Internal API

Here we document the odds and ends that are more helpful for creating your own interfaces or listeners but generally shouldn’t be required to interact with python-can.

Extending the BusABC class

Concrete implementations must implement the following:
  • send() to send individual messages
  • _recv_internal() to receive individual messages (see note below!)
  • set the channel_info attribute to a string describing the underlying bus and/or channel
They might implement the following:
  • flush_tx_buffer() to allow discarding any messages yet to be sent
  • shutdown() to override how the bus should shut down
  • _send_periodic_internal() to override the software based periodic sending and push it down to the kernel or hardware.
  • _apply_filters() to apply efficient filters to lower level systems like the OS kernel or hardware.
  • _detect_available_configs() to allow the interface to report which configurations are currently available for new connections.
  • state() property to allow reading and/or changing the bus state.

Note

TL;DR: Only override _recv_internal(), never recv() directly.

Previously, concrete bus classes had to override recv() directly instead of _recv_internal(), but that has changed to allow the abstract base class to handle in-software message filtering as a fallback. All internal interfaces now implement that new behaviour. Older (custom) interfaces might still be implemented like that and thus might not provide message filtering:

Concrete instances are usually created by can.Bus which takes the users configuration into account.

Bus Internals

Several methods are not documented in the main can.BusABC as they are primarily useful for library developers as opposed to library users. This is the entire ABC bus class with all internal methods:

class can.BusABC(channel, can_filters=None, **kwargs)[source]

Bases: object

The CAN Bus Abstract Base Class that serves as the basis for all concrete interfaces.

This class may be used as an iterator over the received messages.

Construct and open a CAN bus instance of the specified type.

Subclasses should call though this method with all given parameters as it handles generic tasks like applying filters.

Parameters:
  • channel (Any) – The can interface identifier. Expected type is backend dependent.
  • can_filters (Optional[Sequence[Union[CanFilter, CanFilterExtended]]]) – See set_filters() for details.
  • kwargs (dict) – Any backend dependent configurations are passed in this dictionary
RECV_LOGGING_LEVEL = 9

Log level for received messages

__init__(channel, can_filters=None, **kwargs)[source]

Construct and open a CAN bus instance of the specified type.

Subclasses should call though this method with all given parameters as it handles generic tasks like applying filters.

Parameters:
  • channel (Any) – The can interface identifier. Expected type is backend dependent.
  • can_filters (Optional[Sequence[Union[CanFilter, CanFilterExtended]]]) – See set_filters() for details.
  • kwargs (dict) – Any backend dependent configurations are passed in this dictionary
__iter__()[source]

Allow iteration on messages as they are received.

>>> for msg in bus:
...     print(msg)
Yields:Message msg objects.
Return type:Iterator[Message]
__str__()[source]

Return str(self).

Return type:str
__weakref__

list of weak references to the object (if defined)

_apply_filters(filters)[source]

Hook for applying the filters to the underlying kernel or hardware if supported/implemented by the interface.

Parameters:filters (Optional[Sequence[Union[CanFilter, CanFilterExtended]]]) – See set_filters() for details.
static _detect_available_configs()[source]

Detect all configurations/channels that this interface could currently connect with.

This might be quite time consuming.

May not to be implemented by every interface on every platform.

Return type:List[AutoDetectedConfig]
Returns:an iterable of dicts, each being a configuration suitable for usage in the interface’s bus constructor.
_matches_filters(msg)[source]

Checks whether the given message matches at least one of the current filters. See set_filters() for details on how the filters work.

This method should not be overridden.

Parameters:msg (Message) – the message to check if matching
Return type:bool
Returns:whether the given message matches at least one filter
_recv_internal(timeout)[source]

Read a message from the bus and tell whether it was filtered. This methods may be called by recv() to read a message multiple times if the filters set by set_filters() do not match and the call has not yet timed out.

New implementations should always override this method instead of recv(), to be able to take advantage of the software based filtering provided by recv() as a fallback. This method should never be called directly.

Note

This method is not an @abstractmethod (for now) to allow older external implementations to continue using their existing recv() implementation.

Note

The second return value (whether filtering was already done) may change over time for some interfaces, like for example in the Kvaser interface. Thus it cannot be simplified to a constant value.

Parameters:

timeout (float) – seconds to wait for a message, see send()

Return type:

Tuple[Optional[Message], bool]

Returns:

  1. a message that was read or None on timeout
  2. a bool that is True if message filtering has already been done and else False

Raises:
_send_periodic_internal(msgs, period, duration=None)[source]

Default implementation of periodic message sending using threading.

Override this method to enable a more efficient backend specific approach.

Parameters:
  • msgs (Union[Sequence[Message], Message]) – Messages to transmit
  • period (float) – Period in seconds between each message
  • duration (Optional[float]) – The duration between sending each message at the given rate. If no duration is provided, the task will continue indefinitely.
Return type:

CyclicSendTaskABC

Returns:

A started task instance. Note the task can be stopped (and depending on the backend modified) by calling the stop() method.

channel_info = 'unknown'

a string describing the underlying bus and/or channel

filters

Modify the filters of this bus. See set_filters() for details.

