pyudev-0.22.0/doc/guide.rst

User guide
==========

.. currentmodule:: pyudev

This guide gives an introduction in how to use pyudev for common operations
like device enumeration or monitoring:

.. contents::

A detailed reference is provided in the :doc:`API documentation <api/index>`.


Getting started
---------------

Import pyudev and verify that you're using the latest version:

>>> import pyudev
>>> pyudev.__version__
u'0.16'
>>> pyudev.udev_version()
181

This prints the version of pyudev itself and of the underlying libudev_.


A note on versioning
--------------------

pyudev supports libudev_ 151 or newer, but still tries to cover the most recent
libudev_ API completely.  If you are using older libudev_ releases, some
functionality of pyudev may be unavailable, simply because libudev_ is too old
to support a specific feature.  Whenever this is the case, the minimum required
version of udev is noted in the documentation (see
:attr:`Device.is_initialized` for an example).  If no version is specified for
an attribute or a method, it is available on all supported libudev_ versions.
You can check the version of the underlying libudev_ with
:func:`pyudev.udev_version()`.


Enumerating devices
-------------------

A common use case is to enumerate available devices, or a subset thereof.  But
before you can do anything with pyudev, you need to establish a "connection" to
the udev device database first.  This connection is represented by a library
:class:`Context`:

>>> context = pyudev.Context()

The :class:`Context` is the central object of pyudev and libudev_.  You will
need a :class:`Context` object for almost anything in pyudev.  With the
``context`` you can now enumerate the available devices:

>>> for device in context.list_devices(): # doctest: +ELLIPSIS
...     device
...
Device(u'/sys/devices/LNXSYSTM:00')
Device(u'/sys/devices/LNXSYSTM:00/LNXCPU:00')
Device(u'/sys/devices/LNXSYSTM:00/LNXCPU:01')
...

By default, :meth:`list_devices()` yields all devices available on the system
as :class:`Device` objects, but you can filter the list of devices with keyword
arguments to enumerate all available partitions for example:

>>> for device in context.list_devices(subsystem='block', DEVTYPE='partition'):
...    print(device)
...
Device(u'/sys/devices/pci0000:00/0000:00:0d.0/host2/target2:0:0/2:0:0:0/block/sda/sda1')
Device(u'/sys/devices/pci0000:00/0000:00:0d.0/host2/target2:0:0/2:0:0:0/block/sda/sda2')
Device(u'/sys/devices/pci0000:00/0000:00:0d.0/host2/target2:0:0/2:0:0:0/block/sda/sda3')

The choice of the right filters depends on the use case and generally requires
some knowledge about how udev classifies and categorizes devices.  This is out
of the scope of this guide.  Poke around in ``/sys/`` to get a feeling for the
udev-way of device handling, read the udev documentation or one of the
tutorials in the net.

The keyword arguments of :meth:`list_devices()` provide the most common filter
operations.  You can apply other, less common filters by calling one of the
``match_*`` methods on the :class:`Enumerator` returned by of
:meth:`list_devices()`.


Accessing individual devices directly
-------------------------------------

If you just need a single specific :class:`Device`, you don't need to enumerate
all devices with a specific filter criterion.  Instead, you can directly create
:class:`Device` objects from a device path (:meth:`Devices.from_path()`), by
from a subsystem and device name (:meth:`Devices.from_name()`) or from a device
file (:meth:`Devices.from_device_file()`).  The following code gets the
:class:`Device` object for the first hard disc in three different ways:

>>> pyudev.Devices.from_path(context, '/sys/block/sda')
Device(u'/sys/devices/pci0000:00/0000:00:0d.0/host2/target2:0:0/2:0:0:0/block/sda')
>>> pyudev.Devices.from_name(context, 'block', 'sda')
Device(u'/sys/devices/pci0000:00/0000:00:0d.0/host2/target2:0:0/2:0:0:0/block/sda')
>>> pyudev.Devices.from_device_file(context, '/dev/sda')
Device(u'/sys/devices/pci0000:00/0000:00:0d.0/host2/target2:0:0/2:0:0:0/block/sda')

As you can see, you need to pass a :class:`Context` to both methods as
reference to the udev database from which to retrieve information about the
device.

.. note::

   The :class:`Device` objects created in the above example refer to the same
   device.  Consequently, they are considered equal:

   >>> pyudev.Devices.from_path(context, '/sys/block/sda') == pyudev.Devices.from_name(context, 'block', 'sda')
   True

   Whereas :class:`Device` objects referring to different devices are unequal:

   >>> pyudev.Devices.from_name(context, 'block', 'sda') == pyudev.Devices.from_name(context, 'block', 'sda1')
   False


Querying device information
---------------------------

As you've seen, :class:`Device` represents a device in the udev database.  Each
such device has a set of "device properties" (not to be confused with Python
properties as created by :func:`property()`!) that describe the capabilities
and features of this device as well as its relationship to other devices.

