在Python中形成字节串

Question

I am creating a method in Python whereby it will take a number which will form a byte string that will then get sent to the Arduino. However whenever I try, the escape character is always included in the final byte string.

Here is the snippet of the code I am using:

num = 5
my_str = '\\x4' + str(num)
my_str.encode('utf-8')

Result:

b'\\x45'

I tried another method:

num2 = 5
byte1 = b'\\x4'
byte2 = bytes(str(num2), 'ISO-8859-1')
new_byte = byte1 + byte2
new_byte

Result:

b'\\x45'

Trying yet in a different way:

num = 5
u = chr(92) + 'x4' + str(num)
u.encode('ISO-8859-1')

Result:

b'\\x45'

I would like to get the byte string to be b'\\x45' without the escape character but not really sure what I have missed. I will appreciate any pointers on how I can achieve this.

Answer 1

Your problem is that you have already escaped the backslash. It is not recommended to construct a literal using an unknown variable, especially if there's a simpler way, which there is:

def make_into_bytes(n):
    return bytes([64 + n])

print(make_into_bytes(5))

This outputs

b'E'

Note that this isn't a bug, as this is simply the value of 0x45:

>>> b'\x45'
b'E'

The way this function works is basically just doing it by hand. Prepending '4' to a hex string (of length 1) is the same as adding 4 * 16 to it, which is 64. I then construct a bytes object out of this. Note that I assume n is an integer, as in your code. If n should be a digit like 'a' , this would be the integer 10 .

If you want it to work on hex digits, rather than on integer digits, you would need to change it to this:

def make_into_bytes(n):
    return bytes([64 + int(n, 16)])

print(make_into_bytes('5'))
print(make_into_bytes('a'))

with output

b'E'
b'J'

This quite simply converts the digit from base 16 first.

Answer 2

You can use the built-in function chr() to convert an integer to the corresponding character:

>>> chr(0x40 + 5)
'E'

Alternatively, if you just one to get the n-th letter of the alphabet, it might be more readable to use str.ascii_uppercase

>>> string.ascii_uppercase[5 - 1]
'E'

Note that the results in this answer are strings in Python 3, not bytes objects. Simply calling .encode() on them will convert them to bytes .