When using for in and function call with Tkinter the functions arguments value is only showing the last element in the list?

Question

I'm trying to call same function with different argument with correspond to for in and buttons in Tkinter python, and when I click the other buttons the value given by the function is the last calls value . I'm a js dev where have used foreach and array with similar thing.

apps=["k","c","d"] 
for app in apps:
        btn = tk.Button(innerFrame, text=" {}".format(app), command=(
            lambda: runThis(app)))
        btn.pack()
       
def runThis(val, i):
    print("Value of the btn {}".format(val))

the expected output when I click each button is

Value of the btn k
Value of the btn c
Value of the btn d

but what I get is

Value of the btn d
Value of the btn d
Value of the btn d

Does this answer your question? [List comprehension and lambdas in Python](https://stackoverflow.com/questions/45925683/list-comprehension-and-lambdas-in-python) — quamrana, Jul 31 '20 at 12:43
Be aware that the function in your example `runThis` accepts two params `val, i` but you are supplying only one in the button's lambda part `lambda: runThis(app)`. — Wereii, Jul 31 '20 at 12:51

nagyl · Accepted Answer · 2020-07-31T12:54:11.553

Since app is a pointer to an object, and it gets overwritten in the loop, the last element in the list will be the value tk stores.

btn = tk.Button(innerFrame, text=name, command=lambda app=app: runThis(app))

This copies the object, therefore app won't get overwritten in your loop.

Think about it this way. In your loop:

#first loop
app = "k"
function(points to -> app -> points to "k") #first

#second loop
app = "c"
function(points to -> app -> points to "c") #first
function(points to -> app -> points to "c") #second

#third loop
app = "d"
function(points to -> app -> points to "d") #first
function(points to -> app -> points to "d") #second
function(points to -> app -> points to "d") #third

Therefore you need to copy the content of app, to avoid overwriting already existing values.

Might as well check out [`functools.partial`](https://docs.python.org/3/library/functools.html#functools.partial) instead of lambdas for this usecase. — Wereii, Jul 31 '20 at 12:50

When using for in and function call with Tkinter the functions arguments value is only showing the last element in the list?

1 Answers1