Issues with Bytes from a Microcontroller in Python

问题

I am using Python to read micro controller values in a windows based program. The encodings / byte decodings and values have begun to confuse me. Here is my situation:

In the software, I am allowed to call a receive function once per byte received by the Python interpreter, once per line (not quite sure what that is) or once per message which I assume is the entire transmission from the micro controller.

I am struggling with the best way to decode these values. The microcontroller is putting out specific values that correlate to a protocol. For example, calling a function that is supposed to return the hex values:

F0, 79, (the phrase standard_firmata.pde) [then] F7

returns:

b'\xf0y\x02\x03S\x00t\x00a\x00n\x00d\x00a\x00r\x00d\x00F\x00i\x00r\x00m\x00a\x00t\x00a\x00.\x00i\x00n\x00o\x00\xf7'

When set to "once per message" . This is what I want, I can see that the correct values are being sent, but there are too man \x00 values included (they are after every byte it seems). Additionally, the second byte is 0ywhen it is supposed to be 79. It seems like it printed its value in ASCII when all the others were in hex.

How can I ignore all these null characters and make everything in the right format (I am fine with normal hex values)

回答1:

When Python represents a bytes value, it'll use the ASCII representation for anything that has a printable character. Thus the hex 0x79 byte is indeed represented by a y:

>>> b'\x79'
b'y'

Using ASCII characters makes the representation more readable, but doesn't affect the contents. You can use \x.. hex and ASCII notations interchangeably when creating bytes values.

The data appears to encode a UTF-16 message, little endian:

>>> data = b'\xf0y\x02\x03S\x00t\x00a\x00n\x00d\x00a\x00r\x00d\x00F\x00i\x00r\x00m\x00a\x00t\x00a\x00.\x00i\x00n\x00o\x00\xf7'
>>> data[4:-1].decode('utf-16-le')
'̂StandardFirmata.ino'

UTF 16 uses 2 bytes per character, and for ASCII (and Latin 1) codepoints that means that each 2nd byte is a null.

You can use simple comparisons to test for message types:

if data[:2] == b'\xf0\x79':
    assert data[-1] == 0xf7, "Message did not end with F7 closing byte"
    version = tuple(data[2:4])
    message = data[4:-1].decode('utf-16-le')

来源：https://stackoverflow.com/questions/25480639/issues-with-bytes-from-a-microcontroller-in-python