bit_length returns the number of bits necessary to represent an integer in binary, excluding the sign and leading zeros. So

(x.bit_length() + 7) // 8

will just give you the number of bytes necessary to represent that integer x. You could also write something like

from math import ceil
ceil(x.bit_length() / 8)

to get the same number.

The method to_bytes() requires this byte length as its first argument.

To account for x == 0, use the max function to ensure that the number of bytes is at least one:

x.to_bytes(length=(max(x.bit_length(), 1) + 7) // 8, byteorder='little')

in python 3:

>>> a=b'\x00\x00\x00\x00\x07\x80\x00\x03'
>>> b = list(a)
>>> b
[0, 0, 0, 0, 7, 128, 0, 3]
>>> c = bytes(b)
>>> c
b'\x00\x00\x00\x00\x07\x80\x00\x03'
>>>

b"\x00d" means 2 bytes: \x00 and d but for values which are correct char codes (which are "printable") Python displays chars instead of codes. It makes it more readable when it may have readable text.

And chr(0x64) gives d

But if you use .hex() then you can get string 0064, and with .hex(":") you can get 00:64 (but it can't use .hex("\\x"))

If you want only codes \x... then you may have to convert it to string on your own (for example using for-loop and f-string)

a = 100
b = a.to_bytes(2, "big")

print('chr(0x64) :', chr(0x64))
print('b         :', b)
print('b.hex()   :', b.hex())
print('b.hex(":"):', b.hex(':'))

# ------------------------------------

items = []

for value in b:
    items.append( f'\\x{value:02x}' )

text = "".join(items)

print('text:', text)

# shorter
print('text:', "".join(f'\\x{q:02x}' for q in b) )

Result:

chr(0x64) : d
b         : b'\x00d'
b.hex()   : 0064
b.hex(":"): 00:64

text: \x00\x64
text: \x00\x64

Decode the bytes object to produce a string:

>>> b"abcde".decode("utf-8")
'abcde'

The above example assumes that the bytes object is in UTF-8, because it is a common encoding. However, you should use the encoding your data is actually in!