How can I pass object-specific values to a function from objects created in a loop?

Question

I have searched deeply for a solution to this problem, but, being new to Python and tkinter, it is difficult to find solutions to problems in a language you are largely unfamiliar with. I hope this is not a repeat question.

Suppose I create a series of buttons in tkinter, each of which will sequentially display a letter of the alphabet, defined as such:

from string import ascii_uppercase as alphabet
from tkinter import Button
blockLetterButtons = []
for i in range(0,26):
    blockLetterButtons.append(Button(competitorSelectionFrame, 
    text=alphabet[i], command=some_function(alphabet[i])))

Somewhere below this in the for loop is a .grid(...) for all of these.

The function that is being referenced in the command attribute is as the following:

def some_function(letter):
    print(letter)

So I recognize two problems with my code:

1. The function in the command attribute is executing immediately upon running the code, because I am specifying a function call rather than a function name.
2. Even if that was the proper way to call the function with a parameter, using alphabet[i] wouldn't work, because it would pass whatever the value of i is upon click, and not whatever i was upon execution of the for loop.

So my questions are the following:

1. How can I specify a function in the command attribute and pass to that function with a parameter?
2. How can I specify that parameter to be the letter of the alphabet specific to that button, without defining a button explicitly for every letter?

I am fairly certain I could use a list comprehension to solve the second problem, perhaps a dictionary with the key equal to the letter and the value equal to the button, but then I still wouldn't know how to get the key value from the button on its own.

Answer 1

Currying is the practice of passing some arguments to a function to produce a new function which can be called later but "remembers" its earlier arguments. You can use this technique to pass the state.

def some_function(letter):
    def f():
        print(letter)
    return f

...

command = some_function(alphabet[i])

So some_function(alphabet[i])() would call the function, but only using one set of parentheses simply stores the value for later.

Answer 2

You should use a higher order function (a function that returns a function). To demonstrate, I will not use tkinter but the principle is the same:

In [25]: def some_func_maker(letter):
    ...:     def some_func():
    ...:         print(letter)
    ...:     return some_func
    ...:

In [26]: buttons = []

In [27]: from string import ascii_uppercase as alphabet

In [28]: for i in range(26):
    ...:     buttons.append(some_func_maker(alphabet[i]))
    ...:

In [29]: for b in buttons: b()
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z

Note, you can inline this if you don't want to keep a some_func_maker around using a lambda:

In [37]: buttons = []
    ...: for c in alphabet:
    ...:     buttons.append(
    ...:         (lambda x: lambda: some_function(x))(c)
    ...:     )
    ...:

In [38]: for b in buttons: b()
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z

A common trick is that is less verbose is to use a default-argument with your lambda, however, I consider this more hacky, although it is quite common in Python:

In [43]: buttons = []
    ...: for c in alphabet:
    ...:     buttons.append(
    ...:         lambda x=c: some_function(x)
    ...:     )
    ...:

In [44]: buttons[0]()
A

In [45]: buttons[1]()
B

In [46]: buttons[-1]()
Z

Note also, it is consider an antipattern to use for i in range(n): some_iterable[i] when you can simply iterate over the iterable instead. Usually, you just want the actual value, so you just do for x in some_iterable.