# Source code for gpiozero.pins

    # vim: set fileencoding=utf-8:
    #
    # GPIO Zero: a library for controlling the Raspberry Pi's GPIO pins
    #
    # Copyright (c) 2015-2021 Dave Jones <dave@waveform.org.uk>
    # Copyright (c) 2018 Rick Ansell <rick@nbinvincible.org.uk>
    # Copyright (c) 2018 Mike Kazantsev <mk.fraggod@gmail.com>
    # Copyright (c) 2016 Andrew Scheller <github@loowis.durge.org>
    #
    # SPDX-License-Identifier: BSD-3-Clause

    from __future__ import (
        unicode_literals,
        absolute_import,
        print_function,
        division,
        )
    str = type('')

    from weakref import ref
    from collections import defaultdict
    from threading import Lock

    from ..devices import Device
    from ..exc import (
        PinInvalidFunction,
        PinSetInput,
        PinFixedPull,
        PinUnsupported,
        PinSPIUnsupported,
        PinPWMUnsupported,
        PinEdgeDetectUnsupported,
        SPIFixedClockMode,
        SPIFixedBitOrder,
        SPIFixedSelect,
        SPIFixedWordSize,
        SPIFixedRate,
        GPIOPinInUse,
        )

    class Factory(object):
        """
        Generates pins and SPI interfaces for devices. This is an abstract
        base class for pin factories. Descendents *must* override the following
        methods:

        * :meth:`ticks`
        * :meth:`ticks_diff`

        Descendents *may* override the following methods, if applicable:

        * :meth:`close`
        * :meth:`reserve_pins`
        * :meth:`release_pins`
        * :meth:`release_all`
        * :meth:`pin`
        * :meth:`spi`
        * :meth:`_get_pi_info`
        """
        def __init__(self):
            self._reservations = defaultdict(list)
            self._res_lock = Lock()

        def reserve_pins(self, requester, *pins):
            """
            Called to indicate that the device reserves the right to use the
            specified *pins*. This should be done during device construction.  If
            pins are reserved, you must ensure that the reservation is released by
            eventually called :meth:`release_pins`.
            """
            with self._res_lock:
                for pin in pins:
                    for reserver_ref in self._reservations[pin]:
                        reserver = reserver_ref()
                        if reserver is not None and requester._conflicts_with(reserver):
                            raise GPIOPinInUse('pin %s is already in use by %r' %
                                               (pin, reserver))
                    self._reservations[pin].append(ref(requester))

        def release_pins(self, reserver, *pins):
            """
            Releases the reservation of *reserver* against *pins*.  This is
            typically called during :meth:`~gpiozero.Device.close` to clean up
            reservations taken during construction. Releasing a reservation that is
            not currently held will be silently ignored (to permit clean-up after
            failed / partial construction).
            """
            with self._res_lock:
                for pin in pins:
                    self._reservations[pin] = [
                        ref for ref in self._reservations[pin]
                        if ref() not in (reserver, None) # may as well clean up dead refs
                        ]

        def release_all(self, reserver):
            """
            Releases all pin reservations taken out by *reserver*. See
            :meth:`release_pins` for further information).
            """
            # Yes, this would be more efficient if it simply regenerated the
            # reservations list without any references to reserver instead of
            # (in release_pins) looping over each pin individually. However, this
            # then causes a subtle bug in LocalPiFactory which does something
            # horribly naughty (with good reason) and makes its _reservations
            # dictionary equivalent to a class-level one.
            self.release_pins(reserver, *self._reservations)

        def close(self):
            """
            Closes the pin factory. This is expected to clean up all resources
            manipulated by the factory. It it typically called at script
            termination.
            """
            pass

        def pin(self, spec):
            """
            Creates an instance of a :class:`Pin` descendent representing the
            specified pin.

            .. warning::

                Descendents must ensure that pin instances representing the same
                hardware are identical; i.e. two separate invocations of
                :meth:`pin` for the same pin specification must return the same
                object.
            """
            raise PinUnsupported(  # pragma: no cover
                "Individual pins are not supported by this pin factory")

        def spi(self, **spi_args):
            """
            Returns an instance of an :class:`SPI` interface, for the specified SPI
            *port* and *device*, or for the specified pins (*clock_pin*,
            *mosi_pin*, *miso_pin*, and *select_pin*).  Only one of the schemes can
            be used; attempting to mix *port* and *device* with pin numbers will
            raise :exc:`SPIBadArgs`.
            """
            raise PinSPIUnsupported(  # pragma: no cover
                'SPI not supported by this pin factory')

        def ticks(self):
            """
            Return the current ticks, according to the factory. The reference point
            is undefined and thus the result of this method is only meaningful when
            compared to another value returned by this method.