Common device properties are also available as properties of a :class:`Device`
object.  For instance, you can directly query the :attr:`device_node` and the
:attr:`device_type` of block devices:

>>> for device in context.list_devices(subsystem='block'):
...     print('{0} ({1})'.format(device.device_node, device.device_type))
...
/dev/sr0 (disk)
/dev/sda (disk)
/dev/sda1 (partition)
/dev/sda2 (partition)
/dev/sda3 (partition)

For other udev properties, :class:`Device` provides a mapping interface
to access the device properties by means of its properties attribute.

>>> for device in context.list_devices(subsystem='block'):
...    print('{0} ({1})'.format(device.properties['DEVNAME'], device.properties['DEVTYPE']))
...
/dev/sr0 (disk)
/dev/sda (disk)
/dev/sda1 (partition)
/dev/sda2 (partition)
/dev/sda3 (partition)

.. warning::

   When filtering devices, you have to use the device property names.  The
   names of corresponding properties of :class:`Device` will generally **not**
   work.  Compare the following two statements:

   >>> [device.device_node for device in context.list_devices(subsystem='block', DEVTYPE='partition')]
   [u'/dev/sda1', u'/dev/sda2', u'/dev/sda3']
   >>> [device.device_node for device in context.list_devices(subsystem='block', device_type='partition')]
   []


But you can also query many device properties that are not available as Python
properties on the :class:`Device` object with a convenient mapping interface,
like the filesystem type.  :class:`Device` provides a convenient mapping
interface for this purpose:

>>> for device in context.list_devices(subsystem='block', DEVTYPE='partition'):
...     print('{0} ({1})'.format(device.device_node, device.properties.get('ID_FS_TYPE')))
...
/dev/sda1 (ext3)
/dev/sda2 (swap)
/dev/sda3 (ext4)

.. note::

   Such device specific properties may not be available on devices.  Either use
   ``get()`` to specify default values for missing properties, or be prepared
   to catch :exc:`~exceptions.KeyError`.

Most device properties are computed by udev rules from the driver- and
device-specific "device attributes".  The :attr:`Device.attributes` mapping
gives you access to these attributes, but generally you should not need these.
Use the device properties whenever possible.


Examing the device hierarchy
----------------------------

A :class:`Device` is part of a device hierarchy, and can have a
:attr:`~Device.parent` device that more or less resembles the physical
relationship between devices.  For instance, the :attr:`~Device.parent` of
partition devices is a :class:`Device` object that represents the disc the
partition is located on:

>>> 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

Generally, you should not rely on the direct parent-child relationship between
two devices.  Instead of accessing the parent directly, search for a parent
within a specific subsystem, e.g. for the parent ``block`` device, with
:meth:`~Device.find_parent()`:

>>> 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

This also save you the tedious work of traversing the device tree manually, if
you are interested in grand parents, like the name of the PCI slot of the SCSI
or IDE controller of the disc that contains a partition:

>>> for device in context.list_devices(subsystem='block', DEVTYPE='partition'):
...    print('{0} attached to PCI slot {1}'.format(device.device_node, device.find_parent('pci')['PCI_SLOT_NAME']))
...
/dev/sda1 attached to PCI slot 0000:00:0d.0
/dev/sda2 attached to PCI slot 0000:00:0d.0
/dev/sda3 attached to PCI slot 0000:00:0d.0


