如何在Python中进行groupby列表理解(从C#/LINQ的角度)

Question

我正在尝试切换到只使用python。我在C#中使用得非常广泛的是LINQ。在这个练习中，目标是获得键值对的集合，键是每个月，值是该月的消息数量，我如何使用python做这样的事情，或者可能是更好的方法呢？

class MainClass
{
    public static void Main (string[] args)
    {
        string[] months = { "jan", "feb", "mar", "apr", "may", "jun", "jul", "aug", "sep", "oct", "nov", "dec" };

        var log = LineReader ();
        Dictionary<string, int> cumulativeMonths = new Dictionary<string, int> ();

        months.ToList ()
            .ForEach (f => {
                cumulativeMonths.Add(f, log.GroupBy(g => g.Split(' ').First().ToLower())
                    .Where(w => w.Key == f).ToList().Count());

            });                                         
    }
    public static IEnumerable<string> LineReader()
    {
        Console.WriteLine ("Hello World!");
        using (StreamReader sr = new StreamReader (File.OpenRead ("/var/log/messages"))) {

            while (!sr.EndOfStream) {

                yield return sr.ReadLine ();
            }
        }
    }
}

测试输入：

Feb 18 02:51:36 laptop rsyslogd: [origin software="rsyslogd" swVersion="8.4.2" x-pid="2952" x-info="http://www.rsyslog.com"] start
Feb 18 02:51:36 laptop kernel: Adaptec aacraid driver 1.2-0[30300]-ms
Feb 18 02:51:36 laptop kernel: megaraid cmm: 2.20.2.7 (Release Date: Sun Jul 16 00:01:03 EST 2006)
Feb 18 02:51:36 laptop kernel: megaraid: 2.20.5.1 (Release Date: Thu Nov 16 15:32:35 EST 2006)
Feb 18 02:51:36 laptop kernel: megasas: 06.805.06.00-rc1 Thu. Sep. 4 17:00:00 PDT 2014
Feb 18 02:51:36 laptop kernel: qla2xxx [0000:00:00.0]-0005: : QLogic Fibre Channel HBA Driver: 8.07.00.16-k.
Feb 18 02:51:36 laptop kernel: Emulex LightPulse Fibre Channel SCSI driver 10.4.8000.0.
Feb 18 02:51:36 laptop kernel: Copyright(c) 2004-2014 Emulex.  All rights reserved.
Feb 18 02:51:36 laptop kernel: aic94xx: Adaptec aic94xx SAS/SATA driver version 1.0.3 loaded
Feb 18 02:51:36 laptop kernel: ACPI: bus type USB registered

测试输出将是字典：{Jan: 64562，Feb: 38762} ....

Answer 1

这比你以前做的要简单，而且在Python中也很容易：

with open('/var/log/messages', 'r') as f:
    cumulative_months = {}
    for line in f:
        key = line.split()[0].lower()
        cumulative_months[key] = cumulative_months.get(key, 0) + 1

with类似于C#的using，当文件超出作用域时，它会关闭文件。python文件对象可以用作迭代器。它会一次读取并返回一行，直到它到达EOF。(它实际上读取的行略多于一行，参见documentation)。

或者，正如m.wasowski所指出的，您可以将collections.Counter类用于这种类型的任务，以使事情变得更容易、更快。

Answer 2

您可以使用collections.Counter字典：

from collections import Counter
with open('yourfile') as f:
    count = Counter (line.split()[0] for line in f)

对于任何错误，我深表歉意，这是在手机上写的:)

Answer 3

是的，这就是我自己想出来的，我想知道是否有更优雅的(一行的)方法来解决这个问题：

fh = open("/var/log/messages", encoding = "ISO-8859-1")
fh.seek(0)
febMessages = [x for x in fh if x.split(' ')[0].lower() == 'feb']
len(febMessages)