_lcm_extra.py

# See `ExecuteExtraPythonCode` in `pydrake_pybind.h` for usage details and
# rationale.


# This is a python reimplementation of drake::lcm::Subscriber.  We reimplement
# (rather than bind) the C++ class because we want message() to be a python LCM
# message, not a C++ object.
class Subscriber:
    """Subscribes to and stores a copy of the most recent message on a given
    channel, for some message type.  This class does NOT provide any mutex
    behavior for multi-threaded locking; it should only be used in cases where
    the governing DrakeLcmInterface.HandleSubscriptions is called from the same
    thread that owns all copies of this object.
    """

    def __init__(self, lcm, channel, lcm_type):
        """Creates a subscriber and subscribes to `channel` on `lcm`.

        Args:
            lcm: LCM service instance (a pydrake.lcm.DrakeLcmInterface).
            channel: LCM channel name.
            lcm_type: Python class generated by lcmgen.
        """
        self.lcm_type = lcm_type
        self.clear()
        lcm.Subscribe(channel=channel, handler=self._handler)

    def clear(self):
        self.count = 0
        self.raw = []
        self.message = self.lcm_type()

    def _handler(self, raw):
        self.count += 1
        self.raw = raw
        self.message = self.lcm_type.decode(raw)