What's the use of using a lambda function to call a function?

Question

So, as the title suggest, I don't know what's the use of using a lambda function to call another function, especially when putting that lambda function as a keyword argument in a widget such as a button.

def multiply(x,y):
    print(x*y)

window = Tk()

x = 1
y = 1
button = Button(window, text='button' command= lambda x=x, y=y : multiply(x,y))
button.pack()

window.mainloop()

I've been trying to play with it myself. I tried not to use a lambda function to call another function but it's not working as I expected it to be.

Answer 1

What you want is that when you click on your button, your function multiply gets called with the argument "x" and "y"

So achieve this, you need to create a pointer to the function multiply with the right argument, which you can achieve using a lambda expression.

if you were to do button = Button(window, text='button' command= multiply(x,y))

you are not assigning a pointer to a function to the command argument, but instead directly calling multiply(x,y)

you can try this yourself by modify your code

Let's see an example

def print_value(i):
    print(i)


lambda_list = []
normal_list = []

for i in range(0,4):
    normal_list.append(print_value(i))

if you do this, you'll see that 0,1,2,3 gets printed, because I'm calling the function print_value

Now if I do this :

def print_value(i):
    print(i)


lambda_list = []
normal_list = []

for i in range(0,4):
    lambda_list.append(lambda x=i : print_value(x))

Nothing gets printed, because I added pointer to the function inside my list through lambda function, and not directly called the function

but now if I continue my example by doing this

for lambda_func in lambda_list:
    lambda_func()

0,1,2,3 gets printed because I'm now calling my function pointer with the argument I gave to it (through lambda x=i)

now last example

If I were to do

lambda_list = []

for i in range(0,4):
    lambda_list.append(lambda: print_value(i))


for lambda_func in lambda_list:
    lambda_func()

What would be the result? answer : 3 3 3 3

Because when you do lambda x=i : print_value(x) you are capturing the current value of " i " , but when you do lambda: print_value(i), it always references the same "i" (the one of the for loop), ence why you may need to do lambda x=x, y=y : multiply(x,y) in your case instead of lambda: multiply(x,y)

Hope this is clear.