tkinter: Adjust Label text based on changed OptionMenu

Question

In a canvas there is a button 'new entry' that starts a Toplevel. It has a submit Button, a cancel Button, an OptionMenu, and a Label. The objective is that the selected value from the OptionMenu changes the Label text. I didn't add the code for the canvas.

def Test(value):
    p = value
    print(p) # a test
    E2.config(text=str(p))

def Vullen_Toplevel_Aankoop_EUR(i):
    top = Toplevel()
    top.geometry("%dx%d%+d%+d" % (600, 800, 250, 125))

    BSubmit = Button(top, text="Submit", fg='green')
    BSubmit.grid(row=1, column=0, sticky="ew", columnspan=1, padx=20)

    BCancel = Button(top, text="Cancel", fg='red', command=top.destroy)
    BCancel.grid(row=10, column=0, sticky="ew", columnspan=1, padx=20)

    p = None
    E2 = None

    E1_opt = ["A","B","C"]
    E1_var = StringVar(top)
    E1_var.set(E1_opt[0])
    W1 = OptionMenu(top, E1_var, *E1_opt, command=Test)
    W1.grid(row=2, column=1, sticky="ew")

    E2 = Label(top, text='test')
    E2.grid(row=4, column=2, sticky="ew")

The default value of variable p is 'empty'. The OptionMenu triggers command=Test which prints the new value for p on the console. So far so good.

I tried to update Label E2 with: E2.config(test=str(p)). Unfortunately the error message:

NameError: name 'E2' is not defined

Although not added to the code here, I tried the following:

Define E2 Label as 'empty' or as the Label just below where I defined 'p', the error is the same.
Define E2 Label in def Test(value) but here - of course - the error is that Toplevel() top is not defined.
Instead of command=Test I tried command=lambda: Test(top, E1_var.get()) and changed the Test method to def Test(top, value) but then the error is: TypeError: () takes 0 positional arguments but 1 was given

I am kind of out of ideas. Would you have insights to share? Thanks.

E2 is local variable which exists only in one function. Define it outside all function - ie. E2 = None and then you will have global variable. And then you can inform functions to use this variable by using global E2 inside Vullen_Toplevel_Aankoop_EUR (because you use = to assign value). In Test you don't have to use global E2 because you don't use = and it will use external E2 automatically. — furas
@furas To be honest, I don't get if confirmed the approach or not. Both p and E2 are defined in Vullen_Toplevel_Aankoop_EUR (now as = None, code example updated) and can receive with [=] in Test the [value] variable. But still E2 as defined a Button, later set in Vullen_Toplevel_Aankoop_EUR is still not defined in Test. I moved the E2=None definition outside the Toplevel() function but the result remains the same. Perhaps you clarify your statement a bit more. Thanks. — John J. Johnson
read commend again: define E2 = None OUTSIDE FUCTIONS - and use global E2 INSIDE FUNCTIONS. — furas

furas furas · Accepted Answer · 2017-01-24T21:43:22

Create global variable outside all functions

E2 = None

Use global E2 to inform function than you want to assign value to global/external variable E2. Without global it will create local variable E2 and it doesn't assign to global one.

def Vullen_Toplevel_Aankoop_EUR(i):
    global E2 # you need it because you use `E2 = ...`

    E2 = Label(top, text='test')

Full

# create global variable
E2 = None

def Test(value):
    #global E2 # you don't need it because you don't use `E2 = ...`

    E2.config(text=value)

def Vullen_Toplevel_Aankoop_EUR(i):
    global E2 # you need it because you use `E2 = ...`

    E2 = Label(top, text='test')

# you can create also after functions
#E2 = None

BTW: shorter

 top.geometry("600x800+250+125")

tkinter: Adjust Label text based on changed OptionMenu

1 Answers