Python 使用比较器功能进行排序
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/12749398/
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
Using a comparator function to sort
提问by user1427661
So I'm working with a few pre-existing comparators that compare certain values in two tuples and return true if the first is greater than the second, false if otherwise. Here's the code for one of them:
所以我正在使用一些预先存在的比较器，它们比较两个元组中的某些值，如果第一个大于第二个则返回 true，否则返回 false。这是其中之一的代码：
def cmpValue(subInfo1, subInfo2):
"""
Returns True if value in (value, work) tuple subInfo1 is GREATER than
value in (value, work) tuple in subInfo2
"""
# TODO...
if subInfo1[0] > subInfo2[0]:
return True
else:
return False
Now, I have a dictionary that has numerous tuple entries of the type being compared above. I want to sort them all in reverse order, but I don't really understand how I would accomplish that. I was thinking something like:
现在，我有一本字典，其中包含许多上面比较的类型的元组条目。我想以相反的顺序对它们进行排序，但我真的不明白我将如何做到这一点。我在想这样的事情：
sortedDict = sorted(subjects, key=comparator, reverse = True)
But I don't know what to pass into the comparator because each comparator takes two arguments (subInfo1, subInfo2). I cannot change the comparator functions.
但是我不知道要传递给比较器的内容是什么，因为每个比较器都有两个参数（subInfo1、subInfo2）。我无法更改比较器功能。
采纳答案by Fred Foo
You're passing the comparator as the keyfunction. You should be passing it as the cmp, wrapped in some kind of function that turns it into a proper comparator.
您将比较器作为key函数传递。您应该将它作为cmp, 包装在某种函数中，将其转换为适当的比较器。
def make_comparator(less_than):
def compare(x, y):
if less_than(x, y):
return -1
elif less_than(y, x):
return 1
else:
return 0
return compare
sortedDict = sorted(subjects, cmp=make_comparator(cmpValue), reverse=True)
(Although actually, you should be using key functions:
（尽管实际上，您应该使用关键功能：
sorted(subjects, operator.itemgetter(0), reverse=True)
Also note that sortedDictwill not actually be a dict, so the name is rather confusing.)
另请注意，sortedDict实际上不会是 a dict，因此名称相当混乱。）
回答by kaya3
In Python 3 there is no cmpargument for the sortedfunction (nor for list.sort).
在 Python 3cmp中，sorted函数没有参数（也没有 for list.sort）。
According to the docs, the signature is now sorted(iterable, *, key=None, reverse=False), so you have to use a keyfunction to do a custom sort. The docs suggest:
根据 docs，签名现在是sorted(iterable, *, key=None, reverse=False)，所以你必须使用一个key函数来进行自定义排序。文档建议：
Use
functools.cmp_to_key()to convert an old-style cmpfunction to a keyfunction.
用
functools.cmp_to_key()一个老式的转换CMP功能的关键功能。
Here's an example:
下面是一个例子：
>>> def compare(x, y):
... return x[0] - y[0]
...
>>> data = [(4, None), (3, None), (2, None), (1, None)]
>>> from functools import cmp_to_key
>>> sorted(data, key=cmp_to_key(compare))
[(1, None), (2, None), (3, None), (4, None)]
However, your function doesn't conform to the old cmpfunction protocol either, since it returns Trueor False. To convert cmpValueinto the appropriate form, you can do:
但是，您的函数也不符合旧的cmp函数协议，因为它返回True或False。要转换cmpValue为适当的形式，您可以执行以下操作：
def predicate_to_cmp(predicate):
def cmp(x, y):
if predicate(x, y):
return 1
elif x == y:
return 0
else:
return -1
return cmp
Then you can do:
然后你可以这样做：
>>> your_key = cmp_to_key(predicate_to_cmp(cmpValue))
>>> sorted(data, key=your_key)
[(1, None), (2, None), (3, None), (4, None)]
