如何将参数传递给StringVar的回调？

Question

I have set up an Entry widget using tkinter and am using a StringVar in order to trace any changes to it. However, when I try to use the callback function, I find that I am unable to pass any arguments to it.

Here is what I have tried:

hpstring = StringVar()
hpstring.trace("w", hptrace)
hpstring.set("0")
hpbox = tk.Entry(frame, textvariable=hpstring)
hpbox.pack()

def hptrace(*args):
    print(hpstring)

This fails because hpstring is not defined.

hpstring = StringVar()
hpstring.trace("w", hptrace(hpstring))
hpstring.set("0")
hpbox = tk.Entry(frame, textvariable=hpstring)
hpbox.pack()

def hptrace(*args):
    print(hpstring)

This also somehow fails.

hpstring = StringVar()
hpstring.trace("w", lambda hpstring: hptrace(hpstring))
hpstring.set("0")
hpbox = tk.Entry(frame, textvariable=hpstring)
hpbox.pack()

def hptrace(*args):
    print(hpstring)

This time, it fails because "() takes 1 positional argument but 3 were given".

Is there any way to have my callback function (hptrace) take hpstring as an argument?

Answer 1

Is there any way to have my callback function (hptrace) take hpstring as an argument?

You can use lambda or functools.partial . The important thing to remember is that the function called by the trace is called with three arguments , so whatever command you give to trace needs to accept those three arguments.

Using lambda , your code should look something like this if you want to send only hpstring to hptrace :

hpstring.trace("w", lambda var_name, var_index, operation: hptrace(hpstring))

Note: you need to define hptrace before you attempt to use it, which the code in your question fails to do.