xref: /dports/devel/py-pyusb/pyusb-1.0.2/docs/tutorial.rst

==========================
Programming with PyUSB 1.0
==========================

Let me introduce myself
=======================

PyUSB 1.0 is a Python_ library allowing easy USB_ access. PyUSB provides
several features:

100% written in Python:
    Unlike the 0.x version, which is written in C, 1.0 version is written in
    Python. This allows Python programmers with no background in C to
    understand better how PyUSB works.
Platform neutrality:
    1.0 version implements a frontend-backend scheme. This isolates the API
    from system specific implementation details. The glue between the two
    layers is the ``IBackend`` interface. PyUSB comes with builtin backends for
    libusb 0.1, libusb 1.0 and OpenUSB.  You can write your own backend if you
    desire to.
Portability:
    PyUSB should run on any platform with Python >= 2.4, ctypes_ and at least
    one of the supported builtin backends.
Easiness:
    Communicating with an USB_ device has never been so easy! USB is a complex
    protocol, but PyUSB has good defaults for most common configurations.
Support for isochronous transfers:
    PyUSB supports isochronous transfers if the underlying backend supports it.

Although PyUSB makes USB programming less painful, it is assumed in this
tutorial that you have a minimal USB protocol background. If you don't know
anything about USB, I recommend you the excellent Jan Axelson's book **USB
Complete**.

Enough talk, let's code!
========================

Who's who
---------

First of all, let's give an overview on the PyUSB modules. PyUSB modules are
under the ``usb`` package, with the following modules:

======= ===========
Content Description
------- -----------
core    The main USB module.
util    Utility functions.
control Standard control requests.
legacy  The 0.x compatibility layer.
backend A subpackage containing the builtin backends.
======= ===========

For example, to import the ``core`` module, type the following::

    >>> import usb.core
    >>> dev = usb.core.find()

Let's get it started
--------------------

Following is a simplistic program that sends the 'test' string to the first OUT
endpoint found:

.. code-block:: python

    import usb.core
    import usb.util

    # find our device
    dev = usb.core.find(idVendor=0xfffe, idProduct=0x0001)

    # was it found?
    if dev is None:
        raise ValueError('Device not found')

    # set the active configuration. With no arguments, the first
    # configuration will be the active one
    dev.set_configuration()

    # get an endpoint instance
    cfg = dev.get_active_configuration()
    intf = cfg[(0,0)]

    ep = usb.util.find_descriptor(
        intf,
        # match the first OUT endpoint
        custom_match = \
        lambda e: \
            usb.util.endpoint_direction(e.bEndpointAddress) == \
            usb.util.ENDPOINT_OUT)

    assert ep is not None

    # write the data
    ep.write('test')

The first two lines import PyUSB package modules. ``usb.core`` is the main
module, and ``usb.util`` contains utility functions. The next command searches
for our device and returns an instance object if it is found. If not, ``None``
is returned.  After, we set the configuration to use. Note that no argument
indicating what configuration we want was supplied. As you will see, many PyUSB
functions have defaults for most common devices. In this case, the
configuration set is the first one found.

Then, we look for the endpoint we are interested. We search for it inside the
first interface we have. After finding the endpoint, we send the data to it.

If we know the endpoint address in advance, we could just call the ``write``
function from the device object:

.. code-block:: python

    dev.write(1, 'test')

Here we write the string 'test' at the endpoint address *1*.
All these functions will be detailed in the following sections.

What's wrong?
-------------

Every function in PyUSB raises an exception in case of an error. Besides the
`Python standard exceptions
<http://docs.python.org/library/exceptions.html>`__, PyUSB defines the
``usb.core.USBError`` for USB related errors.

You can also use the PyUSB log functionality. It uses the `logging
<http://docs.python.org/library/logging.html>`__ module. To enable it, define
the environment variable ``PYUSB_DEBUG`` with one of the following level
names: ``critical``, ``error``, ``warning``, ``info`` or ``debug``.

By default the messages are sent to `sys.stderr
<http://docs.python.org/library/sys.html>`__.  If you want to, you can redirect
log messages to a file by defining the ``PYUSB_LOG_FILENAME`` environment
variable. If its value is a valid file path, messages will be written to it,
otherwise it will be sent to ``sys.stderr``.

