Python库之pyudev (一)

库pyudev是libudev的python封装，libudev提拱了对本地设备的列举与查询API。

１．安装

pip install pyudev

2. 使用

2.1 开始

导入pyudev，验证库版本。

In [1]: import pyudev
In [2]: print pyudev.__version__
0.21.0
In [3]: print pyudev.udev_version()
229

 2.2 列举（Enumerate)设备

• 创建上下文（Context）对象。Context是pyudev的中心对象，在pyudev程序中几乎都会需要它。

In [5]: context = pyudev.Context()

• 列举全部设备。

In [6]: for device in context.list_devices():
   ...:     print device
   ...:
Device(u'/sys/devices/LNXSYSTM:00')
......
Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata1/ata_port/ata1')
Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata1/host0')
Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata1/host0/scsi_host/host0')
Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata1/link1/ata_link/link1')
Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata1/link1/dev1.0/ata_device/dev1.0')

• 也可给带参数使用list_devices()，从而对设备进行过滤选择。设置过滤条件需要了解linux系统是如何对设备进行分类的。有２种过滤方法：一是使用关键参数（keyword arguments），二是使用自定义过滤器函数对象(matcher_*。
  • 使用keyword arguments。

In [8]: for device in context.list_devices(subsystem='block', DEVTYPE='partition'):
   ...:     print device
   ...:
Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata5/host4/target4:0:0/4:0:0:0/block/sda/sda1')
Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata5/host4/target4:0:0/4:0:0:0/block/sda/sda2')
Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata5/host4/target4:0:0/4:0:0:0/block/sda/sda3')
Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata5/host4/target4:0:0/4:0:0:0/block/sda/sda4')
......

• • 使用matcher_*函数对象。例子略...

2.3 直接访问设备

可通过设备路径\子系统＋设备名\设备文件等三种方法来直接创建设备对象。

In [10]: pyudev.Devices.from_path(context, '/sys/block/sda')
Out[10]: Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata5/host4/target4:0:0/4:0:0:0/block/sda')
In [11]: pyudev.Devices.from_name(context, 'block', 'sda')
Out[11]: Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata5/host4/target4:0:0/4:0:0:0/block/sda')
In [12]: pyudev.Devices.from_device_file(context, '/dev/sda')
Out[12]: Device(u'/sys/devices/pci0000:00/0000:00:1f.2/ata5/host4/target4:0:0/4:0:0:0/block/sda')

这三种方法所获取的对象是同一个设备。

In [16]: pyudev.Devices.from_device_file(context, '/dev/sda') == pyudev.Devices.from_name(context, 'block', 'sda')
Out[16]: True
In [17]: pyudev.Devices.from_name(context, 'block', 'sda') == pyudev.Devices.from_path(context, '/sys/block/sda')
Out[17]: True

2.4 查询设备属性

由列举设备或直接访问所返回的设备对象（Device）都对应于udev数据库中的一个设备，每个设备有“属性”，以描述设备的性能、特点，与其它设备的关系，...等等。查询设备的属性有三种方法：

• 像普通python对象一样获取属性。如下面的代码打印出所有块设备（block)的device_node和device_type属性。

In [19]: for device in context.list_devices(subsystem='block'):
   ....:     print '{0}({1})'.format(device.device_node, device.device_type)
   ....:
/dev/sda(disk)
/dev/sda2(partition)
/dev/sda3(partition)
/dev/sda4(partition)
/dev/sda5(partition)
/dev/sda6(partition)
/dev/sda7(partition)
/dev/sda8(partition)
/dev/sda9(partition)
/dev/loop0(disk)
/dev/loop1(disk)
/dev/loop2(disk)
......

• 以类似字典的方法获取属性。

In [20]: for device in context.list_devices(subsystem='block'):
   ....:     print '({0}({1})'.format(device['DEVNAME'], device['DEVTYPE'])
   ....:
(/dev/sda(disk)
(/dev/sda1(partition)
(/dev/sda2(partition)
(/dev/sda3(partition)
(/dev/sda4(partition)
(/dev/sda5(partition)
(/dev/sda6(partition)
(/dev/sda7(partition)
(/dev/sda8(partition)
(/dev/sda9(partition)
(/dev/loop0(disk)
(/dev/loop1(disk)
(/dev/loop2(disk)
......

• 通过接口函数get()访问设备属性

In [24]: for device in context.list_devices(subsystem='block', DEVTYPE='partition'):
   ....:     print '{0}({1})'.format(device.device_node, device.get('ID_FS_TYPE'))
   ....:
/dev/sda1(ntfs)
/dev/sda2(vfat)
/dev/sda3(vfat)
/dev/sda4(None)
/dev/sda5(ntfs)
/dev/sda6(ntfs)
/dev/sda7(ntfs)
/dev/sda8(None)
/dev/sda9(swap)
......