Monitoring devices
------------------

Synchronous monitoring
~~~~~~~~~~~~~~~~~~~~~~

The Linux kernel emits events whenever devices are added, removed (e.g. a USB
stick was plugged or unplugged) or have their attributes changed (e.g. the
charge level of the battery changed).  With :class:`pyudev.Monitor` you can
react on such events, for example to react on added or removed mountable
filesystems:

>>> monitor = pyudev.Monitor.from_netlink(context)
>>> monitor.filter_by('block')
>>> for device in iter(monitor.poll, None):
...     if 'ID_FS_TYPE' in device:
...         print('{0} partition {1}'.format(device.action, device.get('ID_FS_LABEL')))
...
add partition MULTIBOOT
remove partition MULTIBOOT

After construction of a monitor, you can install an event filter on the monitor
using :meth:`~Monitor.filter_by()`.  In the above example only events from the
``block`` subsystem are handled.

.. note::

   Always prefer :meth:`~Monitor.filter_by()` and
   :meth:`~Monitor.filter_by_tag()` over manually filtering devices (e.g. by
   ``device.subsystem == 'block'`` or ``tag in device.tags``).  These methods
   install the filter on the *kernel side*.  A process waiting for events is
   thus only woken up for events that match these filters.  This is much nicer
   in terms of power consumption and system load than executing filters in the
   process itself.

Eventually, you can receive events from the monitor.  As you can see, a
:class:`Monitor` is iterable and synchronously yields occurred events.  If you
iterate over a :class:`Monitor`, you will synchronously receive events in an
endless loop, until you raise an exception, or ``break`` the loop.

This is the quick and dirty way of monitoring, suitable for small scripts or
quick experiments.  In most cases however, simply iterating over the monitor is
not sufficient, because it blocks the main thread, and can only be stopped if
an event occurs (otherwise the loop is not entered and you have no chance to
``break`` it).


Asynchronous monitoring
~~~~~~~~~~~~~~~~~~~~~~~

For such use cases, pyudev provides asynchronous monitoring with
:class:`MonitorObserver`.  You can use it to log added and removed mountable
filesystems to a file, for example:

>>> monitor = pyudev.Monitor.from_netlink(context)
>>> monitor.filter_by('block')
>>> def log_event(device):
...    if 'ID_FS_TYPE' in device.properties:
...        with open('filesystems.log', 'a+') as stream:
...            print('{0} - {1}'.format(device.action, device.get('ID_FS_LABEL')), file=stream)
...
>>> observer = pyudev.MonitorObserver(monitor, callback=log_event)
>>> observer.start()

The ``observer`` gets a callback (``log_event()`` in this case) which is
asynchronously invoked on every event emitted by the underlying ``monitor``
after the observer has been started using :meth:`~threading.Thread.start()`.

.. warning::

   The callback is invoked from a *different* thread than the one in which the
   ``observer`` was created.  Be sure to protect access to shared resource
   properly when you access them from the callback (e.g. by locking).

The ``observer`` can be stopped at any moment using :meth:`~MonitorObserver.stop()``:

>>> observer.stop()

.. warning::

   Do *not* call :meth:`~MonitorObserver.stop()` from the event handler,
   neither directly nor indirectly.  Use :meth:`~MonitorObserver.send_stop()`
   if you need to stop monitoring from inside the event handler.


GUI toolkit integration
~~~~~~~~~~~~~~~~~~~~~~~

If you're using a GUI toolkit, you already have the event system of the GUI
toolkit at hand.  pyudev provides observer classes that seamlessly integration
in the event system of the GUI toolkit and relieve you from caring with
synchronisation issues that would occur with thread-based monitoring as
implemented by :class:`MonitorObserver`.

pyudev supports all major GUI toolkits available for Python:

- Qt_ 5 using :mod:`pyudev.pyqt5`
- Qt_ 4 using :mod:`pyudev.pyqt4` for the PyQt4_ binding or :mod:`pyudev.pyside`
  for the PySide_ binding
- PyGtk_ 2 using :mod:`pyudev.glib`
- wxWidgets_ and wxPython_ using :mod:`pyudev.wx`

Each of these modules provides an observer class that observers the monitor
asynchronously and emits proper signals upon device events.

For instance, the above example would look like this in a PySide_ application:

>>> from pyudev.pyside import QUDevMonitorObserver
>>> monitor = pyudev.Monitor.from_netlink(context)
>>> observer = QUDevMonitorObserver(monitor)
>>> observer.deviceEvent.connect(log_event)
>>> monitor.start()

Device objects as booleans
~~~~~~~~~~~~~~~~~~~~~~~~~~
The use of a Device object in a boolean context as a shorthand for a comparison
with None is an error.

The Device class inherits from the abstract Mapping class, as it maps udev
property names to their values. Consequently, if a Device object has no udev
properties, an unusual but not impossible occurance, the object is
interpreted as False in a boolean context.

.. _pypi: https://pypi.python.org/pypi/pyudev
.. _libudev: http://www.kernel.org/pub/linux/utils/kernel/hotplug/libudev/
.. _Qt: http://qt.io/developers/
.. _PyQt5: https://riverbankcomputing.co.uk/software/pyqt/intro
.. _PyQt4: https://riverbankcomputing.co.uk/software/pyqt/intro
.. _PySide: http://wiki.qt.io/PySide
.. _PyGtk: http://www.pygtk.org/
.. _wxWidgets: http://wxwidgets.org
.. _wxPython: http://www.wxpython.org