Python lambda函数下划线-冒号语法说明? [英] Python lambda function underscore-colon syntax explanation?

问题描述

在下面的Python脚本中，"aDict"是字典，在lambda函数中"_:_ [0]"有什么作用?

In the following Python script where "aDict" is a dictionary, what does "_: _[0]" do in the lambda function?

sorted(aDict.items(), key=lambda _: _[0])

推荐答案

让我们分开.

1)假设您有一个字典，di:

1) Suppose you have a dict, di:

di={'one': 1, 'two': 2, 'three': 3}

2)现在，假设您想要其每个键，值对:

2) Now suppose you want each of its key, value pairs:

 >>> di.items()
 [('three', 3), ('two', 2), ('one', 1)]

3)现在您要对它们进行排序(因为字典是无序的):

3) Now you want to sort them (since dicts are unordered):

>>> sorted(di.items())
[('one', 1), ('three', 3), ('two', 2)]

请注意，元组按字典顺序排序-由元组第一个元素中的文本排序.这等效于一系列元组的t[0].

Notice that the tuples are sorted lexicographically -- by the text in the first element of the tuple. This is a equivalent to the t[0] of a series of tuples.

假设您希望按数字对它进行排序.您将使用key函数:

Suppose you wanted it sorted by the number instead. You would you use a key function:

>>> sorted(di.items(), key=lambda t: t[1])
[('one', 1), ('two', 2), ('three', 3)]

您拥有sorted(aDict.items(), key=lambda _: _[0])的语句只是使用_作为变量名.它也不执行任何操作，因为aDict.items()会生成元组，并且如果您不使用键，则无论如何它都将按元组的第一个元素进行排序.您的示例中的关键功能完全没用.

The statement you have sorted(aDict.items(), key=lambda _: _[0]) is just using _ as a variable name. It also does nothing, since aDict.items() produces tuples and if you did not use a key it sorts by the first element of the tuple anyway. The key function in your example is completely useless.

可能需要考虑该表单的一个用例(元组除外).如果您使用的是字符串，那么您将按第一个字符进行排序，而忽略其余字符:

There might be a use case for the form (other than for tuples) to consider. If you had strings instead, then you would be sorting by the first character and ignoring the rest:

>>> li=['car','auto','aardvark', 'arizona']
>>> sorted(li, key=lambda c:c[0])
['auto', 'aardvark', 'arizona', 'car']

对比:

>>> sorted(li)
['aardvark', 'arizona', 'auto', 'car']

但是我仍然不会在lambda中使用_. _的使用是对具有最小机会产生副作用的throway变量的使用. Python的命名空间通常会使这种担心不再是真正的烦恼.

I still would not use _ in the lambda however. The use of _ is for a throway variable that has minimal chance of side-effects. Python has namespaces that mostly makes that worry not a real worry.

考虑:

>>> c=22
>>> sorted(li, key=lambda c:c[0])
['auto', 'aardvark', 'arizona', 'car']
>>> c
22

c的值得以保留，因为lambda内有本地名称空间.

The value of c is preserved because of the local namespace inside the lambda.

但是(在Python 2.x而非Python 3.x下)可能是一个问题:

However (under Python 2.x but not Python 3.x) this can be a problem:

>>> c=22
>>> [c for c in '123']
['1', '2', '3']
>>> c
'3'

因此，在列表理解或元组扩展等情况下，(轻型)约定开始对变量使用_，在这种情况下，您不必担心践踏一个名称.消息是:如果将其命名为_，那么我就不在乎它了.

So the (light) convention became using _ for a variable either in the case of a list comprehension or a tuple expansion, etc where you worry less about trampling on one of your names. The message is: If it is named _, I don't really care about it except right here...

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