Return type:Optional[Sequence[Union[CanFilter, CanFilterExtended]]]
flush_tx_buffer()[source]

Discard every message that may be queued in the output buffer(s).

recv(timeout=None)[source]

Block waiting for a message from the Bus.

Parameters:timeout (Optional[float]) – seconds to wait for a message or None to wait indefinitely
Return type:Optional[Message]
Returns:None on timeout or a Message object.
Raises:can.CanError – if an error occurred while reading
send(msg, timeout=None)[source]

Transmit a message to the CAN bus.

Override this method to enable the transmit path.

Parameters:
  • msg (Message) – A message object.
  • timeout (Optional[float]) – If > 0, wait up to this many seconds for message to be ACK’ed or for transmit queue to be ready depending on driver implementation. If timeout is exceeded, an exception will be raised. Might not be supported by all interfaces. None blocks indefinitely.
Raises:

can.CanError – if the message could not be sent

send_periodic(msgs, period, duration=None, store_task=True)[source]

Start sending messages at a given period on this bus.

The task will be active until one of the following conditions are met:

  • the (optional) duration expires
  • the Bus instance goes out of scope
  • the Bus instance is shutdown
  • BusABC.stop_all_periodic_tasks() is called
  • the task’s CyclicTask.stop() method is called.
Parameters:
  • msgs (Union[Sequence[Message], Message]) – Messages to transmit
  • period (float) – Period in seconds between each message
  • duration (Optional[float]) – Approximate duration in seconds to continue sending messages. If no duration is provided, the task will continue indefinitely.
  • store_task (bool) – If True (the default) the task will be attached to this Bus instance. Disable to instead manage tasks manually.
Return type:

CyclicSendTaskABC

Returns:

A started task instance. Note the task can be stopped (and depending on the backend modified) by calling the task’s stop() method.

Note

Note the duration before the messages stop being sent may not be exactly the same as the duration specified by the user. In general the message will be sent at the given rate until at least duration seconds.

Note

For extremely long running Bus instances with many short lived tasks the default api with store_task==True may not be appropriate as the stopped tasks are still taking up memory as they are associated with the Bus instance.

set_filters(filters=None)[source]

Apply filtering to all messages received by this Bus.

All messages that match at least one filter are returned. If filters is None or a zero length sequence, all messages are matched.

Calling without passing any filters will reset the applied filters to None.

Parameters:filters (Optional[Sequence[Union[CanFilter, CanFilterExtended]]]) –

A iterable of dictionaries each containing a “can_id”, a “can_mask”, and an optional “extended” key.

>>> [{"can_id": 0x11, "can_mask": 0x21, "extended": False}]

A filter matches, when <received_can_id> & can_mask == can_id & can_mask. If extended is set as well, it only matches messages where <received_is_extended> == extended. Else it matches every messages based only on the arbitration ID and mask.

shutdown()[source]

Called to carry out any interface specific cleanup required in shutting down a bus.

state

Return the current state of the hardware

Return type:BusState
stop_all_periodic_tasks(remove_tasks=True)[source]

Stop sending any messages that were started using bus.send_periodic.

Note

The result is undefined if a single task throws an exception while being stopped.

Parameters:remove_tasks (bool) – Stop tracking the stopped tasks.

About the IO module

Handling of the different file formats is implemented in can.io. Each file/IO type is within a separate module and ideally implements both a Reader and a Writer. The reader usually extends can.io.generic.BaseIOHandler, while the writer often additionally extends can.Listener, to be able to be passed directly to a can.Notifier.

Adding support for new file formats

This assumes that you want to add a new file format, called canstore. Ideally add both reading and writing support for the new file format, although this is not strictly required.

  1. Create a new module: can/io/canstore.py (or simply copy some existing one like can/io/csv.py)
  2. Implement a reader CanstoreReader (which often extends can.io.generic.BaseIOHandler, but does not have to). Besides from a constructor, only __iter__(self) needs to be implemented.
  3. Implement a writer CanstoreWriter (which often extends can.io.generic.BaseIOHandler and can.Listener, but does not have to). Besides from a constructor, only on_message_received(self, msg) needs to be implemented.
  4. Add a case to can.io.player.LogReader’s __new__().
  5. Document the two new classes (and possibly additional helpers) with docstrings and comments. Please mention features and limitations of the implementation.
  6. Add a short section to the bottom of doc/listeners.rst.
  7. Add tests where appropriate, for example by simply adding a test case called class TestCanstoreFileFormat(ReaderWriterTest) to test/logformats_test.py. That should already handle all of the general testing. Just follow the way the other tests in there do it.
  8. Add imports to can/__init__py and can/io/__init__py so that the new classes can be simply imported as from can import CanstoreReader, CanstoreWriter.

