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98 lines
3.1 KiB
ReStructuredText
98 lines
3.1 KiB
ReStructuredText
.. BACpypes tutorial lesson 1
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Sample 2 - Who-Is/I-Am Counter
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==============================
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This sample application builds on the first sample by overriding the default
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processing for Who-Is and I-Am requests, counting them, then continuing on
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with the regular processing.
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The description of this sample will be about the parts that are different from
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sample 1.
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Counters
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--------
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Python has a excellent *defaultdict* datatype from the *collections* module
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that is perfect for this application. It is very easy to use::
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>>> from collections import defaultdict
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>>> x = defaultdict(int)
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The essential idea is that you can treat some key as having a default value
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if it doesn't exist, so rather than doing this::
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>>> x['a'] = x.get('a', 0) + 1
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You can do this::
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>>> x['a'] += 1
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Processing Service Requests
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---------------------------
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When an instance of the :class:`app.Application` receives a request it attempts
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to look up a function based on the message. So when a WhoIsRequest APDU is
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received, there should be a do_WhoIsRequest function.
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The beginning is going to be standard boiler plate function header::
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def do_WhoIsRequest(self, apdu):
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"""Respond to a Who-Is request."""
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if _debug: SampleApplication._debug("do_WhoIsRequest %r", apdu)
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The middle is going to process the data in the request::
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# build a key from the source and parameters
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key = (str(apdu.pduSource),
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apdu.deviceInstanceRangeLowLimit,
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apdu.deviceInstanceRangeHighLimit,
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)
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# count the times this has been received
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whoIsCounter[key] += 1
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And the end of the function is going to call back to the standard application
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processing::
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# pass back to the default implementation
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BIPSimpleApplication.do_WhoIsRequest(self, apdu)
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The do_IAmRequest function is similer::
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def do_IAmRequest(self, apdu):
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"""Given an I-Am request, cache it."""
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if _debug: SampleApplication._debug("do_IAmRequest %r", apdu)
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It uses a diferent key, but counts them the same::
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# build a key from the source, just use the instance number
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key = (str(apdu.pduSource),
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apdu.iAmDeviceIdentifier[1],
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)
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# count the times this has been received
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iAmCounter[key] += 1
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And has an identical call to the base class::
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# pass back to the default implementation
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BIPSimpleApplication.do_IAmRequest(self, apdu)
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Printing Results
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----------------
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By building the key out of elements in a useful order, it is simple enough
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to sort the dictionary items and print them out, and being able to unpack
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the key in the for loop is a nice feature of Python::
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print "----- Who Is -----"
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for (src, lowlim, hilim), count in sorted(whoIsCounter.items()):
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print "%-20s %8s %8s %4d" % (src, lowlim, hilim, count)
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print
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Pairing up the requests and responses can be a useful excersize, but in most
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cases the I-Am response from a device will be a unicast message directly back
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to the requestor, so relying on broadcast traffic to analyze device and
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address binding is not as useful as it used to be.
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