            The format of the time is also arbitrary, as is whether the time wraps
            after a certain duration. Ticks should only be compared using the
            :meth:`ticks_diff` method.
            """
            raise NotImplementedError

        def ticks_diff(self, later, earlier):
            """
            Return the time in seconds between two :meth:`ticks` results. The
            arguments are specified in the same order as they would be in the
            formula *later* - *earlier* but the result is guaranteed to be in
            seconds, and to be positive even if the ticks "wrapped" between calls
            to :meth:`ticks`.
            """
            raise NotImplementedError

        def _get_pi_info(self):
            return None  # pragma: no cover

        pi_info = property(
            lambda self: self._get_pi_info(),
            doc="""\
            Returns a :class:`PiBoardInfo` instance representing the Pi that
            instances generated by this factory will be attached to.

            If the pins represented by this class are not *directly* attached to a
            Pi (e.g. the pin is attached to a board attached to the Pi, or the pins
            are not on a Pi at all), this may return :data:`None`.
            """)

    class Pin(object):
        """
        Abstract base class representing a pin attached to some form of controller,
        be it GPIO, SPI, ADC, etc.

        Descendents should override property getters and setters to accurately
        represent the capabilities of pins. Descendents *must* override the
        following methods:

        * :meth:`_get_function`
        * :meth:`_set_function`
        * :meth:`_get_state`

        Descendents *may* additionally override the following methods, if
        applicable:

        * :meth:`close`
        * :meth:`output_with_state`
        * :meth:`input_with_pull`
        * :meth:`_set_state`
        * :meth:`_get_frequency`
        * :meth:`_set_frequency`
        * :meth:`_get_pull`
        * :meth:`_set_pull`
        * :meth:`_get_bounce`
        * :meth:`_set_bounce`
        * :meth:`_get_edges`
        * :meth:`_set_edges`
        * :meth:`_get_when_changed`
        * :meth:`_set_when_changed`
        """

        def __repr__(self):
            return "<Pin>"  # pragma: no cover

        def close(self):
            """
            Cleans up the resources allocated to the pin. After this method is
            called, this :class:`Pin` instance may no longer be used to query or
            control the pin's state.
            """
            pass

        def output_with_state(self, state):
            """
            Sets the pin's function to "output" and specifies an initial state
            for the pin. By default this is equivalent to performing::

                pin.function = 'output'
                pin.state = state

            However, descendents may override this in order to provide the smallest
            possible delay between configuring the pin for output and specifying an
            initial value (which can be important for avoiding "blips" in
            active-low configurations).
            """
            self.function = 'output'
            self.state = state

        def input_with_pull(self, pull):
            """
            Sets the pin's function to "input" and specifies an initial pull-up
            for the pin. By default this is equivalent to performing::

                pin.function = 'input'
                pin.pull = pull

            However, descendents may override this order to provide the smallest
            possible delay between configuring the pin for input and pulling the
            pin up/down (which can be important for avoiding "blips" in some
            configurations).
            """
            self.function = 'input'
            self.pull = pull

        def _get_function(self):
            raise NotImplementedError

        def _set_function(self, value):
            raise NotImplementedError

        function = property(
            lambda self: self._get_function(),
            lambda self, value: self._set_function(value),
            doc="""\
            The function of the pin. This property is a string indicating the
            current function or purpose of the pin. Typically this is the string
            "input" or "output". However, in some circumstances it can be other
            strings indicating non-GPIO related functionality.

            With certain pin types (e.g. GPIO pins), this attribute can be changed
            to configure the function of a pin. If an invalid function is
            specified, for this attribute, :exc:`PinInvalidFunction` will be
            raised.
            """)

        def _get_state(self):
            raise NotImplementedError

        def _set_state(self, value):
            raise PinSetInput(  # pragma: no cover
                "Cannot set the state of pin %r" % self)

        state = property(
            lambda self: self._get_state(),
            lambda self, value: self._set_state(value),
            doc="""\
            The state of the pin. This is 0 for low, and 1 for high. As a low level
            view of the pin, no swapping is performed in the case of pull ups (see
            :attr:`pull` for more information):

            .. code-block:: text

                HIGH - - - - >       ,----------------------
                                     |
                                     |
                LOW  ----------------'

            Descendents which implement analog, or analog-like capabilities can
            return values between 0 and 1. For example, pins implementing PWM
            (where :attr:`frequency` is not :data:`None`) return a value between
            0.0 and 1.0 representing the current PWM duty cycle.

