python - Python 可变对象中的引用与赋值

Question

作业:

>>> a = ['spam']
>>> b = ['spam']
>>> a is b
False

引用:

>>> c = ['spam']
>>> d = c
>>> c is d
True

1. 以上两者有什么区别？
2. 为什么赋值结果False？
3. 为什么引用结果 True？

Answer 1

您的第一个代码段创建了两个 不同的唯一列表对象。因此 a is b 返回 false，因为 a 和 b 指向不同的对象:

          +------+
a ------> | list |
          +------+

          +------+
b ------> | list |
          +------+

Your second snippet creates a single list object, and points both c and d to that objects, hence c is d return true:

          +------+
c ------> | list | <------ d
          +------+

---

Note the following, from http://docs.python.org/3/reference/datamodel.html:

Every object has an identity, a type and a value. An object’s identity never changes once it has been created; you may think of it as the object’s address in memory. The is operator compares the identity of two objects; the id() function returns an integer representing its identity.

So is and == are very different; while the former compares object identity, the latter compares object values. Indeed, == tests in your snippets would return true.

---

Given the explanation above, it may come as a surprise that that the story is slightly different with strings:

>>> a = 'str'
>>> b = 'str'
>>> 
>>> a is b
True

这是由于 string interning ，它出现在 CPython 中(即它是特定于实现的)。因此，如果相同的字符串文字出现在两个不同的地方，则相同的字符串对象将用于两者(有限制)。

这在 "Python string interning" 中有更详细的解释。 .