc, but it works.

def setMaxWidth(stringList, element):
    f = tkFont.nametofont(element.cget("font"))
    zerowidth=f.measure("0")
    w=max([f.measure(i) for i in stringList])/zerowidth

    element.config(width=w)

Out of curiosity, I wanted to know how the adjusted width would differ from the character length of each string in my data. Here's a simple loop doing that, and empirical data from my strings.

    for i in stringList:
        print len(i), float(f.measure(i))/float(zerowidth)

The data:

4 3.83333333333
24 20.1666666667
22 18.5
11 11.1666666667
12 11.3333333333
14 12.6666666667

Left column=string length. Right column=calculated width. In other words, a naive len(max(opts, key=len)) would give a bigger-than-needed widget in all cases. (This is of course to be expected.) I've also compared the computed widths of the widget using special cases of repeated W as well as repeated . and in all cases, these artificial tests string test agree well with the width requested by the widget "normally" with no width manually set.

a slightly simpler solution is to put the option menu in a frame, and then set the size of the frame in pixels. Saves a little math at the expense of one extra widget. — Bryan Oakley, Feb 17 '14 at 15:50
@BryanOakley I could add an extra widget for each `OptionMenu`, but that seems both more complex and even hackier/uglier than this. This seems more like "set and forget", despite the extra math. — nitro2k01, Feb 17 '14 at 23:39
In Python 3 tkFont is renamed to font and you will have to cast the resulting float in to a Int but otherwise it works like a charm — Svavelsyra, Sep 03 '19 at 14:24

score 2 · Answer 2 · answered Feb 17 '14 at 12:33

If your GUI uses a fixed-width font, you could get the length of the longest element in the list that populates your OptionMenu, and set the width based on that.

from Tkinter import *

root = Tk()

opts = ['bacon', 'cereal', 'eggs']

oMenuWidth = len(max(opts, key=len))

v = StringVar(root)
v.set(opts[0])
oMenu = OptionMenu(root, v, *opts)
oMenu.config(width=oMenuWidth)
oMenu.grid()

mainloop()

I suppose even if it wasn't fixed length, you could perform a math operation on the width before implementing it, to get it close to right.

score 1 · Answer 3 · answered Feb 17 '14 at 11:55

Those are your two choices: set a fixed width, or rely on the width of the cell to control the width of the widget. Setting a non-arbitrary fixed width should be trivial, since you're the one setting the contents of the widget. Simply keep track of the widest item that you put in the widget, and use that width as the desired width for the widget.

Tkinter OptionMenu: setting the width by the longest menu item in the list

3 Answers3