Tkinter entry text not updated when using .set() (but textvariable is)

I am having trouble setting a default value for a text entry widget in a Python 3.9 tkinter application. I am using a StringVar as the text variable, and changing it using .set().

I am trying to pre-set the date (and time).

If I do it in a class, the box is empty, even though the value has been changed:

class test:
    def __init__(self, root):
        # Content frame widget
        mainframe = ttk.Frame(root, padding="3 3 12 12")
        mainframe.grid(column=0, row=0, sticky=(N, W, E, S))
        
        ttk.Label(root, text="Date/Time").grid(
            column=1, row=3, sticky=E, padx=(5, 0)
        )
        self.date_value = StringVar()
        self.date_value.set(datetime.today().strftime("%Y-%m-%d"))
        ttk.Entry(root, width=10, textvariable=self.date_value).grid(column=2, row=3, sticky=E)
        print(self.date_value.get())

root = Tk()
test(root)
root.mainloop()

If I do it outside the class it works.

root = Tk()

mainframe = ttk.Frame(root, padding="3 3 12 12")
mainframe.grid(column=0, row=0, sticky=(N, W, E, S))

ttk.Label(root, text="Date/Time").grid(
    column=1, row=3, sticky=E, padx=(5, 0)
)
date_value = StringVar()
date_value.set(datetime.today().strftime("%Y-%m-%d"))
ttk.Entry(root, width=10, textvariable=date_value).grid(column=2, row=3, sticky=E)
print(date_value.get())

root.mainloop()

What's weird is I do sometimes see it work in the first example, when there's some other error in my code that causes a subsequent error it correctly shows with the date filled.

I'm sure I'm missing something very obvious but would love to know what it is!

It is because the instance of test() is not store in a variable, so it is garbage collected after it is created.

Just assign the instance to a variable:

...
root = Tk()
app = test(root)
root.mainloop()