lambda working oddly with tkinter

问题:

I am trying to make a calculator using Tkinter. I have been trying to clean up the code a bit but have run into a snag. When creating buttons, I use the following code:

Button(self,text="1",command=lambda: self.addkey("1"),width=self.defaultwidth,height=self.defaultheight).grid(row=5, column=0)
Button(self,text="2",command=lambda: self.addkey("2"),width=self.defaultwidth,height=self.defaultheight).grid(row=5, column=1)
Button(self,text="3",command=lambda: self.addkey("3"),width=self.defaultwidth,height=self.defaultheight).grid(row=5, column=2)

With the following being the command called

def addkey(self,key):
            # Adds a given key to the display
            if len(self.displaytext) + len(key) <= self.maxlength:
                self.displaytext += key
                self.display["text"] = self.displaytext

When buttons 1, 2, and 3 are pressed in that order, the following is the output:

123

I have been trying to clean up the code so that it looks more like:

for i in range(3):
    Button(self,text=str(i+1),command=lambda: self.addkey(str(i+1)),width=self.defaultwidth,height=self.defaultheight).grid(row=5, column=i)

This adds the buttons fine, but when 1, 2, and 3 are pressed in that order, the following shows up on the screen:

333

I was wondering if I missed something or this is simply not possible.

回答1:

Ah, scoping. When you do this:

command=lambda: self.addkey(str(i))

You're not "resolving" the i to a number right there and then. You're just telling the lambda to reference i when it's invoked, afterwards.

At any time past the end of the for loop, i = 3 (last value), so all of your lambdas get 3 when they ask for i.

If I'm not mistaken, you can add a function as means of indirection, and it will appropriately "capture" i from the surrounding scope.

def add_key_f(i):
    return lambda self: self.addkey(i)