如何在 Python 3 中使用 cmp()？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/22490366/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How to use cmp() in Python 3?
提问by BenFire
I cannot get the command cmp()to work.
我无法使命令cmp()起作用。
Here is the code:
这是代码：
a = [1,2,3]
b = [1,2,3]
c = cmp(a,b)
print (c)
I am getting the error:
我收到错误：
Traceback (most recent call last):
File "G:\Dropbox\Code\a = [1,2,3]", line 3, in <module>
c = cmp(a,b)
NameError: name 'cmp' is not defined
[Finished in 0.1s]
采纳答案by DSM
As mentioned in the comments, cmpdoesn't exist in Python 3. If you really want it, you could define it yourself:
正如评论中提到的，cmp在 Python 3 中不存在。如果你真的想要它，你可以自己定义它：
def cmp(a, b):
return (a > b) - (a < b)
which is taken from the original What's New In Python 3.0. It's pretty rare -- though not unheard of -- that it's really needed, though, so you might want to think about whether it's actually the best way to do whatever it is you're up to.
它取自原始的 Python 3.0 中的新增功能。不过，这种情况非常罕见——尽管并非闻所未闻——但确实需要它，因此您可能需要考虑这是否真的是做任何事情的最佳方式。
回答by maxin
In Python 3.x you can import operatorand use operator module's eq(), lt(), etc... instead of cmp()
在Python 3.x，然后才能import operator和使用操作模块eq()，lt()等...而不是cmp()
回答by Peter Chen
If a or b is a class object, then the above answers will have the compilation error as below: For example: a is Class Clock: File "01_ClockClass_lab16.py", line 14, in cmp return (a > b) - (a < b) TypeError: '>' not supported between instances of 'Clock' and 'Clock'
如果 a 或 b 是类对象，则上述答案将出现如下编译错误： 例如：a 是类时钟：文件“01_ClockClass_lab16.py”，第 14 行，cmp return (a > b) - (a < b) 类型错误：“时钟”和“时钟”的实例之间不支持“>”
Change the type with int() to remove the error:
使用 int() 更改类型以消除错误：
def cmp(a, b): return (int(a) > int(b)) - (int(a) < int(b))
def cmp(a, b): return (int(a) > int(b)) - (int(a) < int(b))
回答by Leo
When the sign is needed, probably safest alternative is using math.copysign:
当需要符号时，可能最安全的替代方法是使用 math.copysign：
import math
ang = -2
# alternative for cmp(ang, 0):
math.copysign(1, ang)
# Result: -1
In particular if ang is of np.float64 type because of depreciation of the '-' operator. Example:
特别是如果 ang 是 np.float64 类型，因为 '-' 运算符的折旧。例子：
import numpy as np
def cmp_0(a, b):
return (a > b) - (a < b)
ang = np.float64(-2)
cmp_0(ang, 0)
# Result:
# DeprecationWarning: numpy boolean subtract, the `-` operator, is deprecated,
# use the bitwise_xor, the `^` operator, or the logical_xor function instead.
instead one could use:
相反，可以使用：
def cmp_0(a, b):
return bool(a > b) - bool(a < b)
ang = np.float64(-2)
cmp(ang, 0)
# Result: -1
回答by Chidi
adding to @maxin's answer, in python 3.x, if you want to compare two lists of tuples
aand b
添加到@maxin 的答案中python 3.x，如果您想比较两个元组列表
a和b
import operator
a = [(1,2),(3,4)]
b = [(3,4),(1,2)]
# convert both lists to sets before calling the eq function
print(operator.eq(set(a),set(b))) #True
