python - 为什么容器的最大尺寸有符号位？

Question

我在 Python 3.6 中检查了 sys.maxsize 的帮助:

>>> help(sys)
[...]
maxsize -- the largest supported length of containers.

测试它:

In [10]: '{:,}'.format(sys.maxsize)
Out[10]: '9,223,372,036,854,775,807'

In [11]: math.log2(sys.maxsize)
Out[11]: 63.0

它是 63 位，表示前导符号位。但是，容器的长度不能为负数。

这是怎么回事？

Answer 1

容器的最大大小在 Python 2.5 中从 2³¹-1 增加到 2⁶³-1。 PEP 353: Using ssize_t as the index type ，引入了变化，says :

Why not size_t

An initial attempt to implement this feature tried to use size_t. It quickly turned out that this cannot work: Python uses negative indices in many places (to indicate counting from the end). Even in places where size_t would be usable, too many reformulations of code where necessary, e.g. in loops like:

for(index = length-1; index >= 0; index--)

This loop will never terminate if index is changed from int to size_t.

因此，限制源于使用特定于 Python 的“索引”类型的决定，为了简化负索引的处理，将其定义为有符号 (ssize_t) 而不是无符号 (size_t) 很方便。