Where are you?
--------------

The ``find()`` function in the ``core`` module is used to find and enumerate
devices connected to the system. For example, let's say that our device has a
vendor ID equal to `0xfffe` and product ID equals to `0x0001`. If we would like
to find it, we proceed in this way:

.. code-block:: python

    import usb.core

    dev = usb.core.find(idVendor=0xfffe, idProduct=0x0001)
    if dev is None:
        raise ValueError('Our device is not connected')

That's it, the function will return an ``usb.core.Device`` object representing
our device. If the device is not found, it returns ``None``. Actually, you can
use any field of the Device Descriptor_ you desire. For example, what if we
would like to discover if there is a USB printer connected to the system?  This
is very easy:

.. code-block:: python

    # actually this is not the whole history, keep reading
    if usb.core.find(bDeviceClass=7) is None:
        raise ValueError('No printer found')

The 7 is the code for the printer class according to the USB spec.
Hey, wait, what if I want to enumerate all printers present? No problem:

.. code-block:: python

    # this is not the whole history yet...
    printers = usb.core.find(find_all=True, bDeviceClass=7)

    # Python 2, Python 3, to be or not to be
    import sys
    sys.stdout.write('There are ' + len(printers) + ' in the system\n.')

What happened? Well, it is time for a little explanation... ``find`` has a
parameter called ``find_all`` that defaults to False. When it is false [#]_,
``find`` will return the first device found that matches the specified criteria
(more on that soon). If you give it a *true* value, ``find`` will instead
return a list with all devices matching the criteria.  That's it! Simple, isn't
it?

Finished? No! I have not told you the whole history: many devices actually put
their class information in the Interface Descriptor_ instead of the Device
Descriptor_. So, to really find all printers connected to the system, we would
need to transverse all configurations, and then all interfaces and check if one
of the interfaces has its `bInterfaceClass` field equal to 7. If you are a
`programmer <http://en.wikipedia.org/wiki/Laziness>`__ like me, you might be
wondering if there is an easier way to do that. The answer is yes, there is.
First, let's give a look on the final code to find all printers connected:

.. code-block:: python

    import usb.core
    import usb.util
    import sys

    class find_class(object):
        def __init__(self, class_):
            self._class = class_
        def __call__(self, device):
            # first, let's check the device
            if device.bDeviceClass == self._class:
                return True
            # ok, transverse all devices to find an
            # interface that matches our class
            for cfg in device:
                # find_descriptor: what's it?
                intf = usb.util.find_descriptor(
                                            cfg,
                                            bInterfaceClass=self._class
                                    )
                if intf is not None:
                    return True

            return False

    printers = usb.core.find(find_all=1, custom_match=find_class(7))

The ``custom_match`` parameter accepts any callable object that receives the
device object. It must return true for a matching device, and false for a
non-matching device. You can also combine ``custom_match`` with device fields
if you want:

.. code-block:: python

    # find all printers that belongs to our vendor:
    printers = usb.core.find(find_all=1, custom_match=find_class(7), idVendor=0xfffe)

Here we are only interested in the printers of the `0xfffe` vendor.

Describe yourself
-----------------

Ok, we've found our device, but before talking to it, we would like to know
more about it, you know, configurations, interfaces, endpoints, transfer
types...

If you have a device, you can access any device descriptor fields as object
properties:

.. code-block:: python

    >>> dev.bLength
    >>> dev.bNumConfigurations
    >>> dev.bDeviceClass
    >>> # ...

To access the configurations available in the device, you can iterate over the
device:

.. code-block:: python

    for cfg in dev:
        sys.stdout.write(str(cfg.bConfigurationValue) + '\n')

In the same way, you can iterate over a configuration to access the interfaces,
and iterate over the interfaces to access their endpoints. Each kind of object
has as attributes the fields of the respective descriptor. Let's see an
example:

.. code-block:: python

    for cfg in dev:
        sys.stdout.write(str(cfg.bConfigurationValue) + '\n')
        for intf in cfg:
            sys.stdout.write('\t' + \
                             str(intf.bInterfaceNumber) + \
                             ',' + \
                             str(intf.bAlternateSetting) + \
                             '\n')
            for ep in intf:
                sys.stdout.write('\t\t' + \
                                 str(ep.bEndpointAddress) + \
                                 '\n')

You can also use the subscript operator to access the descriptors randomly,
like this:

.. code-block:: python

    >>> # access the second configuration
    >>> cfg = dev[1]
    >>> # access the first interface
    >>> intf = cfg[(0,0)]
    >>> # third endpoint
    >>> ep = intf[2]

As you can see, the index is zero-based. But wait! There is something weird in
the way I access an interface... Yes, you are right, the subscript operator in
the Configuration accepts a sequence of two items, with the first one being the
index of the Interface and the second one, the alternate setting. So, to access
the first interface, but its second alternate setting, we write ``cfg[(0,1)]``.

