Bytes Command Function Not Working for PySerial

user_13147436 0
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I have a function which takes a set of values and converts it into a bytes string. For example, I need:

input_array = ['E9', '01','06','57','4A','01','F4','01','01','EF']
## needs to become b'\xE9\x01\x06\x57\x4A\x01\xF4\x01\x01\xEF'

The function I have is:

def string_to_command(inp): 
    new_string = ''
    for i in inp:
        new_string += r'\x' + i
    return new_string.encode('latin-1')

When I print both commands:

print(string_to_command(input_array))
print(b'\xE9\x01\x06\x57\x4A\x01\xF4\x01\x01\xEF')
# OUTPUT b'\\xE9\\x01\\x06\\x57\\x4A\\x01\\xF4\\x01\\x01\\xEF'
# OUTPUT b'\xe9\x01\x06WJ\x01\xf4\x01\x01\xef'

I am not sure what is going on here. The last one with b"" actually commands my output device properly, the other does not. How do I fix this problem?

pyserial python
3 Answers

You can convert to int first, then to bytes.

>>> a = ['E9', '01','06','57','4A','01','F4','01','01','EF']
>>> bytes(int(x, base=16) for x in a)
b'\xe9\x01\x06WJ\x01\xf4\x01\x01\xef'
user_36200030

You can join a list of bytes converted from the original array with latin-1 encoding. So adding \x as join sequence, it will output the desired bytes object:

input_array = ['E9', '01','06','57','4A','01','F4','01','01','EF']

print(b''.join([bytes.fromhex(x) for x in input_array]))

Output:

b'\xe9\x01\x06WJ\x01\xf4\x01\x01\xef'
user_138197140

The last one with b"" actually commands my output device properly, the other does not. How do I fix this problem?

I think what is going on here is that you are trying to add \x to every value and then encoding it from there. The problem with that is 'E9' and \x'E9' are not the same:

>>> 'E9'.encode()
b'E9'
>>> '\xE9'.encode()
b'\xc3\xa9' <-- Not the same
>>>

One method I like is bytes.fromhex(s), where s is your 'hex string'.

input_array = ['E9', '01','06','57','4A','01','F4','01','01','EF']

# use "".join(input_array) to get all values combined
# into one string
res = bytes.fromhex( "".join(input_array) )

Outputs :
b'\xe9\x01\x06WJ\x01\xf4\x01\x01\xef'

user_130600580
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