            If a pin is currently configured for input, and an attempt is made to
            set this attribute, :exc:`PinSetInput` will be raised. If an invalid
            value is specified for this attribute, :exc:`PinInvalidState` will be
            raised.
            """)

        def _get_pull(self):
            return 'floating'  # pragma: no cover

        def _set_pull(self, value):
            raise PinFixedPull(  # pragma: no cover
                "Cannot change pull-up on pin %r" % self)

        pull = property(
            lambda self: self._get_pull(),
            lambda self, value: self._set_pull(value),
            doc="""\
            The pull-up state of the pin represented as a string. This is typically
            one of the strings "up", "down", or "floating" but additional values
            may be supported by the underlying hardware.

            If the pin does not support changing pull-up state (for example because
            of a fixed pull-up resistor), attempts to set this property will raise
            :exc:`PinFixedPull`. If the specified value is not supported by the
            underlying hardware, :exc:`PinInvalidPull` is raised.
            """)

        def _get_frequency(self):
            return None  # pragma: no cover

        def _set_frequency(self, value):
            if value is not None:
                raise PinPWMUnsupported(  # pragma: no cover
                    "PWM is not supported on pin %r" % self)

        frequency = property(
            lambda self: self._get_frequency(),
            lambda self, value: self._set_frequency(value),
            doc="""\
            The frequency (in Hz) for the pin's PWM implementation, or :data:`None`
            if PWM is not currently in use. This value always defaults to
            :data:`None` and may be changed with certain pin types to activate or
            deactivate PWM.

            If the pin does not support PWM, :exc:`PinPWMUnsupported` will be
            raised when attempting to set this to a value other than :data:`None`.
            """)

        def _get_bounce(self):
            return None  # pragma: no cover

        def _set_bounce(self, value):
            if value is not None:  # pragma: no cover
                raise PinEdgeDetectUnsupported(
                    "Edge detection is not supported on pin %r" % self)

        bounce = property(
            lambda self: self._get_bounce(),
            lambda self, value: self._set_bounce(value),
            doc="""\
            The amount of bounce detection (elimination) currently in use by edge
            detection, measured in seconds. If bounce detection is not currently in
            use, this is :data:`None`.

            For example, if :attr:`edges` is currently "rising", :attr:`bounce` is
            currently 5/1000 (5ms), then the waveform below will only fire
            :attr:`when_changed` on two occasions despite there being three rising
            edges:

            .. code-block:: text

                TIME 0...1...2...3...4...5...6...7...8...9...10..11..12 ms

                bounce elimination   |===================| |==============

                HIGH - - - - >       ,--. ,--------------. ,--.
                                     |  | |              | |  |
                                     |  | |              | |  |
                LOW  ----------------'  `-'              `-'  `-----------
                                     :                     :
                                     :                     :
                               when_changed          when_changed
                                   fires                 fires

            If the pin does not support edge detection, attempts to set this
            property will raise :exc:`PinEdgeDetectUnsupported`. If the pin
            supports edge detection, the class must implement bounce detection,
            even if only in software.
            """)

        def _get_edges(self):
            return 'none'  # pragma: no cover

        def _set_edges(self, value):
            raise PinEdgeDetectUnsupported(  # pragma: no cover
                "Edge detection is not supported on pin %r" % self)

        edges = property(
            lambda self: self._get_edges(),
            lambda self, value: self._set_edges(value),
            doc="""\
            The edge that will trigger execution of the function or bound method
            assigned to :attr:`when_changed`. This can be one of the strings
            "both" (the default), "rising", "falling", or "none":

            .. code-block:: text

                HIGH - - - - >           ,--------------.
                                         |              |
                                         |              |
                LOW  --------------------'              `--------------
                                         :              :
                                         :              :
                Fires when_changed     "both"         "both"
                when edges is ...     "rising"       "falling"

            If the pin does not support edge detection, attempts to set this
            property will raise :exc:`PinEdgeDetectUnsupported`.
            """)

        def _get_when_changed(self):
            return None  # pragma: no cover

        def _set_when_changed(self, value):
            raise PinEdgeDetectUnsupported(  # pragma: no cover
                "Edge detection is not supported on pin %r" % self)

        when_changed = property(
            lambda self: self._get_when_changed(),
            lambda self, value: self._set_when_changed(value),
            doc="""\
            A function or bound method to be called when the pin's state changes
            (more specifically when the edge specified by :attr:`edges` is detected
            on the pin). The function or bound method must accept two parameters:
            the first will report the ticks (from :meth:`Factory.ticks`) when
            the pin's state changed, and the second will report the pin's current
            state.