Now it's time to we learn a powerful way to find descriptors, the
``find_descriptor`` utility function. We have already seen it in the printer
finding example.  ``find_descriptor`` works in almost the same way as ``find``,
with two exceptions:

* ``find_descriptor`` receives as its first parameter the parent descriptor
  that you will search on.
* There is no ``backend`` [#]_ parameter.

For example, if we have a configuration descriptor ``cfg`` and want to find all
alternate settings of the interface 1, we do so:

.. code-block:: python

    import usb.util
    alt = usb.util.find_descriptor(cfg, find_all=True, bInterfaceNumber=1)

Notice that ``find_descriptor`` is in the ``usb.util`` module. It also accepts
the early described ``custom_match`` parameter.

Dealing with multiple identical devices
***************************************

Sometimes you may have two identical devices connected to the computer. How can
you differentiate them? ``Device`` objects come with two additional attributes
which are not part of the USB Spec, but are very useful: ``bus`` and
``address`` attributes. First of all, it is worth it to say that these
attributes come from the backend and a backend is free to not support them, in
which case they are set to ``None``. That said, these attributes represent the
bus number and bus address of the device and, as you might already have
imagined, can be used to differentiate two devices with the same ``idVendor``
and ``idProduct`` attributes.

How am I supposed to work?
--------------------------

USB devices after connection must be configured through a few standard
requests.  When I started to study USB_ spec, I found myself confused with
descriptors, configurations, interfaces, alternate settings, transfer types and
all this stuff... And worst, you cannot simply ignore them, a device does not
work without setting a configuration, even if it has just one! PyUSB tries to
make your life as easy as possible. For example, after getting your device
object, one of the first things you need to do before communicating with it is
issuing a ``set_configuration`` request. The parameter for this request is the
``bConfigurationValue`` of the configuration you are interested on.  Most
devices have no more than one configuration, and tracking the configuration
value to use is annoying (although most code I have seen simply hardcodes it).
Therefore, in PyUSB, you can just issue a ``set_configuration`` call with no
arguments. In this case, it will set the first configuration found (if your
device has just one, you don't need to worry about the configuration value at
all). For example, let's imagine you have a device with one configuration
descriptor with its `bConfigurationValue` field equals to 5 [#]_, the following
calls below will work equally:

.. code-block:: python

    >>> dev.set_configuration(5)
    # or
    >>> dev.set_configuration() # we assume the configuration 5 is the first one
    # or
    >>> cfg = util.find_descriptor(dev, bConfigurationValue=5)
    >>> cfg.set()
    # or
    >>> cfg = util.find_descriptor(dev, bConfigurationValue=5)
    >>> dev.set_configuration(cfg)

Wow! You can use a ``Configuration`` object as a parameter to
``set_configuration``!  Yes, and also it has a ``set`` method to configure
itself as the current configuration.

The other setting you might or might not have to configure is the interface
alternate setting. Each device can have only one activated configuration at a
time, and each configuration may have more than one interface, and you can use
all interfaces at the same time. You better understand this concept if you
think of an interface as a logical device. For example, let's imagine a
multifunction printer, which is at the same time a printer and a scanner. To
keep things simple (or at least as simple as we can), let's consider that it
has just one configuration. As we have a printer and a scanner, the
configuration has two interfaces, one for the printer and one for the scanner.
A device with more than one interface is called a composite device. When you
connect your multifunction printer to your computer, the Operating System would
load two different drivers: one for each "logical" peripheral you have [#]_.

What about the alternate setting? Good you asked. An interface has one or more
alternate settings. An interface with just one alternate setting is considered
to not having an alternate setting [#]_. Alternate settings are for interfaces
what configurations are for devices, i.e, for each interface, you can have only
one alternate setting active. For example, USB spec says that a device cannot
have an isochronous endpoint in its primary alternate setting [#]_, so a
streaming device must have at least two alternate settings, with the second one
having the isochronous endpoint(s). But as opposed to configurations,
interfaces with just one alternate setting don't need to be set [#]_. You
select an interface alternate setting through the ``set_interface_altsetting``
function:

.. code-block:: python

    >>> dev.set_interface_altsetting(interface = 0, alternate_setting = 0)

.. warning::
    The USB spec says that a device is allowed to return an error in case it
    receives a SET_INTERFACE request for an interface that has no additional
    alternate settings. So, if you are not sure if either the interface has more
    than one alternate setting or it accepts a SET_INTERFACE request,
    the safest way is to call ``set_interface_altsetting`` inside an
    try-except block, like this:

    .. code-block:: python

        try:
            dev.set_interface_altsetting(...)
        except USBError:
            pass

You can also use an ``Interface`` object as parameter to the function, the
``interface`` and ``alternate_setting`` parameters are automatically inferred
from ``bInterfaceNumber`` and ``bAlternateSetting`` fields. Example:

.. code-block:: python

    >>> intf = find_descriptor(...)
    >>> dev.set_interface_altsetting(intf)
    >>> intf.set_altsetting() # wow! Interface also has a method for it

.. warning::
    The ``Interface`` object must belong to the active configuration descriptor.

