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PORetrieve data/exception from a handler function
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  1. PORetrieve data/exception from a handler function
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    copied!<p>I'm trying to use Python and pywinusb-0.3.2 to read data from a USB weighing scale when a valid request hits the web server (<code>requested_serial_number</code> is coming from the query string).</p> <p>So I have the following code inside my <code>do_GET</code>:</p> <pre><code>all_hids = hid.find_all_hid_devices() if all_hids: for index, device in enumerate(all_hids): if device.serial_number == requested_serial_number: device.open() try: device.set_raw_data_handler(scales_handler) while True: sleep(0.1) except GotStableWeightException as e: self.do_HEAD(200) self.send_body(e.value) </code></pre> <p>And this is my scales_handler (plus custom exception definition):</p> <pre><code>def scales_handler(data): print("Byte 0: {0}".format(data[0])) print("Byte 1: {0}".format(data[1])) print("Byte 2: {0}".format(data[2])) print("Byte 3: {0}".format(data[3])) print("Byte 4: {0}".format(data[4])) print("Byte 5: {0}".format(data[5])) print("All: {0}".format(data)) if data[1] == 4: raise GotStableWeightException(data[4] + (256 * data[5])) class GotStableWeightException(Exception): def __init__(self, value): self.value = value def __str__(self): return repr(self.value) </code></pre> <p>So the process is:</p> <ol> <li>Python starts listening for requests on port 8000 using http.server's <code>HTTPServer</code> and <code>BaseHTTPRequestHandler</code></li> <li>A request comes in and is routed to my <code>do_GET</code> function.</li> <li>The <code>do_GET</code> performs some basic validation and then searches for a matching USB HID (my first code sample).</li> <li>If it finds a match, it opens the device and passes in a data handler.</li> </ol> <p><strong>What I'm trying to achieve:</strong></p> <p>I want to allow the data handler to keep reading from the scale until <code>data[1] == 4</code> (which indicates a stable reading from the scale). At that point, I want to retrieve this reading in my <code>do_GET</code> and send it to the waiting HTTP client, close the device and end the function.</p> <p>However, my <code>GotStableWeightException</code> isn't caught inside my <code>do_GET</code>, I think because it is thrown in a different thread. I'm new to programming with multiple threads, and I'm having trouble working out how I can get the result back to <code>do_GET</code> once it matches the <code>data[1] == 4</code> condition.</p> <p><strong>EDIT</strong></p> <p><strong>What I get:</strong></p> <p>Here's the output I see in the terminal from <code>scales_handler</code>:</p> <pre><code>Byte 0: 3 Byte 1: 4 Byte 2: 2 Byte 3: 0 Byte 4: 204 Byte 5: 0 All: [3, 4, 2, 0, 204, 0] Exception in thread Thread-1: Traceback (most recent call last): File "C:\Python33\lib\threading.py", line 639, in _bootstrap_inner self.run() File "C:\Python33\lib\site-packages\pywinusb-0.3.2-py3.3.egg\pywinusb\hid\core.py", line 886, in run hid_object._process_raw_report(raw_report) File "C:\Python33\lib\site-packages\pywinusb-0.3.2-py3.3.egg\pywinusb\hid\helpers.py", line 64, in new_function return function_target(*args, **kw) File "C:\Python33\lib\site-packages\pywinusb-0.3.2-py3.3.egg\pywinusb\hid\core.py", line 714, in _process_raw_report self.__raw_handler(helpers.ReadOnlyList(raw_report)) File "C:\USB HID Scales\server.py", line 28, in scales_handler raise GotStableWeightException(data[4] + (256 * data[5])) GotStableWeightException: 204 </code></pre>
 

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