            .. warning::

                Depending on hardware support, the state is *not guaranteed to be
                accurate*. For instance, many GPIO implementations will provide
                an interrupt indicating when a pin's state changed but not what it
                changed to. In this case the pin driver simply reads the pin's
                current state to supply this parameter, but the pin's state may
                have changed *since* the interrupt. Exercise appropriate caution
                when relying upon this parameter.

            If the pin does not support edge detection, attempts to set this
            property will raise :exc:`PinEdgeDetectUnsupported`.
            """)

    class SPI(Device):
        """
        Abstract interface for `Serial Peripheral Interface`_ (SPI)
        implementations. Descendents *must* override the following methods:

        * :meth:`transfer`
        * :meth:`_get_clock_mode`

        Descendents *may* override the following methods, if applicable:

        * :meth:`read`
        * :meth:`write`
        * :meth:`_set_clock_mode`
        * :meth:`_get_lsb_first`
        * :meth:`_set_lsb_first`
        * :meth:`_get_select_high`
        * :meth:`_set_select_high`
        * :meth:`_get_bits_per_word`
        * :meth:`_set_bits_per_word`

        .. _Serial Peripheral Interface: https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus
        """

        def read(self, n):
            """
            Read *n* words of data from the SPI interface, returning them as a
            sequence of unsigned ints, each no larger than the configured
            :attr:`bits_per_word` of the interface.

            This method is typically used with read-only devices that feature
            half-duplex communication. See :meth:`transfer` for full duplex
            communication.
            """
            return self.transfer([0] * n)

        def write(self, data):
            """
            Write *data* to the SPI interface. *data* must be a sequence of
            unsigned integer words each of which will fit within the configured
            :attr:`bits_per_word` of the interface. The method returns the number
            of words written to the interface (which may be less than or equal to
            the length of *data*).

            This method is typically used with write-only devices that feature
            half-duplex communication. See :meth:`transfer` for full duplex
            communication.
            """
            return len(self.transfer(data))

        def transfer(self, data):
            """
            Write *data* to the SPI interface. *data* must be a sequence of
            unsigned integer words each of which will fit within the configured
            :attr:`bits_per_word` of the interface. The method returns the sequence
            of words read from the interface while writing occurred (full duplex
            communication).

            The length of the sequence returned dictates the number of words of
            *data* written to the interface. Each word in the returned sequence
            will be an unsigned integer no larger than the configured
            :attr:`bits_per_word` of the interface.
            """
            raise NotImplementedError

        @property
        def clock_polarity(self):
            """
            The polarity of the SPI clock pin. If this is :data:`False` (the
            default), the clock pin will idle low, and pulse high. Setting this to
            :data:`True` will cause the clock pin to idle high, and pulse low. On
            many data sheets this is documented as the CPOL value.

            The following diagram illustrates the waveform when
            :attr:`clock_polarity` is :data:`False` (the default), equivalent to
            CPOL 0:

            .. code-block:: text

                       on      on      on      on      on      on      on
                      ,---.   ,---.   ,---.   ,---.   ,---.   ,---.   ,---.
                CLK   |   |   |   |   |   |   |   |   |   |   |   |   |   |
                      |   |   |   |   |   |   |   |   |   |   |   |   |   |
                ------'   `---'   `---'   `---'   `---'   `---'   `---'   `------
                idle       off     off     off     off     off     off       idle

            The following diagram illustrates the waveform when
            :attr:`clock_polarity` is :data:`True`, equivalent to CPOL 1:

            .. code-block:: text

                idle       off     off     off     off     off     off       idle
                ------.   ,---.   ,---.   ,---.   ,---.   ,---.   ,---.   ,------
                      |   |   |   |   |   |   |   |   |   |   |   |   |   |
                CLK   |   |   |   |   |   |   |   |   |   |   |   |   |   |
                      `---'   `---'   `---'   `---'   `---'   `---'   `---'
                       on      on      on      on      on      on      on
            """
            return bool(self.clock_mode & 2)

        @clock_polarity.setter
        def clock_polarity(self, value):
            self.clock_mode = self.clock_mode & (~2) | (bool(value) << 1)

        @property
        def clock_phase(self):
            """
            The phase of the SPI clock pin. If this is :data:`False` (the default),
            data will be read from the MISO pin when the clock pin activates.
            Setting this to :data:`True` will cause data to be read from the MISO
            pin when the clock pin deactivates. On many data sheets this is
            documented as the CPHA value. Whether the clock edge is rising or
            falling when the clock is considered activated is controlled by the
            :attr:`clock_polarity` attribute (corresponding to CPOL).

