比较运算符在字典上做了什么？

Question

我遇到了这个:

>>> d1 = {"john":40, "peter":45}
>>> d2 = {"john":466, "peter":45}
>>> d1 > d2
False

比较运算符在比较两个dicts时是做什么的False？它是如何输出的？

Answer 1

由于这些dicts长度相等,我们找到相应值不相等的最小键,即'john'.然后将该词与该键的值进行比较.

演示:

>>> d1 = {"john":40, "peter":45}
>>> d2 = {"john":466, "peter":45}
>>> d1 < d2
True
>>> d2['john'] = 39
>>> d1 < d2
False

自从20多年前Guido的承诺以来,这个基本思想基本没有改变:

$ git show a0a69b8
commit a0a69b8b429f3d4c91f1c432247cfda017505976
Author: Guido van Rossum <guido@python.org>
Date:   Thu Dec 5 21:55:55 1996 +0000

    Experimental new implementation of dictionary comparison.  This
    defines that a shorter dictionary is always smaller than a longer one.
    For dictionaries of the same size, the smallest differing element
    determines the outcome (which yields the same results as before,
    without explicit sorting).

它没有记录,并且在Python 3中删除了dict比较代码,所以对于任何重要的事情我都不会依赖它.相关的CPython源码在这里.

有趣的事实:显然,在旧版本的Python中,某些字典比较可能会使运行时崩溃,甚至可能导致Windows 98重启.嘿.