Python for 循环是否按引用工作? [英] Do Python for loops work by reference?

问题描述

当在 Python 中使用 for 循环迭代列表中的项目时，改变 item(下面)会改变 items?

When using a for loop in Python to iterate over items in a list, will changing item (below) change the corresponding item in items?

for item in items:
    item += 1

items 中的每个 item 会递增还是保持与循环之前相同?

Will each item in items be incremented or remain the same as before the loop?

[注意:我会对 Python 2.7 和 3.x 感兴趣]

[Note: I would be interested in Python 2.7 and 3.x]

推荐答案

不，Python 中的变量不是指针.

No, variables in Python are not pointers.

它们改为引用堆上的对象，并且分配给变量不会更改引用的对象，而是变量.变量和对象就像绑在气球上的标签；分配将标签重新绑定到不同的气球.

They refer to objects on a heap instead, and assigning to a variable doesn't change the referenced object, but the variable. Variables and objects are like labels tied to balloons; assignment reties the label to a different balloon instead.

见这个我以前的回答 进一步探索气球和标签的想法.

See this previous answer of mine to explore that idea of balloons and labels a bit more.

也就是说，一些对象类型实现了特定的就地添加行为.如果对象是可变的(气球本身可以改变)，那么就地添加可以被解释为突变而不是赋值.

That said, some object types implement specific in-place addition behaviour. If the object is mutable (the balloon itself can change), then an in-place add could be interpreted as a mutation instead of an assignment.

因此，对于整数，item += 1 与 item = item + 1 实际上相同，因为整数是不可变的.您必须创建一个新的整数对象并将item标签绑定到该新对象上.

So, for integers, item += 1 is really the same as item = item + 1 because integers are immutable. You have to create a new integer object and tie the item label to that new object.

另一方面，列表是可变的，lst += [other, items] 被实现为一个 lst.__iadd__([other, items]) 并且改变了 lst 气球本身.赋值仍然发生，但它是同一个对象的重新赋值，因为 .__iadd__() 方法只返回 self 而不是一个新对象.我们最终将标签重新绑在同一个气球上.

Lists on the other hand, are mutable and lst += [other, items] is implemented as a lst.__iadd__([other, items]) and that changes the lst balloon itself. An assignment still takes place, but it is a reassigment of the same object, as the .__iadd__() method simply returns self instead of a new object. We end up re-tying the label to the same balloon.

循环只是在每次迭代时为您提供对列表中下一项的引用.它不允许您更改原始列表本身(这只是另一组气球标签)；相反，它为您包含的每个项目提供一个新标签.

The loop simply gives you a reference to the next item in the list on each iteration. It does not let you change the original list itself (that's just another set of balloon labels); instead it gives you a new label to each of the items contained.

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