Python Tkinter - 名称未定义

Question

代码:

def createLetters(frame, startX, startY, width, height, spacing):

    alphabet = ["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"]

    def letterAction(letter):
        letter.destroy()

    for i in range(0, 26):

        if (i >= 9 and i <= 17):
            y = startY +  height + 2 * spacing
            x = startX + ((width + spacing) * (i - 9))

        elif (i >= 17):
            y = startY + 2 * height + 3 * spacing
            x = (width + spacing) / 2 + startX + ((width + spacing) * (i - 18))

        elif (i <= 8):
            y = startY + spacing
            x = startX + ((width + spacing) * i)

        exec(alphabet[i] + " = Button(" + frame + ", text = '" + alphabet[i] + "', command = letterAction(" + alphabet[i] + "))")
        exec(alphabet[i] + ".place(x = " + str(x) + ", y = " + str(y) + ", width = " + str(width) + ", height = " + str(height) + ")")

错误:

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Python33\lib\tkinter\__init__.py", line 1442, in __call__
    return self.func(*args)
  File "E:\Hangman\hangmanTk.py", line 106, in playScreen
    createLetters("playFrame", 175, 250, 50, 50, 0)
  File "E:\Hangman\hangmanTk.py", line 95, in createLetters
    exec(alphabet[i] + " = Button(" + frame + ", text = '" + alphabet[i] + "', command = letterAction(" + alphabet[i] + "))")
  File "<string>", line 1, in <module>
NameError: name 'A' is not defined

我正在尝试创建多个带有循环的 tkinter 按钮。我可以很好地创建按钮，但我似乎无法为它们创建回调。当我尝试时，它告诉我用于按钮的变量未定义。我尝试在定义按钮的位置添加“exec("global "+ Alphabet[i])”，但这并没有改变任何内容。

Answer 1

Using exec is almost always the wrong way to do it, no matter what "it" is.

And creating variables dynamically is almost always the wrong thing to do.

您在使其正常工作时遇到的问题完美地说明了原因。

<小时/>

只需创建一个 dict将名称映射到按钮:

buttons = {}

# ...

letter = alphabet[i]
buttons[letter] = Button(frame, text = letter, command = letterAction(letter))
buttons[letter].place(x = x, y = y, width = width, height = height)

如果你真的想转储dict进入locals() (或者，类似地，self.__dict__或globals()或...)，这很简单。但你没有。您唯一需要使用该变量的地方是在 letterAction 中。功能。所以:

def createLetters(frame, startX, startY, width, height, spacing):

    alphabet = string.ascii_uppercase
    buttons = {}

    def letterAction(letter):
        buttons[letter].destroy()

    for i, letter in enumerate(alphabet):

        if (i >= 9 and i <= 17):
            y = startY +  height + 2 * spacing
            x = startX + ((width + spacing) * (i - 9))

        elif (i >= 17):
            y = startY + 2 * height + 3 * spacing
            x = (width + spacing) / 2 + startX + ((width + spacing) * (i - 18))

        elif (i <= 8):
            y = startY + spacing
            x = startX + ((width + spacing) * i)

        buttons[letter] = Button(frame, text = letter, command = letterAction(letter))
        buttons[letter].place(x = x, y = y, width = width, height = height)

<小时/>

但请注意，这样做是错误的。 command = letterAction(letter) —无论您直接运行它，还是通过 exec 运行它—要打电话letterAction(letter)现在，在创建按钮之前就将其销毁，并返回 None ，然后将其设置为 command 。

您需要lambda: letterAction(letter)或partial(letterAction, letter)来解决这个问题。

另外，您无法编写代码将按钮变量本身传递给 letter ，无论是现在还是以后，因为该变量尚不存在。您必须将字母作为字符串传递，就像我上面所做的那样。

<小时/>

但实际上，如果您仔细想想，您根本不需要这些按钮变量 — 无论是在 dict 中或其他方式。您只需要一种方法将每个按钮绑定(bind)为其自己的回调目标，对吧？有多种方法可以做到这一点，但最明显的方法是类，继承或委托(delegate)给 Button (或者，在本例中，两者都不需要，因为您不需要将其用作按钮，甚至在创建后不需要记住它)。

当我们这样做时，让我们删除一些无关的括号，这样只会让事情更难阅读并解决 17 的问题。似乎属于两个不同的群体......

class SelfDestructiveButton(object):
    def __init__(self, frame, letter, x, y, width, height):
        self.button = Button(frame, text=letter, command=self.command)
        self.button.place(x=x, y=y, width=width, height=height)
    def command(self):
        self.button.destroy()

def createLetters(frame, startX, startY, width, height, spacing):
    for i, letter in enumerate(string.ascii_uppercase):
        if 9 <= i <= 17:
            y = startY +  height + 2 * spacing
            x = startX + ((width + spacing) * (i - 9))
        elif i > 17:
            y = startY + 2 * height + 3 * spacing
            x = (width + spacing) / 2 + startX + ((width + spacing) * (i - 18))
        else:
            y = startY + spacing
            x = startX + ((width + spacing) * i)
        SelfDestructiveButton(frame, letter, x, y, width, height)

使用 if 'J' <= letter <= 'R' 可能会更清楚，因为调试时您将看到的是字母而不是数字。