Python规范中定义的小数缓存还是实现细节？

Question

Python 似乎有一个所谓的“小数字缓存”，用于存储 -5 到 256 范围内的数字。我们可以用以下程序证明这一点：

for i in range(-7, 258):
    if id(i) == id(i + 0):
        print(i, "is cached")
    else:
        print(i, "is not cached")

输出为

-7 is not cached
-6 is not cached
-5 is cached
-4 is cached
[...]
255 is cached
256 is cached
257 is not cached

我想知道这是在 Python 规范中定义的还是一个实现细节。

Answer 1

这是一个实现细节：chapter 3.1 of the Python language reference（所有版本，从 2.7 到 3.9）说：

for immutable types, operations that compute new values may actually return a reference to any existing object with the same type and value

和

E.g., after a = 1; b = 1, a and b may or may not refer to the same object with the value one, depending on the implementation

（强调我的）

Python/C API reference也提到了

The current implementation keeps an array of integer objects for all integers between -5 and 256,

（强调我的）这也很清楚，这可能会在未来发生变化。