如何为循环中创建的多个按钮使用与命令相同的功能？

Question

I am creating a tic-tac-toe board of buttons. Each button, when clicked, should switch to an X or an O on an alternating basis. I have created the buttons inside for loops, and would, if possible, like to avoid initializing them each separately. I have tried passing the button to the function by using lambda, and I have tried eliminating the problem of parameters in commands by calling the buttons self.bttn (as this is all taking place within my modified Frame class). However, in both cases, I come across the problem that, regardless of which of the 9 buttons I click on, the final button created by the for loops responds.

def __build_board(self):
    """ Creates tic-tac-toe board of 9 blank buttons. """
    for row_num in range(3):
        for col_num in range(3):
            self.bttn = Button(self, text = "    ", command = self.__change_button)
            self.bttn.grid(row = row_num + 1, column = col_num, sticky = W)

That's the way that I tried it with self.bttn. self.__change_button merely switches the text within self.bttn to an X or O. And this is the way I tried it with lambda...

def __build_board(self):
    """ Creates tic-tac-toe board of 9 blank buttons. """
    for row_num in range(3):
        for col_num in range(3):
            bttn = Button(self, text = "    ", command = lambda: self.__change_button(bttn))
            bttn.grid(row = row_num + 1, column = col_num, sticky = W)

I think I understand why these ways of going about it do not work. Seemingly, I have commanded all the buttons to change the text of the final button, but I'm at a loss as to how I should get them to each change their own text the way I want. Any help would be greatly appreciated.

Answer 1

This is a tricky problem...

the reason is that the lambda will be tied to the bttn variable , not to its value and therefore all the buttons will act on the last button (because that will be the value of the variable after the loop).

To see this consider:

x = 1
y = lambda : x
print(y()) # will print 1
x = 2
print(y()) # will print 2; lambda captures variables, not values

A possible work-around is:

def __build_board(self):
    """ Creates tic-tac-toe board of 9 blank buttons. """
    for row_num in range(3):
        for col_num in range(3):
            def mkbutton():
                b = Button(self, text = "    ",
                           command = lambda: self.__change_button(b))
                return b
            bttn = mkbutton()
            bttn.grid(row = row_num + 1, column = col_num, sticky = W)

this way each lambda will be tied to its own separate variable b .

Normally this kind of issue can be solved more easily with the frequent pattern:

x = lambda y=y: ...

where the value y is copied as default for an optional parameter of the lambda.

In your case however this approach is not possible because at the time the lambda is evaluated the Button instance doesn't exist yet (so you cannot store it as a default parameter value in the lambda itself).