Why for sentence in add_command does not work properly?

Question

I try this:

def prints(string):
print(string)

from tkinter import *
root = Tk()
menu = Menu(root)
filemenu = Menu(menu, tearoff=0)
List = Menu(filemenu, tearoff=0)

labels=[x for x in range(1,4)]

for i in range(len(labels)):
    List.add_command(label=labels[i], command=lambda: prints(labels[i]))
List.add_command(label="!", command=lambda: prints("!"))
List.add_command(label="@", command=lambda: prints("@"))
List.add_command(label="#", command=lambda: prints("#"))
filemenu.add_cascade(label="List", menu=List)
menu.add_cascade(label="File", menu=filemenu)

root.config(menu=menu)
root.mainloop()

and it show : image

when I clicked !,@,# it print !,@,#

but when I clicked 1,2,3 it print 3,3,3 not 1,2,3

why it occurs?

Try using `functools.partial` instead of `lambda`. – TheLizzard Feb 20 '21 at 17:11 — TheLizzard, Feb 20 '21 at 17:11
Use: `command=lambda i=i: prints(labels[i])`. – Delrius Euphoria Feb 20 '21 at 17:22 — Delrius Euphoria, Feb 20 '21 at 17:22

score 1 · Answer 1 · answered Feb 20 '21 at 17:24

The for loop does not create a separate scope, regarding variable definitions, so the i variable inside your lambda is not local to it, it's defined in an outer scope. This means that it lives beyond the end of the for loop, retaining the last value (3, in your example).

The lambda definition stored the fact that you mean to access i, it did not store the value of i at definition time. The actual access to i to retrieve its value takes place when the lambda is run, not when the lambda is defined, which is why you always see 3.

The usual trick is to capture the value, at definition time, by storing it in a variable that is local to the lambda, typically by adding an argument:

for i in range(len(labels)):
    List.add_command(label=labels[i], command=lambda j=i: prints(labels[j]))

score 0 · Answer 2 · answered Feb 20 '21 at 17:15

0

You have to use partial instead of lambda like this:

def prints(string):
    print(string)

# Import partial
from functools import partial
from tkinter import *
root = Tk()
menu = Menu(root)
filemenu = Menu(menu, tearoff=0)
List = Menu(filemenu, tearoff=0)

labels=[x for x in range(1,4)]

for i in range(len(labels)):
    # Create the command
    command = partial(prints, labels[i])
    List.add_command(label=labels[i], command=command)
List.add_command(label="!", command=lambda: prints("!"))
List.add_command(label="@", command=lambda: prints("@"))
List.add_command(label="#", command=lambda: prints("#"))
filemenu.add_cascade(label="List", menu=List)
menu.add_cascade(label="File", menu=filemenu)

root.config(menu=menu)
root.mainloop()

This is the documentation for functools.partial.

answered Feb 20 '21 at 17:15

TheLizzard

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Wouldn't a simple `command=lambda i=i: prints(labels[i])` just solve it? – Delrius Euphoria Feb 20 '21 at 17:22
I think that partial is just easier and can be split into multiple lines to comply with PEP8. – TheLizzard Feb 20 '21 at 17:26
You don't _have_ to use `functools.partial`. `lambda` will work when used properly. – Bryan Oakley Feb 20 '21 at 18:26
@BryanOakley I dislike `lambda` so that is why I use partial. Sometimes `lambda` doesn't do what I expect it to do (like in this question). – TheLizzard Feb 20 '21 at 18:40
That's fine, it's just that you said the OP *had* to use `functools.partial`, which isn't true. – Bryan Oakley Feb 20 '21 at 18:41

Why for sentence in add_command does not work properly?

2 Answers2