Python:list包含不同元素类型时的list.sort()查询 [英] Python: list.sort() query when list contains different element types
问题描述
问候Pythonic世界.学习Python 3.3的第4天，我遇到了list.sort的一个奇怪属性.
Greetings Pythonic world. Day 4 of learning Python 3.3 and I've come across a strange property of list.sort.
我创建了一个包含五个元素的列表:四个字符串，中间带有一个数字.由于混合类型，尝试使list.sort起作用会产生预期的错误:
I created a list of five elements: four strings, with a number in the middle. Trying to get list.sort to work gave the expected error because of mixing types:
>>> list = ['b', 'a', 3, 'd', 'c']
>>> list.sort()
Traceback (innermost last):
File "<stdin>", line 1, in <module>
TypeError: unorderable types: int() < str()
>>> list
['b', 'a', 3, 'd', 'c']
列表未更改.
但是随后我将数字移到末尾，再次使用list.sort并得到了:
But then I moved the number to the end, used list.sort again, and got this:
>>> list = ['b', 'a', 'd', 'c', 3]
>>> list.sort()
Traceback (innermost last):
File "<stdin>", line 1, in <module>
TypeError: unorderable types: int() < str()
>>> list
['a', 'b', 'c', 'd', 3]
OK，一个错误.但是列表本身已经排序完毕，将数字放在了最后.我在本网站或Langtangen中找不到任何解释.这种行为是否有一些根本原因?在某些情况下会有用吗?
OK, an error. But the list has sorted itself, kicking the number to the end. I couldn't find any explanation for this on this site or in Langtangen. Is there some underlying reason for this behaviour? Would it be useful in some situation?
推荐答案
从Python 3开始文档:
From the Python 3 docs:
此方法仅使用<在适当的位置对列表进行排序.之间的比较 项目.异常不会被抑制-如果有任何比较操作 失败，整个排序操作将失败(并且列表很可能是 处于部分修改状态).
This method sorts the list in place, using only < comparisons between items. Exceptions are not suppressed - if any comparison operations fail, the entire sort operation will fail (and the list will likely be left in a partially modified state).
文档不特别保证任何行为，但是元素很可能会被部分排序.它们在发生异常时所处的顺序完全不同，并且该顺序在不同的实现之间或在程序的两个后续运行中可能(但不太可能)有所不同.
The docs don't guarantee any behaviour in particular, but the elements will more than likely be left part-way sorted. Whetever order they were in when the exception occurred, and this order can vary between implementations, or possibly (but unlikely) two subsequent runs of the program.
如果您想尝试对项目进行排序而不必担心不幸的重新排序，则可以使用sorted内置函数，该函数将返回新列表，而不是修改原始列表.
If you want to try to sort the items without worrying about an unfortunate re-ordering, you can use the sorted builtin function, which will return a new list rather than modify the original.
>>> seq = ['b', 'a', 3, 'd', 'c']
>>> try:
... seq = sorted(seq) # if sorted fails, result won't be assigned
... except Exception: # you may only want TypeError
... pass
...
>>> seq
['b', 'a', 3, 'd', 'c'] # list unmodified
解决所有人都说
to address everyone saying something like
一旦看到两种不同的类型，就会引发异常
我知道您可能知道这种说法过于简单，但是我认为如果不清楚，将会引起混乱.
I know you are probably aware that this kind of statement is an oversimplification, but I think without being clear, it's going to cause confusion.
下面的示例由两个类A和B组成，它们通过各自的__lt__方法相互支持比较.它显示了将这两种类型混合在一起并以list.sort()排序的列表，然后按排序顺序打印，没有引发异常:
The following example consists of two classes A and B which support comparison with each other through their respective __lt__ methods. It shows a list mixed of these two types sorted with list.sort() and then printed in sorted order with no exceptions raised:
class A:
def __init__(self, value):
self.a = value
def __lt__(self, other):
if isinstance(other, B):
return self.a < other.b
else:
return self.a < other.a
def __repr__(self):
return repr(self.a)
class B:
def __init__(self, value):
self.b = value
def __lt__(self, other):
if isinstance(other, A):
return self.b < other.a
else:
return self.b < other.b
def __repr__(self):
return repr(self.b)
seq = [A(10), B(2), A(8), B(16), B(9)]
seq.sort()
print(seq)
此输出为:
[2, 8, 9, 10, 16]
了解这一点的每个细节并不重要.只是为了说明如果所有部分都存在，混合类型列表可以与list.sort()一起使用
it's not vital that you understand every detail of this. It's just to illustrate that a list of mixed types can work with list.sort() if all the pieces are there
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