            The following diagram indicates when data is read when
            :attr:`clock_polarity` is :data:`False`, and :attr:`clock_phase` is
            :data:`False` (the default), equivalent to CPHA 0:

            .. code-block:: text

                    ,---.   ,---.   ,---.   ,---.   ,---.   ,---.   ,---.
                CLK |   |   |   |   |   |   |   |   |   |   |   |   |   |
                    |   |   |   |   |   |   |   |   |   |   |   |   |   |
                ----'   `---'   `---'   `---'   `---'   `---'   `---'   `-------
                    :       :       :       :       :       :       :
                MISO---.   ,---.   ,---.   ,---.   ,---.   ,---.   ,---.
                  /     \\ /     \\ /     \\ /     \\ /     \\ /     \\ /     \\
                -{  Bit  X  Bit  X  Bit  X  Bit  X  Bit  X  Bit  X  Bit  }------
                  \\     / \\     / \\     / \\     / \\     / \\     / \\     /
                   `---'   `---'   `---'   `---'   `---'   `---'   `---'

            The following diagram indicates when data is read when
            :attr:`clock_polarity` is :data:`False`, but :attr:`clock_phase` is
            :data:`True`, equivalent to CPHA 1:

            .. code-block:: text

                    ,---.   ,---.   ,---.   ,---.   ,---.   ,---.   ,---.
                CLK |   |   |   |   |   |   |   |   |   |   |   |   |   |
                    |   |   |   |   |   |   |   |   |   |   |   |   |   |
                ----'   `---'   `---'   `---'   `---'   `---'   `---'   `-------
                        :       :       :       :       :       :       :
                MISO   ,---.   ,---.   ,---.   ,---.   ,---.   ,---.   ,---.
                      /     \\ /     \\ /     \\ /     \\ /     \\ /     \\ /     \\
                -----{  Bit  X  Bit  X  Bit  X  Bit  X  Bit  X  Bit  X  Bit  }--
                      \\     / \\     / \\     / \\     / \\     / \\     / \\     /
                       `---'   `---'   `---'   `---'   `---'   `---'   `---'
            """
            return bool(self.clock_mode & 1)

        @clock_phase.setter
        def clock_phase(self, value):
            self.clock_mode = self.clock_mode & (~1) | bool(value)

        def _get_clock_mode(self):
            raise NotImplementedError  # pragma: no cover

        def _set_clock_mode(self, value):
            raise SPIFixedClockMode(  # pragma: no cover
                "clock_mode cannot be changed on %r" % self)

        clock_mode = property(
            lambda self: self._get_clock_mode(),
            lambda self, value: self._set_clock_mode(value),
            doc="""\
            Presents a value representing the :attr:`clock_polarity` and
            :attr:`clock_phase` attributes combined according to the following
            table:

            +------+-----------------+--------------+
            | mode | polarity (CPOL) | phase (CPHA) |
            +======+=================+==============+
            | 0    | False           | False        |
            +------+-----------------+--------------+
            | 1    | False           | True         |
            +------+-----------------+--------------+
            | 2    | True            | False        |
            +------+-----------------+--------------+
            | 3    | True            | True         |
            +------+-----------------+--------------+

            Adjusting this value adjusts both the :attr:`clock_polarity` and
            :attr:`clock_phase` attributes simultaneously.
            """)

        def _get_lsb_first(self):
            return False  # pragma: no cover

        def _set_lsb_first(self, value):
            raise SPIFixedBitOrder(  # pragma: no cover
                "lsb_first cannot be changed on %r" % self)

        lsb_first = property(
            lambda self: self._get_lsb_first(),
            lambda self, value: self._set_lsb_first(value),
            doc="""\
            Controls whether words are read and written LSB in (Least Significant
            Bit first) order. The default is :data:`False` indicating that words
            are read and written in MSB (Most Significant Bit first) order.
            Effectively, this controls the `Bit endianness`_ of the connection.