Talk to me, honey
-----------------

Now it's time for us to learn how to communicate with USB devices. USB has four
flavors of transfers: bulk, interrupt, isochronous and control. I don't intend
to explain the purpose of each transfer and the differences among them.
Therefore, I assume you know at least the basics of the USB transfers.

Control transfer is the only transfer that has structured data described in the
spec, the others just send and receive raw data from USB point of view. Because
of it, you have a different function to deal with control transfers, all the
other transfers are managed by the same functions.

You issue a control transfer through the ``ctrl_transfer`` method. It is used
both for OUT and IN transfers. The transfer direction is determined from the
``bmRequestType`` parameter.

The ``ctrl_transfer`` parameters are almost equal to the control request
structure. Following is a example of how to do a control transfer [#]_:

.. code-block:: python

    >>> msg = 'test'
    >>> assert dev.ctrl_transfer(0x40, CTRL_LOOPBACK_WRITE, 0, 0, msg) == len(msg)
    >>> ret = dev.ctrl_transfer(0xC0, CTRL_LOOPBACK_READ, 0, 0, len(msg))
    >>> sret = ''.join([chr(x) for x in ret])
    >>> assert sret == msg

In this example, it is assumed that our device implements two custom control
requests that act as a loopback pipe. What you write with the
``CTRL_LOOPBACK_WRITE`` message, you can read with the ``CTRL_LOOPBACK_READ``
message.

The first four parameters are the ``bmRequestType``, ``bmRequest``, ``wValue``
and ``wIndex`` fields of the standard control transfer structure. The fifth
parameter is either the data payload for an OUT transfer or the number of bytes
to read in an IN transfer.  The data payload can be any sequence type that can
be used as a parameter for the array_ ``__init__`` method.  If there is no data
payload, the parameter should be ``None`` (or 0 in case of an IN transfer).
There is one last optional parameter specifying the timeout of the operation.
If you don't supply it, a default timeout will be used (more on that later). In
an OUT transfer, the return value is the number of bytes really sent to the
device. In an IN transfer, the return value is an array_ object with the data
read.

For the other transfers, you use the methods ``write`` and ``read``,
respectively, to write and read data. You don't need to worry about the
transfer type, it is automatically determined from the endpoint address. Here
is our loopback example assuming the we have a loopback pipe in the endpoint
1:

.. code-block:: python

    >>> msg = 'test'
    >>> assert len(dev.write(1, msg, 100)) == len(msg)
    >>> ret = dev.read(0x81, len(msg), 100)
    >>> sret = ''.join([chr(x) for x in ret])
    >>> assert sret == msg

The first and third parameters are equal for both methods, they are the
endpoint address and timeout, respectively. The second parameter is the data
payload (write) or the number of bytes to read (read). The returned data if
either an instance of the array_ object for the ``read`` method or the number
of bytes written for the ``write`` method.

Since beta 2 version, instead of the number of bytes, you can also pass to
``read`` and ``ctrl_transfer`` an array_ object in which the data will be
read into. In this case, the number of bytes to read will be the length of
the array times the ``array.itemsize`` value.

As in ``ctrl_transfer``, the ``timeout`` parameter is optional. When the
``timeout`` is omitted, it is used the ``Device.default_timeout`` property
as the operation timeout.

Control yourself
----------------

Besides the transfers functions, the module ``usb.control`` offers functions
which implement the standard USB control requests and the ``usb.util`` module
has the convenience function ``get_string`` specifically to return string
descriptors.

Additional Topics
=================

Behind every great abstraction, there's a great implementation
--------------------------------------------------------------

In the early days, there was only libusb_. Then came libusb 1.0, and we had
libusb 0.1 and 1.0.  After, they created OpenUSB_, and now we live at the
`Tower of Babel <http://en.wikipedia.org/wiki/Tower_of_Babel>`__ of the USB
libraries [#]_.  How does PyUSB deal with it? Well, PyUSB is a democratic
library, you may choose whichever library you want. Actually, you can write
your own USB library from scratch and tell PyUSB to use it.

The ``find`` function has one more parameter that I haven't told you. It is the
``backend`` parameter. If you don't supply it, it will be used one of the
builtin backends. A backend is an object inherited from
``usb.backend.IBackend``, responsible to implement the operating system
specific USB stuff. As you might guess, the builtins are libusb 0.1, libusb 1.0
and OpenUSB backends.

