使用算术运算符比较无内置类型? [英] Comparing None with built-in types using arithmetic operators?
问题描述
Python 2.7.2 (default, Jun 12 2011, 14:24:46) [MSC v.1500 64 bit (AMD64)] on win32
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>>> None > 0
False
>>> None == 0
False
>>> None < 0
True
- 比较
无
使用为内置类型(在这种情况下为整数)定义的算术运算符? - 语言规范的前两个和第三个比较部分之间的区别(Python的规范 - 你一定是开玩笑:)或者是CPython的实现细节?
- Is comparing
None
using arithmetic operators well defined for built-in types (integers in this case)? - Is the difference between the first two and the third comparison part of language specification (Python's specification - you must be kidding :)) or is it CPython's implementation detail?
推荐答案
None
是 if obj is None:
(或如果obj不是None:
)。
不同类型之间的比较已经从Python 3中删除了好的原因 - 他们是一个常见的错误源,导致混乱。例如
Comparison between different types has been removed from Python 3 for good reasons - they were a common source of errors and lead to confusion. For example
>>> "3" < 4
False
在Python 3中,你会得到一个 TypeError
比较 str
与 int
或任何与无
。
>>> None < 0
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unorderable types: NoneType() < int()
(我的意思是比较,试图确定两个值如果两个对象是不同类型的,它们总是返回 False
。)
(I mean "comparing" in the sense of trying to determine which of two values is larger/smaller. Comparison for equality is OK - it will always return False
if two object are of different types.)
我在文档中找不到此引用,但在学习Python,第4版中,Mark Lutz在第204页写道:
I haven't found a reference in the docs for this, but in Learning Python, 4th edition, Mark Lutz writes on page 204:
[...]比较不同类型的对象(例如,字符串和
列表)工作 - 语言定义不同
类型之间的固定排序,这是确定性的,如果不是美学上令人愉快的。也就是说,
的排序是基于所涉及的类型的名称:所有整数
小于所有字符串,例如,因为int
小于
str
。
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