data frame from received messages pcan

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Originally reported by: Alex Eshoo (Bitbucket: AlexEshoo, GitHub: AlexEshoo)

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I'm trying to get just the data bytes from a received message using pcan. I see that it sets msg.data = theMsg.DATA which is a c_ubtye Array type. I tried to wrap it with bytearray() to convert it to something readable, but it just outputs blank lines. Am I doing something wrong or is there a better way to access just the data bytes of a received message using pcan interface?

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• Bitbucket: https://bitbucket.org/hardbyte/python-can/issue/66

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Original comment by Brian Thorne (Bitbucket: hardbyte, GitHub: hardbyte):

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Hmm, that is not ideal. In the socketcan backend the data is copied across byte by byte from the ctypes array:

[frame.data[i] for i in range(frame.can_dlc)]

I don't see why this won't work for pcan.py too - would you mind trying it out?
Either comment here or add a PR and I'll change the library.

Cheers,
Brian

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Original comment by Alex Eshoo (Bitbucket: AlexEshoo, GitHub: AlexEshoo):

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Unfortunately I don't have my PCAN hardware right now, but I agree something like that might work. I had actually copied the code from the __str__() method of the lower Message class: data_strings.append("{0:02x}".format(self.data[index])) and then I converted the strings to ints using the int(s,16) builtin. I think the method used in the socketcan backend is better though. I don't see why it wouldn't work either, so if I get a chance to sit down with the hardware again I'll give it shot and let you know how it works.

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Original comment by Kevin Albert (Bitbucket: Kevinalb, GitHub: Kevinalb):

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Hello I run into the same problem (in fact Issue #71)

Changing
rx_msg.data = theMsg.DATA
for
rx_msg.data = [theMsg.DATA[i] for i in range(rx_msg.dlc)]

Produce something like this : [23, 0, 112, 0, 128, 7, 51, 71] is that what to be expected?
When using a Notifier and a listener with this data produce an error int the str method of the Message Object :
File "C:\Users\maler\AppData\Local\Programs\Python\Python35\lib\site-packages\can\message.py", line 73, in str
if (self.data is not None) and (self.data.isalnum()):
AttributeError: 'list' object has no attribute 'isalnum'

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Original comment by Mathias Giacomuzzi (Bitbucket: mgiaco, GitHub: mgiaco):

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So should I add a pull request here? Because in pcan.py the line:

#!python
        rx_msg.data = theMsg.DATA

should be replaced by

#!python

        rx_msg.data = bytearray(theMsg.DATA)

But I am not sure if this is the fastest convert.
But this change works for me on Windows 7 with python 3.5 and Peak CAN

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Original comment by Mathias Giacomuzzi (Bitbucket: mgiaco, GitHub: mgiaco):

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I also merged the latest PCANBasic.py with the default.

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Original comment by Kevin Albert (Bitbucket: Kevinalb, GitHub: Kevinalb):

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Would there is still a problem with Message str?
Or is that only me?

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Original comment by Alex Eshoo (Bitbucket: AlexEshoo, GitHub: AlexEshoo):

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@kevin from what you wrote before, your source for rx_msg.data in pcan.py does not match what Mathias suggested. You should try that if you can. I have not been able to try anything yet, though I may get a chance next week.

@mathias I believe I initially tried to output bytearray(theMsg.DATA) and had issues with that, but I may have done something wrong/different. If I get a chance I will try it out and let you know. Sorry for not having tried anything yet, I have been occupied with other things this week.

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Original comment by Alex Eshoo (Bitbucket: AlexEshoo, GitHub: AlexEshoo):

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Brian,

I was able to try out the suggestion you had. Setting:

#!python
rx_msg.data = [theMsg.DATA[i] for i in range(rx_msg.dlc)]

In pcan.py works great. print msg.data outputs a list of data now as expected originally. I think this is the best implementation.

Made PR 23

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Original comment by Mathias Giacomuzzi (Bitbucket: mgiaco, GitHub: mgiaco):

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Alex are you sure? I tried it now and this does not work for me with python 3.5 and peak can. I do not understand why this should work because rx_msg.data is type bytearry() and then you allocate a list to it. Does a list have an isalnum() function?

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Original comment by Alex Eshoo (Bitbucket: AlexEshoo, GitHub: AlexEshoo):

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I cant speak for python 3.5, I am on 2.7. rx_msg.data is actually a ctypes byte array so I get the idea behind wrapping it with bytearray() but when I tried this the output is not readable. On 2.7 I get a nice readable list with the change that Brian suggested. isalnum() does not appear to be a method on list in python 2.7 either, but I don't encounter this exception. Did you make any other changes to the source by any chance?

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Original comment by Mathias Giacomuzzi (Bitbucket: mgiaco, GitHub: mgiaco):

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Sorry, I have my peak can at my office. So I will try it on Monday with py2.7 and py3.5.

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Original comment by Christian Sandberg (Bitbucket: sandberg, GitHub: sandberg):

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Maybe?

#!python
bytearray(theMsg.DATA[:rx_msg.dlc])

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Original comment by Alex Eshoo (Bitbucket: AlexEshoo, GitHub: AlexEshoo):

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I think I see where the code is failing in the Message() class. From Kevin's post the __str__ method on that class has a line: if (self.data is not None) and (self.data.isalnum()): If self.data is a list, python will raise an attribute error when calling isalnum() on it. I did not have the most recent version of message.py, thus my code was working. Why was this added to Message?

#!python

if (self.data is not None) and (self.data.isalnum()):
            try:
                field_strings.append("'{}'".format(self.data.decode('utf-8')))
            except UnicodeError as e:
                pass

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Original comment by Christian Sandberg (Bitbucket: sandberg, GitHub: sandberg):

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I think it makes sense that all interfaces return data as the same type to avoid surprises and bytearray is a logical representation for it. The code in str checks if the data happens to be a string and prints it as well. Not sure how many CAN messages are pure strings though...

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Original comment by Mathias Giacomuzzi (Bitbucket: mgiaco, GitHub: mgiaco):

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So as I said before for me

#!python

rx_msg.data = bytearray(theMsg.DATA)

Works perfect for me - and sure with the latest from the repository. With py2.7 and py3.5