上面三种方法中，推荐使用接口函数来返回设备属性。当试图返回一个并不存在的属性时，get函数可返回指定的缺省值（通过参数设置），也可抛出KeyError异常。

此外，可用Device.attributes来查看设备有哪些属性，尽管大多数时间不需要这么做。

2.5　检索设备层级（hierarchy）

udev中设备是具有层次属性的，即设备之间可能存在父－子关系，如分区设备(partition）就是某磁盘设备的子设备。用Device对象的parent属性可返回其父设备对象。

In [25]: for device in context.list_devices(subsystem='block', DEVTYPE='partition'):
   ....:     print '{0} is located on {1}'.format(device.device_node, device.parent.device_node)
   ....:
/dev/sda1 is located on /dev/sda
/dev/sda2 is located on /dev/sda
/dev/sda3 is located on /dev/sda
/dev/sda4 is located on /dev/sda
/dev/sda5 is located on /dev/sda
/dev/sda6 is located on /dev/sda
/dev/sda7 is located on /dev/sda
/dev/sda8 is located on /dev/sda
/dev/sda9 is located on /dev/sda

除了上面“硬”访问之外，更常用的是用搜索的方法返回父设备，即使用find_parent函数。

In [26]: for device in context.list_devices(subsystem='block', DEVTYPE='partition'):
   ....:     print '{0} is located on {1}'.format(device.device_node, device.find_parent('block').device_node)
   ....:
/dev/sda1 is located on /dev/sda
/dev/sda2 is located on /dev/sda
/dev/sda3 is located on /dev/sda
/dev/sda4 is located on /dev/sda
/dev/sda5 is located on /dev/sda
/dev/sda6 is located on /dev/sda
/dev/sda7 is located on /dev/sda
/dev/sda8 is located on /dev/sda
/dev/sda9 is located on /dev/sda

find_parent使用更加灵活，且能够简化多层设备间的追溯，可直接返回祖父级、曾祖父级...的设备而不需要逐级搜索。如从分区设备直接返回其所在磁盘的IDE控制器或SCSI控制器的PCI插槽的名字。

In [27]: for device in context.list_devices(subsystem='block', DEVTYPE='partition'):
   ....:     print '{0} attatched to PCI slot {1}'.format(device.device_node, device.find_parent('pci')['PCI_SLOT_NAME'])
   ....:
/dev/sda1 attatched to PCI slot 0000:00:1f.2
/dev/sda2 attatched to PCI slot 0000:00:1f.2
/dev/sda3 attatched to PCI slot 0000:00:1f.2
/dev/sda4 attatched to PCI slot 0000:00:1f.2
/dev/sda5 attatched to PCI slot 0000:00:1f.2
/dev/sda6 attatched to PCI slot 0000:00:1f.2
/dev/sda7 attatched to PCI slot 0000:00:1f.2
/dev/sda8 attatched to PCI slot 0000:00:1f.2
/dev/sda9 attatched to PCI slot 0000:00:1f.2

上面的例子直接从partition对象返回其相关的pci对象，打印其'PCI_SLOT_NAME‘属性。

2.6 设备监控（Monitor)

当添加、移除（如插、拔USB设备），或者设备属性变化（如电池充电等级变化）时，Linux核心将发送设备事件以便应用程序进行处理。pyudev程序监控系统设备事件的步骤是：创建监控器（Monitor）对象，设置监控器的过滤器（即设置需要处理的哪些事件），最后启用监控器。启用监控器有二种形式：同步监控、异步监控。

• 同步监控

应用程序收到设备事件后获取系统控制权并进行处理，系统核心挂起等待应用程序处理完毕，应用程序完成处理后返回并交回系统控制权。这种方式效率低，仅适合处理过程简单的情况。

In [28]: monitor = pyudev.Monitor.from_netlink(context)

In [29]: monitor.filter_by('block')

In [30]: for device in iter(monitor.poll, None):
   ....:     if 'ID_FS_TYPE' in device:
   ....:         print '{0} partition {1}'.format(device.action, device.get('ID_FS_LABLE'))
   ....:
add partition None
add partition None

停止同步监控需要从处理过程中break循环或抛出异常（raise exeption)。

此外，为Monitor设置过滤器可以用filter_by()或filter_by_tag()函数。

• 异步监控

应用程序创建监控器、事件处理程序（回调函数），将其插入系统核心的事件响应链，即安装观察器（Observer）。当期望的事件发生时，系统根据事件响应链启动一个新的线程调用回调函数，系统核心本身并不挂起。需要停止监控时，应从处理线程外部调用Obsever的stop函数从系统的事件响应链中观察器，如果要在事件处理程序中停止观察器，应调用send_stop()函数通知Observer进行间接停止。

In [3]: monitor = pyudev.Monitor.from_netlink(context)

In [4]: monitor.filter_by('block')

In [5]: def log_event(action, device):
   ...:     if 'ID_FS_TYPE' in device:
   ...:         with open('filesystem.log', 'a+') as stream:
   ...:             stream.write('{0} - {1}/n'.format(action, device.get('ID_FS_LABLE')))
   ...:             

In [6]: observer = pyudev.MonitorObserver(monitor, log_event)

In [7]: observer.start()

In [8]: observer.stop()

 3. 与GUI库的集成

pyudev库支持多种GUI库，模块对应关系如下：

pyudev.pyqt5<--------->Qt5

pyudev.pyqt4<--------->Qt4

pyudev.glib<------------>PyGtk2

pyudev.wx<------------->wxWidgets / wxPython

例子略...可参考Using pyudev.pyqt5 within PyQt5's event loop