            The following diagram shows the a word containing the number 5 (binary
            0101) transmitted on MISO with :attr:`bits_per_word` set to 4, and
            :attr:`clock_mode` set to 0, when :attr:`lsb_first` is :data:`False`
            (the default):

            .. code-block:: text

                    ,---.   ,---.   ,---.   ,---.
                CLK |   |   |   |   |   |   |   |
                    |   |   |   |   |   |   |   |
                ----'   `---'   `---'   `---'   `-----
                    :     ,-------. :     ,-------.
                MISO:     | :     | :     | :     |
                    :     | :     | :     | :     |
                ----------' :     `-------' :     `----
                    :       :       :       :
                   MSB                     LSB

            And now with :attr:`lsb_first` set to :data:`True` (and all other
            parameters the same):

            .. code-block:: text

                    ,---.   ,---.   ,---.   ,---.
                CLK |   |   |   |   |   |   |   |
                    |   |   |   |   |   |   |   |
                ----'   `---'   `---'   `---'   `-----
                  ,-------. :     ,-------. :
                MISO:     | :     | :     | :
                  | :     | :     | :     | :
                --' :     `-------' :     `-----------
                    :       :       :       :
                   LSB                     MSB

            .. _Bit endianness: https://en.wikipedia.org/wiki/Endianness#Bit_endianness
            """)

        def _get_select_high(self):
            return False  # pragma: no cover

        def _set_select_high(self, value):
            raise SPIFixedSelect(  # pragma: no cover
                "select_high cannot be changed on %r" % self)

        select_high = property(
            lambda self: self._get_select_high(),
            lambda self, value: self._set_select_high(value),
            doc="""\
            If :data:`False` (the default), the chip select line is considered
            active when it is pulled low. When set to :data:`True`, the chip select
            line is considered active when it is driven high.

            The following diagram shows the waveform of the chip select line, and
            the clock when :attr:`clock_polarity` is :data:`False`, and
            :attr:`select_high` is :data:`False` (the default):

            .. code-block:: text

                ---.                                                     ,------
                __ |                                                     |
                CS |      chip is selected, and will react to clock      |  idle
                   `-----------------------------------------------------'

                    ,---.   ,---.   ,---.   ,---.   ,---.   ,---.   ,---.
                CLK |   |   |   |   |   |   |   |   |   |   |   |   |   |
                    |   |   |   |   |   |   |   |   |   |   |   |   |   |
                ----'   `---'   `---'   `---'   `---'   `---'   `---'   `-------

            And when :attr:`select_high` is :data:`True`:

            .. code-block:: text

                   ,-----------------------------------------------------.
                CS |      chip is selected, and will react to clock      |  idle
                   |                                                     |
                ---'                                                     `------

                    ,---.   ,---.   ,---.   ,---.   ,---.   ,---.   ,---.
                CLK |   |   |   |   |   |   |   |   |   |   |   |   |   |
                    |   |   |   |   |   |   |   |   |   |   |   |   |   |
                ----'   `---'   `---'   `---'   `---'   `---'   `---'   `-------
            """)

        def _get_bits_per_word(self):
            return 8  # pragma: no cover

        def _set_bits_per_word(self, value):
            raise SPIFixedWordSize(  # pragma: no cover
                "bits_per_word cannot be changed on %r" % self)

        bits_per_word = property(
            lambda self: self._get_bits_per_word(),
            lambda self, value: self._set_bits_per_word(value),
            doc="""\
            Controls the number of bits that make up a word, and thus where the
            word boundaries appear in the data stream, and the maximum value of a
            word. Defaults to 8 meaning that words are effectively bytes.

            Several implementations do not support non-byte-sized words.
            """)

        def _get_rate(self):
            return 100000  # pragma: no cover

        def _set_rate(self, value):
            raise SPIFixedRate(  # pragma: no cover
                "rate cannot be changed on %r" % self)

        rate = property(
            lambda self: self._get_rate(),
            lambda self, value: self._set_rate(value),
            doc="""\
            Controls the speed of the SPI interface in Hz (or baud).

            Note that most software SPI implementations ignore this property, and
            will raise :exc:`SPIFixedRate` if an attempt is made to set it, as they
            have no rate control (they simply bit-bang as fast as possible because
            typically this isn't very fast anyway, and introducing measures to
            limit the rate would simply slow them down to the point of being
            useless).
            """)