The convert function takes the variable containing the input string, as well as a reference to the Label widget as parameters, so that it can change the value shown by the Label.
convert以包含有输入字符串的变量以及对Label窗口小部件的引用作为参数，以改变Label所显示的值。
If you want to change an immutable object, you don't - you create a new object with the changed value and point your reference to it.
如果您想要改变一个不可变对象的话，您不会改变它，而是使用修改后的值来创建一个新的对象，并把您的引用指向它。
However, you must be aware that these alternatives change the semantics of the interaction from pass-by-value to pass-by-reference.
然而，您必须注意到这些可供选择的方案会将交互方法从按值传递(pass - by - value)改变为按引用传递(pass - by - reference)。
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