IO Utilities

Contains a generic class for file IO.

class can.io.generic.BaseIOHandler(file, mode='rt')[source]

Bases: object

A generic file handler that can be used for reading and writing.

Can be used as a context manager.

Attr Optional[FileLike] file:
 

the file-like object that is kept internally, or None if none was opened

Parameters:
  • file (Union[Io[Any], str, Forwardref, None]) – a path-like object to open a file, a file-like object to be used as a file or None to not use a file at all
  • mode (str) – the mode that should be used to open the file, see open(), ignored if file is None
stop()[source]

Closes the undelying file-like object and flushes it, if it was opened in write mode.

Return type:None
class can.io.generic.MessageReader(file, mode='rt')[source]

Bases: can.io.generic.BaseIOHandler

The base class for all readers.

Parameters:
  • file (Union[Io[Any], str, Forwardref, None]) – a path-like object to open a file, a file-like object to be used as a file or None to not use a file at all
  • mode (str) – the mode that should be used to open the file, see open(), ignored if file is None
class can.io.generic.MessageWriter(file, mode='rt')[source]

Bases: can.io.generic.BaseIOHandler, can.listener.Listener

The base class for all writers.

Parameters:
  • file (Union[Io[Any], str, Forwardref, None]) – a path-like object to open a file, a file-like object to be used as a file or None to not use a file at all
  • mode (str) – the mode that should be used to open the file, see open(), ignored if file is None

Other Utilities

Utilities and configuration file parsing.

can.util.channel2int(channel)[source]

Try to convert the channel to an integer.

Parameters:channel (Union[int, str, None]) – Channel string (e.g. can0, CAN1) or integer
Return type:Optional[int]
Returns:Channel integer or None if unsuccessful
can.util.deprecated_args_alias(**aliases)[source]

Allows to rename/deprecate a function kwarg(s) and optionally have the deprecated kwarg(s) set as alias(es)

Example:

@deprecated_args_alias(oldArg=”new_arg”, anotherOldArg=”another_new_arg”) def library_function(new_arg, another_new_arg):

pass

@deprecated_args_alias(oldArg=”new_arg”, obsoleteOldArg=None) def library_function(new_arg):

pass
can.util.dlc2len(dlc)[source]

Calculate the data length from DLC.

Parameters:dlc (int) – DLC (0-15)
Return type:int
Returns:Data length in number of bytes (0-64)
can.util.len2dlc(length)[source]

Calculate the DLC from data length.

Parameters:length (int) – Length in number of bytes (0-64)
Return type:int
Returns:DLC (0-15)
can.util.load_config(path=None, config=None, context=None)[source]

Returns a dict with configuration details which is loaded from (in this order):

  • config
  • can.rc
  • Environment variables CAN_INTERFACE, CAN_CHANNEL, CAN_BITRATE
  • Config files /etc/can.conf or ~/.can or ~/.canrc where the latter may add or replace values of the former.

Interface can be any of the strings from can.VALID_INTERFACES for example: kvaser, socketcan, pcan, usb2can, ixxat, nican, virtual.

Note

The key bustype is copied to interface if that one is missing and does never appear in the result.

Parameters:
  • path – Optional path to config file.
  • config – A dict which may set the ‘interface’, and/or the ‘channel’, or neither. It may set other values that are passed through.
  • context – Extra ‘context’ pass to config sources. This can be use to section other than ‘default’ in the configuration file.
Returns:

A config dictionary that should contain ‘interface’ & ‘channel’:

{
    'interface': 'python-can backend interface to use',
    'channel': 'default channel to use',
    # possibly more
}

Note None will be used if all the options are exhausted without finding a value.

All unused values are passed from config over to this.

Raises:

NotImplementedError if the interface isn’t recognized

can.util.load_environment_config(context=None)[source]

Loads config dict from environmental variables (if set):

  • CAN_INTERFACE
  • CAN_CHANNEL
  • CAN_BITRATE
  • CAN_CONFIG

if context is supplied, “_{context}” is appended to the environment variable name we will look at. For example if context=”ABC”:

  • CAN_INTERFACE_ABC
  • CAN_CHANNEL_ABC
  • CAN_BITRATE_ABC
  • CAN_CONFIG_ABC
Return type:Dict[str, str]
can.util.load_file_config(path=None, section='default')[source]

Loads configuration from file with following content:

[default]
interface = socketcan
channel = can0
Parameters:
  • path – path to config file. If not specified, several sensible default locations are tried depending on platform.
  • section – name of the section to read configuration from.
can.util.rename_kwargs(func_name, kwargs, aliases)[source]

Helper function for deprecated_args_alias

can.util.set_logging_level(level_name=None)[source]

Set the logging level for the “can” logger. Expects one of: ‘critical’, ‘error’, ‘warning’, ‘info’, ‘debug’, ‘subdebug’