我如何使用Python的itertools.groupby（）？ [英] How do I use Python's itertools.groupby()?

问题描述

我无法找到关于如何实际使用Python的 itertools.groupby（）函数的可理解的解释。我要做的是这样：

I haven't been able to find an understandable explanation of how to actually use Python's itertools.groupby() function. What I'm trying to do is this:

• 列出一个清单 - 在这种情况下，一个物化的孩子 lxml 元素


• 根据某些条件将其划分为组


• 然后分别迭代这些组中的每一个。

• Take a list - in this case, the children of an objectified lxml element
• Divide it into groups based on some criteria
• Then later iterate over each of these groups separately.

我查看了文档，以及示例，但我在尝试将它们应用到一个简单的数字列表之外时遇到了麻烦。

I've reviewed the documentation, and the examples, but I've had trouble trying to apply them beyond a simple list of numbers.

那么，我如何使用 itertools.groupby（）？我应该使用另一种技术吗？指向良好先决条件阅读的指针也将受到赞赏。

So, how do I use of itertools.groupby()? Is there another technique I should be using? Pointers to good "prerequisite" reading would also be appreciated.

推荐答案

重要提示：你必须首先对您的数据进行排序。

我没有得到的部分是示例中的部分建设

The part I didn't get is that in the example construction

groups = []
uniquekeys = []
for k, g in groupby(data, keyfunc):
   groups.append(list(g))    # Store group iterator as a list
   uniquekeys.append(k)

k 是当前的分组键， g 是一个迭代器您可以用来迭代该分组键定义的组。换句话说， groupby 迭代器本身返回迭代器。

k is the current grouping key, and g is an iterator that you can use to iterate over the group defined by that grouping key. In other words, the groupby iterator itself returns iterators.

这是一个例子，使用更清晰的变量名：

Here's an example of that, using clearer variable names:

from itertools import groupby

things = [("animal", "bear"), ("animal", "duck"), ("plant", "cactus"), ("vehicle", "speed boat"), ("vehicle", "school bus")]

for key, group in groupby(things, lambda x: x[0]):
    for thing in group:
        print "A %s is a %s." % (thing[1], key)
    print " "

这将给你输出：

熊是动物。

鸭子是动物。

A bear is a animal.
A duck is a animal.

仙人掌是一种植物。

快艇是一种车辆。

校车是一种车辆。

A speed boat is a vehicle.
A school bus is a vehicle.

在这个例子中，的东西是第一个项目的元组列表在每个元组中是第二个项目所属的组。

In this example, things is a list of tuples where the first item in each tuple is the group the second item belongs to.

groupby（）函数有两个参数：（1）要分组的数据和（2）函数把它分组。

The groupby() function takes two arguments: (1) the data to group and (2) the function to group it with.

这里， lambda x：x [0] 告诉 groupby（）使用每个元组中的第一项作为分组键。

Here, lambda x: x[0] tells groupby() to use the first item in each tuple as the grouping key.

在中语句， groupby 返回三个（键，组迭代器）对 - 每个唯一键一次。您可以使用返回的迭代器来迭代该组中的每个单独项目。

In the above for statement, groupby returns three (key, group iterator) pairs - once for each unique key. You can use the returned iterator to iterate over each individual item in that group.

这是使用列表解析的相同数据的略有不同的示例：

Here's a slightly different example with the same data, using a list comprehension:

for key, group in groupby(things, lambda x: x[0]):
    listOfThings = " and ".join([thing[1] for thing in group])
    print key + "s:  " + listOfThings + "."

这将为您提供输出：

动物：熊和鸭。

植物：仙人掌。

车辆：快艇和校车。

animals: bear and duck.
plants: cactus.
vehicles: speed boat and school bus.

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