问题描述

在 Java 中，我喜欢使用添加到集合"操作返回的布尔值来测试该元素是否已经存在于集合中:

In Java I like to use the Boolean value returned by an "add to the set" operation to test whether the element was already present in the set:

if (set.add("x")) {
   print "x was not yet in the set";
}

我的问题是，在 Python 中有没有这样方便的东西?我试过了

My question is, is there something as convenient in Python? I tried

 z = set()
 if (z.add(y)):
     print something

但它不打印任何东西.我错过了什么吗?谢谢！

But it does not print anything. Am I missing something? Thx!

推荐答案

在 Python 中，set.add() 方法不返回任何内容.您必须使用 not in 运算符:

In Python, the set.add() method does not return anything. You have to use the not in operator:

z = set()
if y not in z: # If the object is not in the list yet...
    print something
z.add(y)

如果您真的需要在添加之前知道对象是否在集合中，只需存储布尔值即可:

If you really need to know whether the object was in the set before you added it, just store the boolean value:

z = set()
was_here = y not in z
z.add(y)
if was_here: # If the object was not in the list yet...
    print something

但是，我认为您不太可能需要它.

However, I think it is unlikely you need it.

这是一个 Python 约定:当一个方法更新某个对象时，它返回 None.你可以忽略这个约定；此外，还有一些在野外"违反它的方法.然而，这是一个普遍的、公认的惯例:我建议您坚持并牢记这一点.

This is a Python convention: when a method updates some object, it returns None. You can ignore this convention; also, there are methods "in the wild" that violate it. However, it is a common, recognized convention: I'd recommend to stick to it and have it in mind.