You can create your own backend and use it. Just inherit from ``IBackend`` and
implement the methods necessary. You might want to take a look at the
``usb.backend`` package documentation to learn how to do that.

Don't be selfish
----------------

Python has what we call *automatic memory management*. This means that the
virtual machine will decide when to release objects from the memory. Under the
hood, PyUSB manages all low level resources it needs to work (interface
claiming, device handles, etc.) and most of the users don't need to worry about
that. But, because of the nondeterministic nature of automatic object
destruction of Python, users cannot predict when the resources allocated will
be released. Some applications need to allocate and free the resources
deterministically.  For these kind of applications, the ``usb.util`` module has
a set of functions to deal with resource management.

If you want to claim and release interfaces manually, you may use the
``claim_interface`` and ``release_interface`` functions. ``claim_interface``
will claim the specified interface if the device has not done it yet. If the
device already claimed the interface, it does nothing.  In a similar way,
``release_interface`` will release the specified interface if it is claimed.
If the interface is not claimed, it does nothing. You can use manual interface
claim to solve the `configuration selection problem
<http://libusb.sourceforge.net/api-1.0/caveats.html>`__ described in the
libusb_ documentation.

If you want to free all resources allocated by the device object (including
interfaces claimed), you can use the ``dispose_resources`` function. It
releases all resources allocated and puts the device object (but not the device
hardware itself) in the state it was at the time when the ``find`` function
returned.

Specifying libraries by hand
----------------------------

In general, a backend is an wrapper on a shared library which implements the
USB access API.  By default, the backend uses the `find_library()
<http://docs.python.org/3/library/ctypes.html#finding-shared-libraries>`_
ctypes_ function. On Linux and other Unix like Operating Systems,
``find_library`` tries to run external programs (like */sbin/ldconfig*, *gcc*
and *objdump*) to find the library file.

On systems where these programs are missing and/or the library cache is
disabled, this function cannot be used. To overcome this limitation, PyUSB
allows you to supply a custom `find_library()` function to the backend.

An example for such scenario would be:

.. code-block:: python

    >>> import usb.core
    >>> import usb.backend.libusb1
    >>>
    >>> backend = usb.backend.libusb1.get_backend(find_library=lambda x: "/usr/lib/libusb-1.0.so")
    >>> dev     = usb.core.find(..., backend=backend)

Notice the `find_library` argument for the `get_backend()` function, in which
you supply a function that is responsible to find the correct library for the
backend.

Old school rules
----------------

If you wrote an application using the old PyUSB API (0.whatever), you may be
asking yourself if you need to update your code to use the new API. Well, you
should, but you don't need to. PyUSB 1.0 comes with the ``usb.legacy``
compatibility module. It implements the older API above the new API. "So, do I
have just to replace my ``import usb`` statement with ``import usb.legacy as
usb`` to get my application working?", you ask. The answer is yes, it will
work, but you don't have to. If you run your application untouched it will just
work, because the ``import usb`` statement will import all public symbols from
``usb.legacy``. If you face a problem, probably you found a bug.

Help me, please
---------------

If you need help, **do not email me**, the mailing list is there for this.
Subscribe instructions can be found at the PyUSB_ website.

.. [#] When I say True or False (capitalized), I mean the respective values of
       the Python language. And when I say true and false, I mean any
       expression in Python which evals to true or false.

.. [#] See backend specific documentation.

.. [#] USB spec does not impose any sequential value to the configuration
       value. The same is true for interface and alternate setting numbers.

.. [#] Actually things are a little more complex, but this simple explanation
       is enough for us.

.. [#] I know it sounds weird.

.. [#] This is because if there is no bandwidth for isochronous transfer at the
       device configuration time, the device can be successfully enumerated.

.. [#] This does not happen for configurations because a device is allowed to
       be in an unconfigured state.

.. [#] In PyUSB, control transfers are only issued in the endpoint 0. It's very
       very very rare a device having an alternate control endpoint (I've never
       seem such device).

.. [#] It's just a joke, don't take it seriously. Many choices is better than
       no choice.

.. _libusb: http://www.libusb.org
.. _OpenUSB: http://sourceforge.net/p/openusb/wiki/Home/
.. _USB: http://www.usb.org
.. _PyUSB: http://pyusb.wiki.sourceforge.net
.. _Python: http://www.python.org
.. _ctypes: http://docs.python.org/library/ctypes.html
.. _Descriptor: http://www.beyondlogic.org/usbnutshell/usb5.htm
.. _array: http://docs.python.org/library/array.html